# Exaust backfiring



## TheGoatFather1965 (Jul 30, 2017)

Hello,

I can’t seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.

My timing is at 14 initial and 35 total.

I have a dual feed carb.

What do you think it may be?


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## cij911 (Oct 25, 2017)

TheGoatFather1965 said:


> Hello,
> 
> I can’t seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.
> 
> ...


Are you logging AFRs when you slam the throttle ? My guess is you are running rich and then the unburnt fuel is exploding in the exhaust....

I am using a wideband to log AFRs right now and it has been interesting....


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## TheGoatFather1965 (Jul 30, 2017)

Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.


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## cij911 (Oct 25, 2017)

TheGoatFather1965 said:


> Hi ,
> 
> I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.


Well I am using the innovate wideband as well (LM2). Under throttle I am more like 12.5 - 13 range at the leanest. If you are seeing an AFR of 15 when WOT then you are tuned too lean IMHO. Optimal AFR is 14.7 (as I am sure you know), but there are a number of other factors to consider and with a carb I doubt you will ever be able to tune the car as precisely as a newer computer controlled fuel injected motor. I generally am happy if the car is at 12 - 13 AFR under WOT.

Obviously when you let off the throttle, your AFR will be much greater than 14.7. I believe I see greater than 20 IIRC while coasting.

Your plugs don't look "bad" to me, but you are definitely not running rich by the looks of them (syncs to your wideband readings). Please confirm when you are hearing the backfire.

Generally a backfire indicates you are running rich (as there is unburnt fuel in the exhaust that ignites) or you have a timing issue. (That said, it can happen if you are too lean as well.)

What is your vacuum at idle? Have you verified you don't have a vacuum leak? What is your AFR at idle?


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> TheGoatFather1965 said:
> 
> 
> > Hi ,
> ...


Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT. 

I double check my timing and it still the same as what i set it at. My initial is 14 and my total @ 35.

I shouldnt have any vacuum leaks i check a while ago but i used to have the same backfiring issue as well. The device im using isnt accurate at idle shows me 25+ but as soon as i raise the rpm to about 1500 starts reading better


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## 64GTOConvertible (Aug 28, 2016)

Plug wires might be getting old. What kind of ignition are you running?


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## cij911 (Oct 25, 2017)

TheGoatFather1965 said:


> Hello,
> 
> As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.


Well I think you are running a bit lean and may not be getting a great combustion. You could try retarding the timing 2* and see if that helps.

25" of vacuum at idle is great (congrats) - that would tell me you don't have a vacuum leak.


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## TheGoatFather1965 (Jul 30, 2017)

64GTOConvertible said:


> Plug wires might be getting old. What kind of ignition are you running?


Hii,

Im running the stock distributor, the spark plug wires are new , taylor thundervolt and new spark plugs installed together about 3 monthes ago.


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> TheGoatFather1965 said:
> 
> 
> > Hello,
> ...


I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?


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## cij911 (Oct 25, 2017)

TheGoatFather1965 said:


> I tried changing the timing but still got the same backfiring.
> 
> Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?


If you retarded timing and still had the backfiring then I'm guessing it is just too lean a mixture and you will need to richen it up a bit.

25" of vacuum would be nice, but I believe is atypical for Pontiacs....

When does your full timing come into play (RPM?) ?


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> TheGoatFather1965 said:
> 
> 
> > I tried changing the timing but still got the same backfiring.
> ...


Im going to retard my timing again and see just to double check, around 4 degrees and start from there.

I have a vaccum guage, im going to see what i get on idle, whats a good number to be above?

I usually set my total @4000 rpm.

Thank you


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## PontiacJim (Dec 29, 2012)

What type/brand/size of carb do you have?
Many things to check as to what it could be. More info might help us ID the problem better or narrow it down, so here is a list and a few more things to consider. 


Does it stumble/bog at any point during acceleration?

What spark plugs/gap are they?

Point gap and dwell setting checked/confirmed?

Checked PCV valve/hose/grommet for operation and/or vacuum leaks?

Headers or cast iron exhaust manifolds?

Any leaks or bad gaskets at the exhaust ports allowing air to be sucked in?

1.5 or 1.65 rocker arm ratio?

Rocker arms/hydraulic lifters adjusted too tight keeping valves slightly open at RPM.

Weak valve spring?

Bad cam lobe? 

With the high vacuum number of 25", maybe it is pulling too much vacuum for the vacuum can on your distributor? Maybe try an adjustable vacuum can which can be used to set the speed at which the vacuum advance pulls in when you let off the gas.

Just for fun, have you tested the vacuum with the air cleaner off? Is is possible the air cleaner is not flowing as it should and has become a restriction?


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## cij911 (Oct 25, 2017)

TheGoatFather1965 said:


> Im going to retard my timing again and see just to double check, around 4 degrees and start from there.
> 
> I have a vaccum guage, im going to see what i get on idle, whats a good number to be above?
> 
> ...


Full disclosure I am an OLD CAR NEWB and have learned a lot recently about mechanical and vacuum advance and Carb design. That said, there are many related aspects to EFI / computer tuning and older style setups.

Lars sent me a very good write up on timing and if I remember correctly my full timing comes in much earlier (more like 3K rpm).

Vacuum at idle - the higher the better but I doubt you will see more than 25" with a decent gauge. From what I have read many GTOs are below 18" and if they have a more aggressive cam will be ~10 - 12".


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> TheGoatFather1965 said:
> 
> 
> > Im going to retard my timing again and see just to double check, around 4 degrees and start from there.
> ...


Hello,

I set it my total @ 4000 because its said that it could be more accurate and set my intial with the distributer advance port disconnected as well.

Ill be really happy with 14-16” but i actually have an aggressive cam!

Im going to test it tomorrow and let you know of the results hopefully.

Where is the highest manifold pressure on the intake?


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## PontiacJim (Dec 29, 2012)

I am also going to throw this one at you just for consideration - your previous posting:

"*455 trouble*s

I know this is a old post , but i thought id let you guys what went on.

Did a compression test all were within 10% of each other, except for one, when i removed the spark plug coolant bursted out of the spark plug hole!!!

So i removed everything, changed the head gasket and basically did a overhaul since i was at it.

And now no more coolant loss and it runs a little better!

Im glad it was only a head gasket."

Did you have the head checked to make sure a valve was not bent? If you had coolant bursting out the spark plug hole when you removed the plug, it is possible that the water in the cylinder could have caused what is sometimes referred to as "hydrolock." Could have bent a valve, pushrod, or ? Just a thought on the popping exhaust problem. 

My brother operates a railroad and one of his engines experienced this. He knew he had one cylinder which had a water leak and it is possible on a train engine to drain it off - but this was done in an attempt to keep the train running until it could be put out of service and repaired. His luck ran out and the engine locked up at idle - hydrolocked. When they pulled down the engine to check the cylinder, the connecting rod he pulled out was pretty bent up. He told me that connecting rods in a diesel engine are pretty big, but water does not compress too well so something has got to give. A new liner, piston, and rod and it's all fixed up and running again.


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## TheGoatFather1965 (Jul 30, 2017)

PontiacJim said:


> What type/brand/size of carb do you have?
> Many things to check as to what it could be. More info might help us ID the problem better or narrow it down, so here is a list and a few more things to consider.
> 
> 
> ...


Hello Pontiac Jim,

I have a thunder series AVS 800 cfm edelbrock Carb.

Doesnt stumble nor bog at acceleration.

Spark plugs are delco R45T’s

Gap checked and confirmed i think at 0.40 if I remember correctly.

PCV valve no leaks! But i still need to do a full vaccum check.

Dougs long tube headers!

The exaust gasket may have a small leak but i dont think so but even before this leak i had the same problem so im 100% sure its not it.

1.5 rocker arms ratio, i tightened them myself.

Cam is fine i dont think its a a bad lobe but how do i check?

For the vacuum as i mentioned above i need to do a vacuum test to be sure.

I have double valve springs that i installed when i installed this cam.

And ill check that once i do the vacuum test!


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## TheGoatFather1965 (Jul 30, 2017)

PontiacJim said:


> I am also going to throw this one at you just for consideration - your previous posting:
> 
> "*455 trouble*s
> 
> ...


Hello,

When i replaced the head gasket i checked the springs, rockers , pushrods, valves and everything and polished and resurfaced the head since i was at it just to be sure.

How do i check if the cam lobes are gone?


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## PontiacJim (Dec 29, 2012)

OK on your heads as you had things checked out - was just a consideration.

Two things. 1.5 rockers and you tightened yourself. How? Factory torque down or are you using poly locks & "zero lashing" them?

Next, since you have been playing around a lot with the carb, throw away the air/fuel gauge for now and adjust it old school. I found this on another site which might help? -- "Idle mixture screws too far in and the butterflys too far open at idle will do it. Drawing on fuel/air from the main circuit and not from the idle circuit, so when you cut the throttle there's excess fuel being drawn in through the main circuit."

Fatten up your idle mixture screws and back out of the idle screw. Don't use a vacuum gauge either. Adjust it by ear. I like to begin by backing out the idle mixture screws 2 1/2 turns each, but in your case back them out about 4 turns to fatten them up. Then back your idle screw out to bring your RPM's down to around 650 RPM's (or whatever the lowest idle RPM your cam like to idle at). Then, one at a time, turn each in until the engine RPM's drop, stop adjusting, and turn the idle mixture screw back out to get the engine back up to 650 RPM's. See if that setting changes anything. I might even try it with 4 turns out and the idle at 650 RPM's and take it for a spin to see if it gets worse or better.

You are going to have to experiment, the old way. If richening it up improves it, then that's the way to go. If it gets worse, then leaning it out may be the way to go and then test that with a drive. The carb also has the step-up springs rated for specific vacuum settings. 25Hg of vacuum at idle seems way too high for a performance cam, but I am no expert on the subject.

There are other adjustments on the carb as well which the online owners manual points out. Check the trouble shooting list near the end. Do you have the correct 4-hole spacer if you have it on a factory Q-jet, or spreadbore type manifold?: http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

Might be worth a call to Edelbrock as they could possibly steer you in the right direction as well. :thumbsup:

Another long shot. Maybe your headers are over scavenging? This may also be cam related based on cam overlap where the intake/exhaust valves are open at the same time. Unburned fuel mixture may be getting sucked out for an instant when you decelerate rather than "force" the exhaust gases out the headers under load when you open up the throttle. I have also read that headers require a step up in carb jetting to richen up the carb when they are used. Just another thing to confuse you. :yesnod:


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## TheGoatFather1965 (Jul 30, 2017)

PontiacJim said:


> OK on your heads as you had things checked out - was just a consideration.
> 
> Two things. 1.5 rockers and you tightened yourself. How? Factory torque down or are you using poly locks & "zero lashing" them?
> 
> ...


Sorry for the later reply!!! I dont know how to turn on notifications on the forum.

As im still having the same problem,

1 - for the rockers, factory torque down.

2- thats how i usually adjust the carb and it seems to be running great and now i keep the idle @700 rpm

3- my manifold is a an edelbrock performer so i dont have to run an adapter.

As for the headers im not sure? How would i know if they were ‘over scavenging’

I did notice something yesterday tho im running a stock style valley pan with a pcv provision but i dont have a grommet installed with the pcv , could this be an issue? So the pcv is plugged directly into the valley pan without a grommet.

This has taken so long and i cant seem to figure it out.

I have now installed an MSD distributor and ignition box, still the same.

Im going to change out the spark plugs as well just to make sure i eliminate the ignition issues.


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## PontiacJim (Dec 29, 2012)

TheGoatFather1965 said:


> Sorry for the later reply!!! I dont know how to turn on notifications on the forum.
> 
> As im still having the same problem,
> 
> ...



You say you have a radical cam. In one of your posts, you stated you had 25" of vacuum at idle. You won't have 25" of vacuum with a radical cam. Now you state you will be happy with 14"-16" of vacuum. I'm confused. :thumbsup:

You could have a bad/worn cam lobe(s).

What is your _real _vacuum reading at idle? 

Run plug gap at .035".

PCV valve needs the rubber grommet in the valley pan to which it plugs into.

What happened to all your AF ratio's and the wideband monitor?

So begin the whole process again and post your numbers.

MSD distributor. You eliminated the factory resistor wire and have it wired for 12volts? Most aftermarket distributors require 12volts, not 8 volts like points. Your instructions for the MSD should say what you need.

Cam Specs ?

New balancer?

Confirmed No.1 piston at TDC and timing mark on balancer at 0?

Did you degree the cam to manufacturer's specs and confirm them?

Initial Timing - with vacuum advance disconnected and plugged at the carb?

At what RPM does your advance begin to move? Should not move until you get to about 1000-1200 RPM's.

What is your total timing in degrees when the distributor advance stops ( vacuum disconnected) ? You stated 4,000 RPM's. Double check. You should see 34-36 degrees total at what RPM?

Some additional reading on the wideband tuning. You may still need to step up on your jetting if you have a radical cam and used the carb right out of the box.

https://www.enginebuildermag.com/2008/07/carburetor-tuning-the-airfuel-equation/

*Mixture, Air/Fuel Equation*
Henry P. Olsen,author

Now that the ignition spark timing advance curves are optimized for the blend of reformulated and/or oxygenated gasoline your customers are using we will now show you how we use tools such as a 5-gas exhaust analyzer and wideband Lambda air/fuel (A/F) meter to tune the mixture.

*The Air/Fuel Mixture*

A lean fuel mixture can cause an engine to have a surge or miss at idle and part throttle stumble on acceleration, leading to engine overheating and lack of power. A rich fuel mixture can cause an engine to “load up” at idle, foul the spark plugs, and also lack power or run sluggish.

If the A/F mixture that is delivered to the engine is excessively rich for too long the engine could leave leftover fuel from the combustion process, washing the oil off the cylinder walls. Without the oil to act as an anti-wear agent, the pistons and rings will make metal-to-metal contact with the cylinder walls. Also, if enough fuel gets past the rings and into the crankcase the oil can become diluted and lose much of its lubricating properties and accelerate engine wear.

Theoretically, the ideal stoichiometric A/F mixture (the chemically ideal mixture of air and fuel that is required to provide a complete burn) for a properly tuned engine running on pure gasoline is 14.7:1; that is, 14.7 lbs. of air to 1 lb. of fuel. However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, plus the fact that fuel may not fully vaporize in the combustion chamber, a 14.7:1 A/F mixture is often too lean for actual operating needs. A more realistic light-load, cruise A/F mixture for a stock carbureted engine running on reformulated unleaded gasoline is in the 14.1:1 range.

The A/F mixture always varies from cylinder to cylinder, therefore we tend to tune the average A/F mixture slightly on the rich side to avoid engine misfire in the leanest cylinder. It is possible to target an A/F mixture leaner than 14.7:1 for maximum fuel economy but this can lead to driveabilty problems if any one cylinder is leaner than the others. The power mixture we target for maximum horsepower is in the 12.2:1 – 13.5:1 A/F range, depending on the engine package and its combustion chamber design.

The original equipment carburetor(s) that came on a muscle or classic vehicle’s engine was tuned for the leaded gasoline of the day, so in most cases the engine will tend to run lean with the reformulated and/or oxygenated unleaded gasoline of today. The gasoline of today also has lower volatility than the leaded gasoline of days past, which will cause most carbureted engines to need a slightly richer A/F mixture at idle and light load part throttle driving conditions to have the same drivability as it had with the leaded gasoline of the ’60s and ’70s.

Back in the 1950s and early 1960s, the car manufactures tended to calibrate their carburetors on the rich side of the ideal A/F mixture needs of the engine with the leaded gasoline of the day. Then starting in the late 1960s, the carburetors were calibrated more toward the lean side of the ideal A/F mixture needs of the engine so the vehicle could pass the exhaust emission standards that were just coming into existence.
The modern reformulated conventional and oxygenated gasoline of today will cause the A/F mixture to shift leaner when compared to the leaded gasoline of the 1960s and 1970s. This means if the A/F mixture was lean with leaded gasoline it will be even leaner with today’s gasoline blends.

The high performance and replacement carburetors sold today are sold with an A/F mixture curve designed for a generic engine; therefore they must be tuned for both the specific engine and the blend of gasoline they will be used with. These aftermarket carburetors should be designed with an A/F mixture that is rich enough for a wide variety of engine packages with different exhaust systems, but this is not always true. Some of the aftermarket carburetors we see need a lot of tuning work to get the A/F mixture correct for the engine’s demands with the reformulated unleaded gasoline of today.

*Air/Fuel Mixture Tuning Guidelines*

Back in the days of leaded gasoline an experienced tuner would adjust the A/F mixture the engine was getting from its carburetor by reading the color the fuel left on the insulator of the spark plug in the exhaust port and in the first 6 inches of the exhaust header. The reformulated unleaded gasoline we have today has made reading spark plugs almost impossible because it leaves little or no color on the spark plug insulator.
However, modern technology has made available at an affordable price both portable 5-gas exhaust gas analyzers and wideband Lambda (“oxygen”) sensor based digital A/F meters that can be used to accurately “read” the A/F mixture in an engine by analyzing the content of the engine’s exhaust gases. These modern tools can allow you to observe what A/F mixture the engine is getting from the fuel system while driving the car in real world conditions at any rpm and load condition.

The ideal A/F ratio for maximum power or fuel economy may be best calculated at the factory with the engine on a dynamometer, but the readings that are available from a 5-gas exhaust gas analyzer allow you to tune the A/F mixture for what your engine needs in real world driving conditions. The readings from an infrared exhaust gas analyzer will indicate A/F ratio, engine misfire, engine combustion efficiency and excessive combustion chamber heat (detonation) by looking at the following exhaust gases:

CO (Carbon Monoxide): The reading from an infrared gas analyzer that we use to determine the air to fuel ratio when the A/F mixture is on the rich side of stoichiometric. (Note: CO is partially burned fuel.)
The other readings that exhaust analyzers provide are:
HC (Hydrocarbons): The amount of unburned fuel in the exhaust (a indicator of an engine misfire).
CO2 (Carbon Dioxide): A gas that is the product of complete combustion (the best A/F mixture gives you the highest CO2 reading). The ideal ignition-timing advance will also create the highest CO2 reading
O2 (Oxygen): A high O2 reading indicates a lean mixture; an exhaust leak or the engine has a “hot” cam. Note: if O2 content is above 2 to 3 percent, air dilution of the exhaust gases being measured is indicated and the accuracy of the all of the gas readings may be negatively affected.
NOx (Oxides of Nitrogen): A gas created by excessive combustion chamber heat. This gas can be used as a precursor to detonation.

The readings you can get from a 5-gas exhaust gas analyzer can help an experienced tuner calculate what A/F mixture and how much ignition spark timing advance the engine needs to perform at its best.

A wideband sensor lambda sensor based A/F meter calculates the A/F mixture by “reading” the unburned combustible content of the exhaust gases (note: a lot of people call the lambda sensor an oxygen sensor but Bosch calls it a lambda sensor). The wideband lambda sensor measures the amount of oxygen that must be added to or subtracted from the exhaust gas to form a stoichiometric gas mix in its reference chamber, the A/F meter then calculates the A/F mixture of the exhaust gas from that value.

The readings you get from a wideband lambda sensor based A/F meter can be quite accurate, but false readings can be created by an exhaust leak, engine misfire, or an engine with a high performance camshaft at lower engine speeds. These false readings are caused by the Lambda sensor misreading the unused oxygen and/or unburned combustibles that are in the exhaust gas mixture

*Tuning with a 5-Gas Analyzer and Wideband Lambda Meter*

The use of a portable 5-gas exhaust gas analyzer and/or a wideband sensor based A/F meter can allow a tuner to observe the A/F mixture the engine is getting from its fuel system at any engine operating condition.

A starting point for A/F mixtures for most mild performance engines is:
• Idle: 1.0% to 3.0 % CO or a 14.1-13.4:1;
• Cruise rpm: 1.0% CO or a 14.1:1 with a mild performance engine; or 1.0% – 3.0% CO or a 14.1 – 13.4:1 with high performance cam; and
• Power mixture and acceleration: 6.0% CO or a 12.5:1 for a “normal” engine or high performance engine with improved combustion chamber design such as a Pro Stock or a NASCAR engine; in some cases you may be able to use a slightly leaner power mixture of 4% CO or a 13.0:1.

When we are tuning fuel systems, we use both infrared exhaust gas analyzer and the wideband Lambda sensor methods. This way we can take advantage of the strengths of both tuning methods. The infrared exhaust gas analyzer, while slower in reaction time than a wideband sensor based A/F meter can actually best determine A/F mixture needs. The misfire rate can be observed with the HC (hydrocarbon) reading.

Efficiency can be observed by the CO2 reading (carbon dioxide) reading, and the NOx reading (oxides of nitrogen) can also be used as a precursor to detonation. A wideband Lambda sensor-based A/F meter systems available from companies such as Innovate Motorsports or FAST have almost no delay, while a 5-gas exhaust gas analyzer has a 6 to 10 second delay.

If the engine you are tuning has an air-gap style intake manifold and/or high performance camshaft you may need to tune the idle and cruise mixtures richer than a stock engine with the same gasoline. The added performance from an air-gap intake manifold and the increased valve overlap from a high performance camshaft can often come at the price of lower fuel vaporization at lower rpm operating conditions.

The richer A/F mixture can help cover up the driveability problems when the fuel is not completely vaporized. The heat the intake manifold gets from the exhaust gas crossover in a conventional intake manifold helps the engine vaporize the fuel as it travels from the carburetor into the cylinders combustion chamber.

*A/F Mixture Delivery Circuits*

A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct A/F mixture for the demands. The accelerator pump system adds fuel as the throttle valves are opened. Tuning the accelerator pump squirter volume and duration is mainly done by trial and error to obtain the best throttle response, but a 12.5:1 A/F mixture is a good place to start.

An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. If the engine you are working on is equipped with a power valve (no metering rods), the main jet size is what determines the A/F mixture that will be delivered to the engine at light-load/cruise speeds.

The power valve restriction (under the power valve) determines what A/F mixture the carburetor will supply when the power valve is open; under high power demands a 6.5? power valve will be open, supplying richer A/F mixture any time the vacuum is below its 6.5? opening point.

Power valves have a reputation for being a weak link in certain designs, but the carburetor can be retrofitted with backfire protection, which will improve reliability. A carburetor that uses metering rods in the primary jets will use the metering rods to change the A/F ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the A/F mixture.

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is to determine what the A/F mixture is at idle through 3,000 rpm. If the cruise mixture is off, first change the jets to get the A/F mixture correct at 2,500-3,000 rpm cruise range. Then check and set the idle mixture. If the A/F mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle jet.

If the mixture is still lean at 1,000 through 1,800 rpm after enlarging the idle jet, the idle channel restriction (if used) may have to be enlarged slightly to allow more fuel to be delivered at part throttle. It is important to note that any changes other than basic adjustments and jet changes should be done by a “carburetor expert” to avoid damaging a vintage carburetor. If the carburetor is damaged a replacement numbers matching carburetor could be quite expensive.

A modular design carburetor, such as a Holley, with a metering block does not use an idle channel restrictor. When we want to richen the part throttle we often must slightly enlarge the idle well in the metering block. When the A/F mixture is too lean at part throttle the engine may miss or stumble on light acceleration and at 5 – 25 mph light throttle cruise conditions.This lean off idle problem has become more prominent as the ethanol content in today’s gasoline is increased and as the gasoline formulation is changed.

If the A/F mixture is too rich at idle and/or part throttle, the idle jet or part throttle idle restriction may be too big. You may need to be replace it with a smaller one. Once you have the idle, part throttle and cruise A/F mixture curves correct, the next step is a road test.

A road test using a portable infrared exhaust gas analyzer and/or a wideband oxygen sensor will allow you to check the cruise speed A/F mixture, followed by a check of the power A/F mixture under load. This type of test allows you to see what the A/F mixture is under real world driving conditions. During this road test you will be able to read and then correct the A/F mixture.

If you see an A/F mixture reading that goes too lean at high engine loads, the first thing to do before you change jet size is to check the fuel pressure. The fuel pressure must stay above 5 psi at wide-open throttle; if not, the carburetor will starve for fuel.
The most common accelerator pump-related complaint we hear is a hesitation on quick acceleration. This hesitation is most often caused by the changes in the gasoline’s volatility and changes in carburetor manufacturing. The accelerator pump duration spring used on most replacement carburetors is not as strong as the spring that was on these same carburetor designs used in the 1960s.

We use an accelerator pump upgrade kit on most Holley modular style carburetors that consists of a stronger duration spring, a 0.031? squirter and a “pink cam” (Ole’s p/n 1330), this makes the accelerator squirt more active.

When we are working on a engine with an Edelbrock Performer or Thunder series carburetor we use an improved accelerator pump (Ole’s p/n 1010). This accelerator pump has a stronger duration spring that allows the pump to be more active and thus help cure most of the accelerator pump related hesitation we see with these carburetors.

*Selecting the Correct Carburetor*

The big four suppliers of 4 barrel carburetors today are Edelbrock, Holley, Quick Fuel and Barry Grant, each of these carburetor designs have strong and weak spots. The carburetor that we would recommend is based on how the customer will drive their car and the engine package that is in the car.

The Carter-designed Edelbrock Thunder and Performer are reliable low maintenance carburetors with great electric chokes but if the driver likes to drive fast around corners they may not bethe best carburetor to select for that application. The off-idle system design in these Carter-designed carburetors can lead to a lean off-idle stumble problem when the engine has a “hot cam” or an air-gap style intake manifold. Enlarging the idle channel restrictor on the 500 thru 650 cfm units will often cure this lean off-idle stumble problem but we have not had the same success solving this lean off-idle stumble problem on the 750 and 800 cfm carburetors of this design.

Modular carburetors manufactured and sold by companies such as Holley, Barry Grant and Quick Fuel are very good carburetors to select when the driver likes to drive fast around corners or when you are tuning for maximum power. Quick Fuel also sells billet metering blocks with changeable idle jets, power channel restrictors and emulsion well restrictions for the Holley style modular carburetors, which allow you to custom tune the fuel curve.
When the customer wants a high performance modular carburetor with an electric choke we often recommend a Holley brand carburetor because their chokes have a choke pull-off built in. When we are tuning a high performance engine with a “hot cam” (over 240 degrees of duration @ .050?) or any engine with an air-gap style intake manifold, we often recommend a race-designed modular carburetor with a four corner idle system.


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## TheGoatFather1965 (Jul 30, 2017)

PontiacJim,

“You say you have a radical cam. In one of your posts, you stated you had 25" of vacuum at idle. You won't have 25" of vacuum with a radical cam. Now you state you will be happy with 14"-16" of vacuum. I'm confused. “

I said i get a reading of 25” because its a faulty guage but gets corrected and drops and read properly when i rev it to 1000+ Rpm.

So i do NOT have 25” of vaccum.

I installed a new cam to see if i have worn out cam lobes but the issue is still here.

‘What is your real vacuum reading at idle?’

- i need to confirm before telling you. However, yesterday i checked my ‘ported vaccum’ on the carb ports. It gave me a reading of 8.5” @ idle and 22-25” @ 4000 rpm.

Run plug gap at .035".
- i need to change the spark plugs and i will gap them at .035 i used to run them at 0.040.

PCV valve needs the rubber grommet in the valley pan to which it plugs into.

- Purchased one and waiting for it to be delivered. Can this affect vaccum or cause leaks on the valley pan?

What happened to all your AF ratio's and the wideband monitor?

- Nothing i just broke the clamp but i can use it on idle and rev instead of street driving.

So begin the whole process again and post your numbers.

MSD distributor. You eliminated the factory resistor wire and have it wired for 12volts? Most aftermarket distributors require 12volts, not 8 volts like points. Your instructions for the MSD should say what you need.

Yes, now wired for 12volts.

Cam Specs ?

- New cam specs (3/4monthes ago) 
230/236 @ 0.50’ duration - 110LBA

New balancer?

-OEM balancer.

Confirmed No.1 piston at TDC and timing mark on balancer at 0?

-YES

Did you degree the cam to manufacturer's specs and confirm them?

-YES

Initial Timing - with vacuum advance disconnected and plugged at the carb?

- YES / MSD has the mechanical advance so the carb was plugged and the initial is set @ 16degrees.

At what RPM does your advance begin to move? 

- starts moving @ 1000-1200 RPM's.

What is your total timing in degrees when the distributor advance stops ( vacuum disconnected) ? You stated 4,000 RPM's. Double check. You should see 34-36 degrees total at what RPM?

I will double check tonight, but it was set @ 35 degrees total @ 4000RPM’s


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## cij911 (Oct 25, 2017)

Well I am confused with your vacuum readings....8.5" vacuum at idle definitely syncs to an aggressive cam.

Jim's post on AF was great and I suspect I am setup a tad too lean at idle / cruise (changed after reading Pontiac's like to run lean).....My guess is your AF ratio is the problem. Get the car to operating temperature and log AF from idle to 3000 rpm as suggested and report back . Good luck


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## PontiacJim (Dec 29, 2012)

*GF*: "- i need to confirm before telling you. However, yesterday i checked my ‘ported vaccum’ on the carb ports. It gave me a reading of 8.5” @ idle and 22-25” @ 4000 rpm."

*PJ*: 8.5" @ idle seems wrong - you don't have a cam that big. Don't use a "ported" line coming off your carb to get your reading. Can you hook to a line that would pull off the manifold? You carb might have a fitting for both "ported" and "manifold" at the front? Sometimes a vacuum T is used at the back of the carb if you have power brakes and you can tap off this. 

Ported vacuum is applied above the throttle plate in the carb so it only has vacuum as you start to add rpm. Manifold vacuum is applied below the throttle plate at idle. The difference between the 2 is the amount of vacuum applied at idle, and just off idle as engine RPM's begin to increase.

*GF*: Can this (PCV valve) affect vaccum or cause leaks on the valley pan?

*PJ*: Yes, it can affect vacuum. Lean A/F ratio and cause poor idle.

*GF*: - New cam specs (3/4monthes ago) 230/236 @ 0.50’ duration - 110LBA

*PJ*: 455, right? What is your compression ratio? The 110 LSA cams are best used for engines with compression below 9 -to-1. I am using a 112LSA cam on my 455 build with just over 9-to-1 compression. 

Is this Comp Cam's XE274? Interesting note at Comp's website with regards to the cam specs, "Valve Adjustment: Zero Lash Plus ½ Turn". I know you installed the heads and adjusted the valves. Are you using the factory torque spec or adjustable valve train with poly locks? 

I ask because I wonder if you may be having an issue with the valve action which could indeed give you the back firing problems if they were not closing correctly.

*PJ*: 

1,000-1,200RPM is good on advance beginning to move.
4,000RPM for total might be a bit much. I might try 3,500RPM to see if any change.
12Volts on MSD is good.
Everything else seems to look OK.

Keep plugging away and keep us informed. :thumbsup:


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## BearGFR (Aug 25, 2008)

Plugs look fine to me, just where they should be and not lean. I doubt it's a lean misfire from that and from what you said your readings were, unless the "other bank" (where you don't have the O2 sensor installed, assuming you're running a single sensor LM2) is way out of whack from the bank where the sensor is. 

Personally, given what you've said so far I'd be very suspicious of it being an ignition problem and not a fuel mixture problem. What happens at very slow/light acceleration, not close to WOT, at 3000+ rpm?.

Bear


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> Well I am confused with your vacuum readings....8.5" vacuum at idle definitely syncs to an aggressive cam.
> 
> Jim's post on AF was great and I suspect I am setup a tad too lean at idle / cruise (changed after reading Pontiac's like to run lean).....My guess is your AF ratio is the problem. Get the car to operating temperature and log AF from idle to 3000 rpm as suggested and report back <img src="http://www.gtoforum.com/images/smilies/smile.gif" border="0" alt="" title="Smile" class="inlineimg" />. Good luck


Alright CJ , will report back soon. Actually i was using the innovate air/fuel guage hooked on my tail pipes so its not so accurate and i do not have bungs on my headers.

Ill try to find a better alternative and come back with results!


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## TheGoatFather1965 (Jul 30, 2017)

PontiacJim said:


> *GF*: "- i need to confirm before telling you. However, yesterday i checked my ‘ported vaccum’ on the carb ports. It gave me a reading of 8.5” @ idle and 22-25” @ 4000 rpm."
> 
> *PJ*: 8.5" @ idle seems wrong - you don't have a cam that big. Don't use a "ported" line coming off your carb to get your reading. Can you hook to a line that would pull off the manifold? You carb might have a fitting for both "ported" and "manifold" at the front? Sometimes a vacuum T is used at the back of the carb if you have power brakes and you can tap off this.
> 
> ...



PJ, 

I have used the manifold port as well with the T fitting and it actually gave me the same 8.5” of vaccum. So like you explained it should be the manifold vaccum.

As for the PCV valve its plugged on the carb and to the valley pan but without a grommet. I think that should reduce the oil leak and hopefully richen up the AFR if its whats causing it to get lean. The grommet is on the way.

Yes,

Factory torque down specs and the cam is indeed the XE274 Comp Cams.

I will set the total @ 3500 again hopefully this weekend ! 

So, i will set the timing again and let you know! 

Im sorry this is taking too long ? but i cant seem to figure it out and hopefully we will get there!


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## TheGoatFather1965 (Jul 30, 2017)

BearGFR said:


> Plugs look fine to me, just where they should be and not lean. I doubt it's a lean misfire from that and from what you said your readings were, unless the "other bank" (where you don't have the O2 sensor installed, assuming you're running a single sensor LM2) is way out of whack from the bank where the sensor is.
> 
> Personally, given what you've said so far I'd be very suspicious of it being an ignition problem and not a fuel mixture problem. What happens at very slow/light acceleration, not close to WOT, at 3000+ rpm?.
> 
> Bear


Bear,

I think the spark plugs are fine but i havent checked im going to replace them soon and im thinking of iridium spark plugs from NGk with a spark plug gap of 0.35’


It might be an ignition problem or the other, 

Nothing happens its fine and smooth its only when im at WOT and above 3000RPM like you said but light driving conditions are good.


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## cij911 (Oct 25, 2017)

Any update ?


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## Lemans guy (Oct 14, 2014)

Backfire is literally....”fire in the back”......an air fuel explosion in the exhaust.......the mixture failed to fire in the cylinder and then is passed out the exhaust valve and the hot manifold and or pipes ignites the mixture......

So I think you are way lean for sure at WOT....I like low 12’s there.......but a lean backfire usually exhibits on rapid deceleration.....like coming fast downhill coming off the throttle, the throttle closes way lean fast and it backfires...

Your mixture is not firing when you have high demand,....could be valves like Pontiac Jim said, but also ignition....a weak coil will do that, cannot meet demand and so spark is so weak the mixture will not fire and you have a fire in the back......

A loose or bad distributor cap can do the same, even a cracked one with a hairline crack....can cause those cylinders to miss under high demand....when they miss.....the mixture moves through etc.

Check coil, or swap a known good one, double check spark plug wires and firing order....remember counter clockwise on Pontiac.....

Eliminate that ignition first.......as the possible source, but you need to richen up a bit, make sure secondaries are opening on demand as well


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Backfire is literally....”fire in the back”......an air fuel explosion in the exhaust.......the mixture failed to fire in the cylinder and then is passed out the exhaust valve and the hot manifold and or pipes ignites the mixture......
> 
> So I think you are way lean for sure at WOT....I like low 12’s there.......but a lean backfire usually exhibits on rapid deceleration.....like coming fast downhill coming off the throttle, the throttle closes way lean fast and it backfires...


So if this (backfiring on deceleration) is the issue, how do you richen? I'm guessing you would need to richen the idle settings as upon deceleration you should be running off the idle circuit ? If that is the case, what AFR should one target for idle ?


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## Lemans guy (Oct 14, 2014)

Right Co,....you have to richen up the idle circuit since the car runs exclusively on the idle circuit from idle to about 2500 Rpms....

Then the primary jets come on. So yes when you come off on a real fast decelerations you should not be spiking to lean.

Best idle for pure gas is 13.8 or so,....it is not that exact, but shoot for close to that. If you are running E-10 shoot a bit lower since the stoic head on E-10 is 14.1.....but 13.4 will run great...and these will also be your light throttle cruise numbers under where your primary’s come in

All your easy driving under say 50mph.....

The next circuit, the primary jets I like just below the idle circuit,........so high 12’s to low 13’s........say 12.7 to 13.3....you can widen it or narrow it and it will itself by temperature,.....but if you are in 12.5 to 13.5 on the primary jets that is ok with 13 being the midpoint...think of it as the correct AFR range with the lower number as in the range but richer and the higher number as in the range but leaner...

For the power circuits, when they are added it drops yet again....they include the accelerator pump shot,....the power valve...and the secondary jets.....here is where you want low 12’s a little high 11’s.....

So like my idle cruise is 13.5,....at say 1500 rpm,...if I stab the throttle the I drop to 12.2’...but just for a few seconds of acceleration...then back to the cruise mixture.....

If pulling a hill the power valve should drop the mixture at lower speeds into that power range.....11.8 to 12.4 say.....

Same with secondaries.....that is the power place......:nerd::nerd::nerd:


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## Lemans guy (Oct 14, 2014)

on your idle circuit you want that relationship between your idle holes and your transfer slot to be symbiotic,...not too much or too little of either...

the carb needs to pull fuel from both and if you have one too tight then it is not pulling the mixture correctly,....the transfer slot is key, remmeber the square or close to it hole...and no bog off idle. likewise the idle mixture screws don't need to be dialed in too tight.....

If you try to run your idle mixture too close to Stoichometric....... 14.7 for pure gas...... 14.1 for E-10....you will be too lean and when the throttle closes rapidly it will spike up lean and cause a backfire...also remmember stoich is a perfect burn for emissions.......not for power in a carbueratoed car.,,we have to be a little richer because we are running a wet intake.......fuel will condense on the walls of the intake, it is not as exact as fuel injection....

that said the car can feel just like fuel injection with these mixtures we talked about, just on the rich side of stoich....

I like to think of the carb circuits as 3........idle.....main......power

idle the leanest of the 3 power the richest..........main in between and it changes all between as we go...although perfect numbers are not there if you curve the carb with a vacumn gauge and a wide band O2 from the cockpit you can watch each circuit as it comes in and goes out...

with my Quick Fuel Carb I can tune any circuit....so for example the Power valve from QFT was a 6.5 and the Power Valve Channel Restrictor (PVCH) was a 59...

while watching my Vacumn and AFR I could tell the Power valve needed to come in earlier or it would almost never get used, and it was way too rich dropping the mixture to a pig rich 10.5!

so by watching when I wanted that power and even bench testing the PV and watching when they open and how much,...I chnged to an 8.5 Power valve and leaned the power valve channel restrictors to 47's....which on the QFT carb you change with a screwdriver just like a jet. In Holley you have to drill out the restrictor.

now when pulling a hill the PV comes on and drops the AFR to the low 12's.......right where it should be and comes on when I need it.

on PV's they start to open at the rated Vac and are closed above that...The QFT PV's have large holes so once hey start to open the restrictors get plenty of fuel and the restrictors control the amount of fuel.

also on the QFT you can change the idle air bleeds and high speed air bleeds with a screwdriver form the top of the carb,...again Holley you would have to drill it.By changing those i can lean or richen the idle circuits...and move the vacumn signal that pulls in the primary jets,....I have the primary jets coming in at 1800 RPM..

i like it lower that 2500............:nerd::nerd::nerd:I can then cruise on the primary jets at 70 mph 2300 Rpm, and about 13.2 AFR.......


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## cij911 (Oct 25, 2017)

THANKS A TON LEMANS GUY ! Super helpful. I am definitely too lean in the idle circuit and will bring that back to low 13s (14s now IIRC).

Any further direction / help on the power valve? I really did not understand how it worked when I rebuilt the trip power and IIRC, there was no adjustment.

Thanks again.

Chris


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Best idle for pure gas is 13.8 or so,....it is not that exact, but shoot for close to that. If you are running E-10 shoot a bit lower since the stoic head on E-10 is 14.1.....but 13.4 will run great...and these will also be your light throttle cruise numbers under where your primary’s come in
> 
> All your easy driving under say 50mph.....
> 
> ...


OK so my idle circuit is a bit weird.....when the car is at temperature and just idling, I see AFR 12.8 - 13.3 range.....But if I take the car out, go WOT, and then let off (no accelerator), I see AFRs 14-15 and back-firing....Any ideas ???


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## Lemans guy (Oct 14, 2014)

Well cmon it maybe that your throttle blades are shutting too awful tight,......of course we want the transfer slot to look like a squar in general, but it has a little more room to open than that without causing an off idle hesitation....

So you may try to just open the throttle screw the width of a screwdriver blade, just barely clockwise.......that will give it just a little more opening so when you come off WOT it does not slam shut so far......do not worry about the reading going lean when that happens, it should just worry about the backfire....if you eliminate that, then you can come back to the mixture screws and go in an eith of a turn on each and take out whatever enrichment the opening of the throttle added....

Just try to message that throttle open a little...

Also is that mixture at idle in drive? Or park?
Are you running pure gas or 10% ethanol?
What is your idle speed in drive?
What is your idle vacuum in drive?

As far as the power valve goes it is just a window that opens once vacuum reaches the hg of the vale,...like a 6.5 PV open at 6.5 hg and allows more fuel to flow thu mini jets called Power Valve Channel restricors PVCR...or metering rods depending on the carb...

That opening controls the volume of fuel added which is delivered thru the booster, same place as the main jets whether they are flowing or not.....valve controls when they open, restrictors or metering rods control how much they flow....if you have a vac gauge and an AFR in cockpit

Watch when vac goes below the valves rating and see how rich it goes, usually too rich unless adjusted.....


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## Lemans guy (Oct 14, 2014)

Chris also go back up and read PJ download of Henry Olsen’s writeup.....he is the carb guy, and is spot on. If you notice he says 13.4 to 14.1 for idle mixture as a range.......exactly...so I set pure gas at 13.8 right in the middle of that range....and of course it varies a little with temp altitude etc, but it will be right....E-10 go a bit lower 13.5......

You mentioned Lars...I go by everything Lars does, he is the tops...I use his vacuum restrictors when curving cabueretors,.....so if you listen to Henry and Lars you will be right....you get this stuff and your backfire will go away....


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## Lemans guy (Oct 14, 2014)

Chris to continue with your backfire issue,..another likely cause is too much timing. I know that you have considered this and discussed it earlier. So when you are at WOT and then come off,....and are running full manifold vac...as I always do, you are then adding that vacuuming timing rapididly back in....this is where the correct vacuum advance total matters....

In the old days with old gas 16 degrees or more vac advance worked......today, 10 degrees is right....or say 8 to 12 degrees. 

An ez test to see if your vac advance is causing the backfire...would be to drive the car until warmed up and then clamp off or plug vac advance.
Then drive the car at speed, it will be more sluggish during the rest but run it up and see if it backfires...if it does too much vac advance timing may be the cause.

Also the aforementioned slight throttle opening, do them separately. But two ez tests that may isolate the backfire.

On the Vac advance if you are running an original points distributor the lowest available vacumn advance can total made has 16 degrees of advance, too much. That has to be reduced. You can use Lars vacumn corrector to get it to 10 degrees. I use them on all the distributors I curve.

If you are running HEI your vacumn can can also be pulling in too much vacumn,...Adjustable vac cans can be limited with a hard stop or the rate of vac adjusted to sore of help. But those adjustable vac cans all pull in 20 or more degrees of advance without mechanical limiting.
If you need the number I can give you a better can for HEI that limits it to 10 degrees.

So what is your timing set-up, base, centrifugal and vac....are you running full manifold vac or ported...HEI or original points?

A lean mixture or too much timing are likely suspects in your decelerations backfiring,....:nerd:


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## cij911 (Oct 25, 2017)

Lemans guy said:


> An ez test to see if your vac advance is causing the backfire...would be to drive the car until warmed up and then clamp off or plug vac advance.
> Then drive the car at speed, it will be more sluggish during the rest but run it up and see if it backfires...if it does too much vac advance timing may be the cause.
> 
> 
> ...


THANK YOU!

I assume you meant remove vac advance from the equation and if it doesn't back fire on deceleration, then I may have too much timing.....

I am running a Pertronix II and I will have to check my total timing, but IIRC total was 34 degrees, initial was 8....I will have to check timing again....

That said, I know I am lean after WOT....When I release the gas pedal (butterfly shut closed), my AFRs jump to 14 - 15 and I backfire.....


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## Lemans guy (Oct 14, 2014)

Yes exactly, remove vac advance for the test,...and see if backfire is gone. 

Spiking to 14 or 15 when throttle closes is not too lean and should not cause a backfire, that is normal. Spiking to 18 to 20 is another matter.

If you vac advance is pulling in 16 degrees.....or more and you are hooked to full manifold vac, when you come off the throttle you are instantly adding that 16 degrees, that will cause a backfire......

You only want 10 degrees there, your 8 base with 34 total is ok will not effect it. Try vac advance test first, if that is it ...it is an easy fix. 

You are not too lean at idle maybe a little rich.....13.4 with E-10 ......13.8 for pure shoot for that,....with a range in there of about 8 degrees...those are the midpoint numbers to shoot for....:nerd:w


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Yes exactly, remove vac advance for the test,...and see if backfire is gone.
> 
> Spiking to 14 or 15 when throttle closes is not too lean and should not cause a backfire, that is normal. Spiking to 18 to 20 is another matter.
> 
> ...


Well I checked timing and my base is in fact 15-16*.....So it appears my timing is too high. I am a bit lost as to how it could be higher than I recall....Maybe it was idling higher and thus slightly more timing ??


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## Lemans guy (Oct 14, 2014)

Yes it could be adding centrifugal timing if the centrifugal advance comes in too low. This could be a result of weaker advance springs. When you try to get it in early before 3000 RPM those springs can get pretty weak, if you turn up the idle they then add a few degrees timing....

So you can reset and start from the beginning,....the last thing I always do is the springs. So you can just put a heavier spring on it from your curve kit and that should bring it above idle....remove and plug vac and recheck....


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Yes it could be adding centrifugal timing if the centrifugal advance comes in too low. This could be a result of weaker advance springs. When you try to get it in early before 3000 RPM those springs can get pretty weak, if you turn up the idle they then add a few degrees timing....
> 
> So you can reset and start from the beginning,....the last thing I always do is the springs. So you can just put a heavier spring on it from your curve kit and that should bring it above idle....remove and plug vac and recheck....


I did add the weak springs to bring timing in sooner.....If the weather permits, I will test timing today....

I will shoot for total timing to be between 32 - 36 degrees.

But before I change anything, I will test the lack of vacuum & backfire test.....

Thanks again for your help!


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## Lemans guy (Oct 14, 2014)

Chris,...when you change springs you want to make sure they start above idle timing,...and come in soon, you may get 28 to 3800RPM..

Usually the last few degrees may come in later,...but most will be in early enough...it is easy to get them tall below 3000 but the light springs then bring them in too soon, at idle.....

Let us know what happens


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Chris,...when you change springs you want to make sure they start above idle timing,...and come in soon, you may get 28 to 3800RPM..
> 
> Usually the last few degrees may come in later,...but most will be in early enough...it is easy to get them tall below 3000 but the light springs then bring them in too soon, at idle.....
> 
> Let us know what happens


I think it was too much advance (primarily).

I tested the setup without the vacuum advance connected and pretty much all backfiring was gone. There was a bit of crackling on deceleration and I noticed it was running a tad leaner at idle (I thought it would have run richer with less timing).

I reduced base timing to 12* (from 16*), re-connected the vacuum advance, richened idle to high 12's / low 13's, and took her out for a spin. No back-firing and she feels strong....

THANKS SO MUCH FOR YOUR HELP!!!


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## Lemans guy (Oct 14, 2014)

Super,...you made it more efficient at idle when you reduced timing, because with your vac you had likely way too much....

Like 16 base and 16 or more vac....32!.....when you went to 16 @ idle it became more efficent......moving leaner, closer to 14.7 Stoich....

Anyway, so you know what the backfire was now too much timing, mostly from too much Vac advance timing....

Do it this way to optimize even more.....find out your centrifugal timing by running it up until it stops advancing and subtract your base setting..

That is your centrifugal timing now subtract that number from 36 and reset your base timing to that number....if centrifugal was 26....

36-26=10......10 is your base,......

Then get your correct vac can see up to pull in 10 degrees from full manifold vac........your idle timing will be 20...or close

..and you will be able to set your mixture as I described previously in this thread,....you are a bit rich in high 12’s Low 13’s...but once you fix the timing a little leaning of the mixture screws should dial it right in.....

Don’t stop because you are getting almost perfect....and the cost is nil for super cool running!:nerd::nerd:


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Super,...you made it more efficient at idle when you reduced timing, because with your vac you had likely way too much....
> 
> Like 16 base and 16 or more vac....32!.....when you went to 16 @ idle it became more efficent......moving leaner, closer to 14.7 Stoich....
> 
> ...


So you are trying to get total timing to 36* ? If so, I suspect I am very close right now....I'll check tomorrow and let you know. Thanks again for your help.


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## Lemans guy (Oct 14, 2014)

Yes you are close,.....but you are likely over on the vacumn can...

Nail down that centrifugal, then set the base.........those 2 together are called total timing.......should be 36...

Vacumn advance is never included in total timing......but should only add 10 more degrees on taday’s Gas...

Once those are set you can play with the springs ez enough,....to get in in where you want it....

You are on it!!.......your car will run strong thruout the RPM bands, cooler and cooler at idle as well.....fast response with light throttle touch and nice smooth acceleration with better gas mileage,.....all good stuff

If you work on the carb AFR like you are doing even better......I curve the Dist and the Carb!


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## Lemans guy (Oct 14, 2014)

Remember to disconnect and plug vac advance when running up engine for checking total advance.....you have light springs in there so you should see it stopping advancing pretty early...


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> Any update ?


Sorry for the late reply, ive been really busy! 

I have good news and i have bad news:

The good news is, i fixed the problem, no backfiring, solid acceleration and no bugging at all.

The bad news: parked it for a month and got a little busy with studies and work. When i started her this weekend, its really smokey and it backfiring but even at idle (sound like gunshots).

So im figuring its just better to rebuild again from the start.


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Yes you are close,.....but you are likely over on the vacumn can...
> 
> Nail down that centrifugal, then set the base.........those 2 together are called total timing.......should be 36...
> 
> ...


Lemans Guy - question for you, regarding timing and centrifugal specifically.

I removed the springs on the distributor when setting timing so I could get the timing to advance early (before 3K), but noticed that the timing continues to climb a bit as the rpms go to 4.5K. My question is, should I set timing such that 36* is the MAX I see, or set it where 36* comes in before 3K without springs?

Also I tested the vacuum can (like both you and Lars suggested) and my can is adding 25*. I can get a new can, but it looks like I need to remove the distributor to swap the vacuum cans .


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## Lemans guy (Oct 14, 2014)

So think of it this wat CJ....total advance and when it comes in are “unrelated” for the moment of the discussion.

Total advance we know is centrifugal +Base and is the most important number......so if it keeps advancing to 4 thousand RPM etc, where it stops is the most important number,...let’s say it stops at 40.....

Now subtract your base, say you had 10, you now know that have 30 degrees of centrifugal advance in the distributor,....your base should then be set at 6......30+6=36......springs have no say in this......

Now if your vac can is pulling 25, I see that all the time by the way, ditch it and get The right vac set up, depending if you are running HEI or not.

I can give you those specifics if you don’t already have them.

You can change the vac can in the car. It is two screws and a little finesse, but easier to do it on the bench.

If your total is way too high, then you have to add a bushing or change the weights and center piece or a combination.

Springs are last, once all this is right then you can experiment with different springs, and they cannot go over your total advance, they just adjust WHEN that total comes in.:nerd:


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## cij911 (Oct 25, 2017)

Lemans guy, that makes sense. But I read elsewhere you wanted a base timing of 10-14 with vacuum of 10, so 24* at idle. The problem is I’d be more like 6-8 * base to limit total to 36*.

Is there a way to correct for this?


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## Lemans guy (Oct 14, 2014)

Cj the most important number is the 36.....so let’s say you have 28 centrifugal and add 8 base for 36....

Now to that 8 base you are going to add 10 degrees of vacumn advance.....so idle will be 18 degrees....it will idle great and run great.

Then just do the springs so it comes in reasonably early, really 2800 to 3600 is good.

You don’t have to have 24 degrees at idle although cars with very radical cams can handle more idle timing. If you have to get then you have to use a bushing on the centrifugal weights or use different weights and center piece.

But if your numbers are like I said above,..I would run it like that if it does not ping or knock, then try turning the base up 2 degrees to 10 base....for total of 38.....andd then your idle would be 20...........10 base and 10 vacumn

Lot’s of Pontiac engines can take that much some more, but compression set up intakes all play a roll.


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> Lemans guy, that makes sense. But I read elsewhere you wanted a base timing of 10-14 with vacuum of 10, so 24* at idle. The problem is I’d be more like 6-8 * base to limit total to 36*.
> 
> Is there a way to correct for this?


My problem was fixed after eliminating everything.

But the problem was a distributor problem, i got an msd distributer with an msd ignition box and set TDC (top dead center) once again.

Then i was playing with the timing.

The stock msd has 18* of mechanical advance (stock spring - black)

So to get 34* of total timing (works with me) i set the initial timing @ 16*.

Got new spark plugs as well.

This is what cured my backfiring issue and the engine started running strong and healthy!


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## cij911 (Oct 25, 2017)

What is your vacuum advance?


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## TheGoatFather1965 (Jul 30, 2017)

cij911 said:


> What is your vacuum advance?


I run an MSD pro billet distributer without vacuum advance.


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## cij911 (Oct 25, 2017)

So more confusion to add to the timing advice / debate....

I have total set to 36*. My distributor has a mechanical advance of 30*, so this means my base is 6*. The vacuum can that came with the distributor adds 25*, so at idle I am at 31*.

If I were to add a vacuum can with just 10*, then my idle timing would be 16* which is way to low.

I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance. Frankly I don't understand how the two setups would be materially different as my setup will have a more aggressive mechanical ramp that should offset the lower base timing.

What I am wondering is if it would be worth it to make all of these changes to the distributor ? Maybe I am running too much timing (e.g. on deceleration / engine braking) thus causing the backfire? But under WOT, the car feels great.


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## PontiacJim (Dec 29, 2012)

*cij911* - "I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance."

PJ - You might want to let this guy have your car and let him set it up for you.

You have been given the correct info by* Lemans guy* as well as *TheGoatFather1965* and don't seem to want to take their advice and I am not sure why not? If you talk to 100 people you are going to get 100 suggestions - only adding to your confusion.

Maybe you want to start from scratch. Pull the distributor and do a rebuild. Check the gear for wear and endplay. Then go from there. With all this difficulty, I'm thinking you have a bad/worn distributor.


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## PontiacJim (Dec 29, 2012)

*cij911*, it would be best if you began a new discussion and title it "Backfiring II." Then supply all the specs on the engine - year, cubes, heads, compression, intake, carb, exhaust, ignition type, and any other modifications that may have been done. Rebuilt or not. What octane gas do you run. 4-speed or automatic. Rear gearing. Are you at sea level or at high altitude like Denver (this does make a difference).

All of these things may not seem important, but they are in knowing what your combo is and then matching your timing and advance curve to the engine/car.

Start from scratch with the info and then help can go forward from there. Then do what is suggested and provide results so your situation can be dialed in and resolved. :thumbsup:


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## cij911 (Oct 25, 2017)

PontiacJim said:


> *cij911* - "I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance."
> 
> PJ - You might want to let this guy have your car and let him set it up for you.
> 
> ...


PJ - He is not local, otherwise I would. He is super knowledgeable and very helpful. That said, I am trying to learn from those of you with more experience.

The distributor is NEW and there are NO problems with it, just that the vacuum can it came with has too much advance 25* vs recommended 10*. Additionally it appears the distributor has more mechanical advance than ideal - 30*, which has my base timing at 6* to get a total advance of 36* coming in before 3500. The car as it sits runs well and pulls hard.

What I am trying to understand is why changing the distributor would help. Basically the recommended setup has base at 16* (vs my 6*) and a 10* vacuum can so at idle it would be 26* (vs. my setup at 31*). My understanding is if a car runs well at 31* (vs 26*) it is even better as it will run cooler at idle. At WOT, I am at 36* (just as the recommended), so really no difference there.

So basically it comes down to partial throttle and transient response (which is a major part of normal driving). My question is am I likely running too much timing? While the recommended distributor has base timing 10* more than mine, I feel that is really a meaningless (as described above). So maybe the concern is I am running too much timing mid rpm range with modest load ? That I could understand, but I am unsure if that is true as I haven't seen any signs. Basically I guess the car could see as much as 51* (full mechanical + full vacuum) vs. the recommended setup as much as 30* (full mechanical of 20* + full vacuum of 10*). So is that the issue ? And if so, what would I see / hear / or feel that would indicate it was too much? (I haven't heard pinging, car feels responsive, and stays under 190* under the hardest driving.)

Thanks for the help.


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## PontiacJim (Dec 29, 2012)

Ok, got it. Started a new discussion titled "Distributor Timing" to begin a fresh discussion and separate your situation from the original post. :thumbsup:

https://www.gtoforum.com/f170/distributor-tuning-134141/


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## Lemans guy (Oct 14, 2014)

So cj if I had your distributor I would put a breast’s bushing in it to limit the travel ad knock that 30 back to 24 or 26.

You would then set the base at 10 or 12 for 26 total. You would then add 10 degrees from a vacumn can for 46 to 48 light throttle cruise.

Once there if no pinging, you cab turn the base up 2 degrees and check if ok you can run 38 total with 22 or 24 degrees idle timing.

You say you must have 26 or more idle timing,...well really you should see it this way a very radical cam CAN take more timing at idle....

But there is not a Mandatory number....24 or 26 at idle is smooth on a hot cam,...20 degrees is smooth on most hot ones.....can you go more?

Maybe but it is a distinction without a difference usually. The total timing is the most important number,....if you want more at idle that you have to take it out of the centrifugal weights,....I have done them 18 centrifugal and 18 base =36....+10 vacumn.......

That is 28 at idle, a real hot cam can take it, but the difference in idle cooling from 24 to 28 is not really of concern. The concern is when guys have base idle at 6 or 8 or 10.......and use ported vac with no more advance at idle.....even at 16 degrees idle timing like you have you will be ok.....you can run it like that....

There is no absolute number, some engines even can take 40 degrees, but you have to be really tuned in, like racers and engine builders like PJ and BigD....they know what their car needs.....b feel and experience..

So don’t get hung up on it must be 26 base etc,..the difference of a few degrees will not likely even be noticeable, as long at the total is right nd no knocking:nerd:


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## Lemans guy (Oct 14, 2014)

Brass bushing ...sorry typo

I missed that notice PJ I was tying the response,maybe it can be moved to the correct forum


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## PontiacJim (Dec 29, 2012)

Lemans guy said:


> Brass bushing ...sorry typo
> 
> I missed that notice PJ I was tying the response,maybe it can be moved to the correct forum


OK, got it done. Just did a simple copy/paste to switch it over. Corrected the typo - I won't even ask. LOL :thumbsup:


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## cij911 (Oct 25, 2017)

Recommendation to all seeing AFRs spiking lean on deceleration (and /or back firing) - check for exhaust leaks.....Ask me how I know .


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