# Back fire Issue



## Goatfreak (Aug 18, 2020)

My 68 GTO seems to have a backfire issue on trying to acceleration. I have complete MSD electronic ignition system with Holley 750cfm double pumper 400ci. Previously had trouble with engine bogging/hesitation on acceleration now have slight hesitation and pretty consistent backfire through carb on acceleration. I have tried all accelerator cams for carb, minor +/- on air/fuel mixtures. Tried going back to R45S plugs from R45TS per reputable source, but found thread issues with #3 and getting R45S plug to start.(?) I was trying to find thread size to chase all the threads, but uncertain of thread size.


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## Lemans guy (Oct 14, 2014)

Hesitation at speed is often a lean bog, and backfires are caused by different things, one being over advanced spark timing.

you changed the accelerator pump cam but maybe you need a bigger nozzle or squirter for the accelerator pump.

also idle mixtures are still active up until primary jets take over usually about 2500 RPM, actually the idle circuit still draws a little but the main feed over 2500 is the primary jets.

A lean bog happens on all carbs it is when the throttle plate opens fast per the demand of your foot, that let’s in a rush of air.......lean and it will cause a bog or hesitation.

to overcome that bog an accelerator pump squirts in more gas and covers that transition,...

so you may need a bigger squirt. Secondaries also open but the throttle plates must be wider open to cause vacumn in the Venturi, center of the carb, then th esscondaries will add fuel as well. If your secondary vacumn spring is too strong,they will not open and you will feel a bog.

It could also be timing issues, but in this case you might look at your fuel systems first, since you have been adjusting them


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## O52 (Jan 27, 2019)

AC 44S was the factory supplied plug for the four barrel 68 GTO (16 Heads). This was upgraded to a R44S in 69. 
But AC 44s is no longer made . 
R45S is the next plug of choice for a stock engine. R43S for a colder plug.

The S suffix means extended firing tip
TS means tapered seat with extended firing tip
R is for resistor plug

The next question is, what number are your heads? And the date code of the heads? This has a lot to do with the plug recommendation and the thread size you need to chase the threads
If you have the stock #16 heads you should be using R45S. These heads use a plug with a metal gasket and are not designed for a TS plug.

Cylinder head number is in the middle over the center exhaust ports. Date code is usually just below the valve cover on the right side of the head


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## Goatfreak (Aug 18, 2020)

Lemans guy said:


> Hesitation at speed is often a lean bog, and backfires are caused by different things, one being over advanced spark timing.
> 
> you changed the accelerator pump cam but maybe you need a bigger nozzle or squirter for the accelerator pump.
> 
> ...


Would it help to open up on fuel mix set screws on side of carb? They were adjusted down a bit. Just a thought.


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## Goatfreak (Aug 18, 2020)

Goatfreak said:


> Would it help to open up on fuel mix set screws on side of carb? They were adjusted down a bit. Just a thought.


Also want to mention that I pulled plugs to clean them and had a lot of carbon on them.


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## pontrc (Mar 18, 2020)

Nice color on that goat! Agree with the others on things to watch but what are your plug gap settings? I was running at 045 with HEI but Pontiac Jim suggests that maybe it’s too much, 035 was recommended


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## Goatfreak (Aug 18, 2020)

pontrc said:


> Nice color on that goat! Agree with the others on things to watch but what are your plug gap settings? I was running at 045 with HEI but Pontiac Jim suggests that maybe it’s too much, 035 was recommended


Yep, I have mine set at 045. Will try 035. Thanks.


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## Goatfreak (Aug 18, 2020)

O52 said:


> AC 44S was the factory supplied plug for the four barrel 68 GTO (16 Heads). This was upgraded to a R44S in 69.
> But AC 44s is no longer made .
> R45S is the next plug of choice for a stock engine. R43S for a colder plug.
> 
> ...


Here is what I found on heads. I only see: R(driver) side #B or 8028; L side #B or 8038. The only other stamping is a 6X over both middle exhaust ports. Looked these numbers up in PHS number crunching info I have and they are all foreign, nothing even close. So not sure #'s of heads. Thanks.


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## Lemans guy (Oct 14, 2014)

Yes you need to read up on the Demon carb procedure and just start from scratch to set those screws....go back to the beginning,...and yes I agree On a street car, even HEI, I like plug gap set at .35.......racing is different considerations, but for reliable starts and smooth running,.035 is a good place as you drive the gap is always wearing wider, not narrower..

so when one starts with a real wide gap as it wears it starts to give you misfires and hard starts. .35 will fire reliably a proper air fuel mix


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## BearGFR (Aug 25, 2008)

Goatfreak said:


> Here is what I found on heads. I only see: R(driver) side #B or 8028; L side #B or 8038. The only other stamping is a 6X over both middle exhaust ports. Looked these numbers up in PHS number crunching info I have and they are all foreign, nothing even close. So not sure #'s of heads. Thanks.


That's a B - B028/B038 Which equates to February 02/03 1978, so those are year model 1978 heads. Reference this link on the Wallace Racing site.
Pontiac V8 Cylinder Heads

Reference the graphic on that page to see where the secondary code is. It will either be 4 or 8 (for 6X). Those heads will have 2.11 intake valves, 1.66 exhaust valves, and a chamber volume if either 93.75 cc's (4) or 100.04 cc's (8)

Fiddling with the idle screws only affects idle mixture. Doing that has no effect on transition, cruise, or wide open.

FYI in case you care: No Pontiac ever left the factory with one of those 400-4 stickers on the air cleaner. That was strictly a Buick thing. Years ago some after market company decided to start making and selling them with the Pontiac arrowhead on them. 

The most common cause of a backfire (actual popping, not just a bog) is a problem with ignition timing. It'd be a good idea to verify what yours is set to, both at idle and what it is at - say - 2500 rpm. If you have a vacuum advance, make the checks with it disabled/plugged. A retarded condition (read: not enough advance) can cause a severe and nasty backfire because the fire is being lit so late that it's still burning vigorously when the intake valve opens, allowing it to ignite the incoming charge while it's still in the intake ports - bang!

Bear


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## O52 (Jan 27, 2019)

Your heads require the R45TS plugs. Thread size is 14mm. Having 1978 heads makes me wonder what year is your engine? Back by the distributor there's a date code on the block with the same format as the heads. Theres also a two letter engine code on the front of the block just below the right cylinder head. That and whether or not your engine has been rebuilt with a decent cam will help us troubleshoot your problems.


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## Goatfreak (Aug 18, 2020)

BearGFR said:


> That's a B - B028/B038 Which equates to February 02/03 1978, so those are year model 1978 heads. Reference this link on the Wallace Racing site.
> Pontiac V8 Cylinder Heads
> 
> Reference the graphic on that page to see where the secondary code is. It will either be 4 or 8 (for 6X). Those heads will have 2.11 intake valves, 1.66 exhaust valves, and a chamber volume if either 93.75 cc's (4) or 100.04 cc's (8)
> ...


I am finding out there's numerous things on this car that I should have done more homework on. Air cleaner no big deal, heads another issue.
Backfire does not happen all the time on acceleration. Would that still be a timing issue? Don't have working timing light anymore, but looks like I need to get one. What are specs with time with and without vac advance? 
Thanks!


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## Goatfreak (Aug 18, 2020)

O52 said:


> Your heads require the R45TS plugs. Thread size is 14mm. Having 1978 heads makes me wonder what year is your engine? Back by the distributor there's a date code on the block with the same format as the heads. Theres also a two letter engine code on the front of the block just below the right cylinder head. That and whether or not your engine has been rebuilt with a decent cam will help us troubleshoot your problems.


Will advise once I get back to GTO. Thanks.


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## Goatfreak (Aug 18, 2020)

Goatfreak said:


> Will advise once I get back to GTO. Thanks.


Letters on engine block YS. Looking into. Number above is 0261613.


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## O52 (Jan 27, 2019)

So far so good. YS is the code for a '68 350 HP 400 automatic, but...
It is also the code for the 1978 180 HP 400. 
The date code by the distributor will verify.

You can also check the engines partial VIN code on the side of the timing cover. It should match the VIN on the dash if the engine block is original to the car.


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## Goatfreak (Aug 18, 2020)

I'll try and get to date code by distributor. A bit handicapped with metal brace in neck with discs removed. Limits motion. Will try timing cover, may be easier.


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## Goatfreak (Aug 18, 2020)

Here is what I can see by distributor; K158. Does that make any sense?


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## O52 (Jan 27, 2019)

K158 - Cast on Nov 15 1968 for a 1969 GTO.

So you have a 69 400 block with '78 heads. One more piece of the puzzle will be the secondary code for the heads.
That will give the chamber size and then you'll have a better idea of what your working with.


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## BearGFR (Aug 25, 2008)

Goatfreak said:


> I am finding out there's numerous things on this car that I should have done more homework on. Air cleaner no big deal, heads another issue.
> Backfire does not happen all the time on acceleration. Would that still be a timing issue? Don't have working timing light anymore, but looks like I need to get one. What are specs with time with and without vac advance?
> Thanks!


Regardless of whether those are 6X-4's or 6X-8's --- That's a good thing. Those larger chambers have the effect of lowering compression and making the engine more friendly to today's pump gas. Once we know specifically if they're -4's or -8's that will enable getting a good estimate of the engine's actual compression ratio (getting it exact requires more information, like which head gaskets, which pistons, exact bore size, piston deck clearance, cc-ing the heads to measure actual chamber volume, etc.)

Setting timing with a light is ALWAYS done with vacuum advance disconnected/disabled and the passage to the intake/carb plugged. Always. The goal of "setting" timing is to establish how much ignition advance is present at different RPM whenever the engine is under heavy load, like at wide open, or pulling up a steep hill. The function of the vacuum advance mechanism is to add "more" advance in situations where the engine is very lightly loaded and not working hard, because that helps with cooling and also fuel economy. The only time you use a timing light with the vacuum advance system "active" is when you want to measure how much advance the vacuum system is capable of adding to what's already present due to the combination of initial+mechanical advance.

"Most" Pontiac open chamber cylinder heads tend to be happy with about 35 degrees total (initial+mechanical) that's all in by around 2500 RPM. Both varieties of 6X's are open chamber heads. The last closed chamber heads I'm aware of were the 670's in 1967. They tended to like more advance, around 40-45 degrees. 

Finding the optimal advance for a specific combination usually means a trip to the drag strip (or a facility with accurate time clocks) and making repeated runs under as much as possible identical conditions until you find the setting that nets the lowest E.T.'s

Bear


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## Goatfreak (Aug 18, 2020)

Will check and get back to you. Thanks.


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## Goatfreak (Aug 18, 2020)

Lemans guy said:


> Hesitation at speed is often a lean bog, and backfires are caused by different things, one being over advanced spark timing.
> 
> you changed the accelerator pump cam but maybe you need a bigger nozzle or squirter for the accelerator pump.
> 
> ...


Info on the Holley 750cfm on car; Primary jet size 70, Primary discharge nozzle 0.028. Secondary jet size 81, discharge nozzle 0.031. Increase both , either or? Suggestions on how much larger? Thanks.


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## Goatfreak (Aug 18, 2020)

BearGFR said:


> Regardless of whether those are 6X-4's or 6X-8's --- That's a good thing. Those larger chambers have the effect of lowering compression and making the engine more friendly to today's pump gas. Once we know specifically if they're -4's or -8's that will enable getting a good estimate of the engine's actual compression ratio (getting it exact requires more information, like which head gaskets, which pistons, exact bore size, piston deck clearance, cc-ing the heads to measure actual chamber volume, etc.)
> 
> Setting timing with a light is ALWAYS done with vacuum advance disconnected/disabled and the passage to the intake/carb plugged. Always. The goal of "setting" timing is to establish how much ignition advance is present at different RPM whenever the engine is under heavy load, like at wide open, or pulling up a steep hill. The function of the vacuum advance mechanism is to add "more" advance in situations where the engine is very lightly loaded and not working hard, because that helps with cooling and also fuel economy. The only time you use a timing light with the vacuum advance system "active" is when you want to measure how much advance the vacuum system is capable of adding to what's already present due to the combination of initial+mechanical advance.
> 
> ...


Secondary code is 8.


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## BearGFR (Aug 25, 2008)

Your jetting sounds "ballpark" --- MAYBE (emphasis on the maybe part) a little on the lean side but don't take that at face value. To tell for sure, you have to either "read" the spark plugs or (better) use an air/fuel meter. There are all kinds of references with photos "out there" about reading plugs. You should be able to find them with a web search engine. The trick is getting a 'good' reading. The additives in pump gas today make it a little difficult to read them with precision, plus you've got to make sure that the engine is fully warmed up and that you take the reading at "the right time". For instance, when tuning for the drag strip the recommendation was to make a pass, then as you go through the timing traps on the top end simultaneously kill the engine/shift to neutral/ keep holding the throttle open -- then brake/coast to a stop and immediately read the plugs. The idea was that you wanted to get as close as you could to reading the plugs as they were 'colored' from the mode of operation that you were tuning for --- WOT under full load in the case of running at the track. 
You might not be able to get it "perfect" on the street by reading plugs but it should give you an idea if you're reasonably close or not. Black and sooty is an obvious overly rich condition. 
This chart might help:


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## Goatfreak (Aug 18, 2020)

What I have found each time I have pulled the plugs are in the area of the (slightly) "carbon fouled" & "too cold" next to cold or rich pictures. I have not checked plugs yet since I dropped plug gap back to .035. Need to start and run for a bit and then check.


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## Goatfreak (Aug 18, 2020)

Got a pic of an air/fuel meter? Not sure I have seen one. Or if I have did not know what it was.
Thanks!


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## Lemans guy (Oct 14, 2014)

When you accelerate at lower speeds like 30 to 50MPH.....you are giving a shot from the primary accelerator pump, the secondary sprinter comes on when those throttle plates open, which would be much further open.

so if you bog at low speed increase the .31 squirter a bit......if you bog from 50 to 75 MPH

that is when you are demanding more and when your secondaries should open in response to more pedal,...if your bog is there try a little bigger squirter there...

it is a trial and error ...an AFR meter can help you fine tune it in better........


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## StuInNH (Apr 17, 2020)

Back when I had my first 68 GTO I had the same problem. Back then, it was a cracked distributor cap.


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## Goatfreak (Aug 18, 2020)

Brand new MSD system. Will check it out though. Thanks.


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## Goatfreak (Aug 18, 2020)

Lemans guy said:


> When you accelerate at lower speeds like 30 to 50MPH.....you are giving a shot from the primary accelerator pump, the secondary sprinter comes on when those throttle plates open, which would be much further open.
> 
> so if you bog at low speed increase the .31 squirter a bit......if you bog from 50 to 75 MPH
> 
> ...


Here's latest findings. Did vac test per discussion with a Holley Tech. He also made a comment on increasing jet size on primarys to help with lean surge. Results are not very good. Low vac. Readings were 9-13 in vac. Did vac test with carb cleaner and nothing found. So possible ignition timing could be issue(?) Vac increase as I increased rpms, but never made it to 20 ins. What else can bring down vac that low? Exhaust was very strong like maybe not complete fire in ignition system. 
The readings of 9-13 is the bounce in vac needle on gauge when taking readings. So can eliminate dwell with MSD system. Plug gap at .035 per specs. Coil and cap are new with MSD system but will still follow up on them. Plug wires are old, so possible they could be part of issue. More thoughts?
Another question, why would engine rpm's increase when closing secondary needle valves?

Have talked with mechanic that has been working on it previously and told him status. Going to meet with him and further discuss.


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## BLK69JUDGE (Jun 10, 2010)

your alternator looks a tad tilted ??


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## Goatfreak (Aug 18, 2020)

Probably angle of pic.


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