# Random backfire



## nuggets (Apr 27, 2011)

Apologies if this is a repeat post, I normally spend days searching the forum but I cannot find anyone who has had this same problem. 

67 400 with stock internals. Pertronix distributor, 12 volt coil, other than that the engine is completely stock. PO had it rebuilt roughly 40k miles ago. Recently I noticed a tick in the valvetrain. Turns out one of the rocker arms was eating into the valve stems due to the rocker nut backing off. This caused some more damage to the rocker shaft and valve spring so in the meantime of getting them refurbished, I switched some 69 heads I had, that are in great condition, onto the engine. These 69 heads are the original heads to the car and also stock. 

Under load, right at 2500-2700 rpm, the engine will backfire through the intake, not the exhaust. This only happens once, and just a small pop, and only at those rpms, and only under load. I can slowly speed up the engine and it's fine. Currently initial timing is at 14 degrees. I've played with timing, and anything under, or over that cause it to backfire way more, still at those rpms. Again, only though the intake. 

I also have hard starting. If I don't get it started right away, it appears the engine will flood. So I wait 5 mins then I can get it started. The carb was rebuilt about a year ago. Quadrajet. And has been working perfect so far. I should note, I can turn the air/fuel mixture screws in all the way and the rpm will barely change. 

Does the head switch now require a jet size change? Would too much fuel cause this random backfire? Or do I have a burnt intake valve? Valves were inspected at install and they seemed fine. 

Sorry for the long post.


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## nuggets (Apr 27, 2011)

Also, if someone could move this to engine tuning, that would be great. It will not let me delete or move this post.


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## Lemans guy (Oct 14, 2014)

Intake backfires are often timing issues. 14 base is one third of your timing equation. You have base, Centrifigal and vacumn timing.

only one is RPM dependent and that is Centrifigal timing. It comes from weights and springs inside the distributor under the rotor cap. If this distributor is very old than your springs may be worn out or have moved off the stems. The weights could be stuck, or rusty.

so first ting is…..pop off the distributor cap and then the rotor and have a look

How does it look? A new curve kit will get you new springs and weights if necessary. Original factory weights are preferable if not worn out. Then you will need to find what your Centrifigal timing actually is. Because you don’t know and you need to know.

So the way to do that is to remove one spring from the weights and put the rotor and cap back on get your timing light and a helper start it up. Now while watching the timing on your advance timing light rev the engine until THE TIMING STOPS ADVANCING note that number.

it may be 35, or 40 or more or less. Now subtract what your base timing was set at, 14 from that number and that is your Centrifigal timing. Let’s say it was 34 subtract 14 your Centrifigal is then 20.

Replace the spring. Removing the spring for the test allows you to not have to over rev the engine, it will reach total Centrifigal advance quickly with one spring. So see what you get.

The curve kit you want is this or similar,…Moroso # 72310 for 1957 to 1975 GM Distributor. With an external coil. From your description sounds like what you have with a Petronix conversion. But if you have a “coil in cap” wide body HEI this won’t fit. There is a different one for that.

get your timing right first, sure could be valve, could be too lean as well. But start with timing.

Centrifigal timing is only effected by RPM’s nothing else. So check it out.Good Luck!


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## Lemans guy (Oct 14, 2014)

PS vacumn timing from your distributor vac can is “Load” dependent. You may need to make sure your vacumn can is working properly as well.


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## nuggets (Apr 27, 2011)

Thank you I'll check this out and see what I can find out! Would this have any effect on the flooding at cold starting issue?


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## Lemans guy (Oct 14, 2014)

No that would be carb and fuel pressure issues. You said if I don’t get it started right away it will flood. But that is because it is not starting and maybe you are pumping the throttle.

You said moving the base timing from 14 either way causes back firing, that should not be. Your timing must be set right, and then you have to examine carb, vacumn leaks and mechanical issues.


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## PontiacJim (Dec 29, 2012)

I would first try and figure out why the rocker arm backed off. I am not sure how a rocker arm can eat into a hardened valve stem? I think you may have other issues.

You may have a valve or 2 that is too tight and not fully closing/seating. You could have a cam/lifter worn out.

If the engine was rebuilt, quite possible rocker arm geometry is off. Not knowing what was done to the engine during the rebuild, I would want to verify pushrod length and the rocker arm wear pattern on top of the valve stem. Stock bottle-neck studs/nuts get torqued to 20-25 ft lbs and their done. If vavle train geometry is off, when you torque them down you could be causing the valve to hang slightly open.

It is possible that wrong/weak valve springs were used or incorrect valve retainer. Popping back through a carb is typically a burnt valve or possibly a vacuum leak. What does your vacuum gauge read when testing engine vacuum?

What do all your spark plugs look like? Any wet plugs? Any that show detonation?

What is your compression? Stock? Dished pistons to lower compression? What octane gas do your run?

40,000 miles - maybe a sloppy timing chain/gears. Check for this using the distributor's rotor and its movement.

Any spark plug wires crossed or touching causing a jump of the spark to the other wire? Is the engine grounded to the frame? Grounded to the body?

Do you have 12 volts at all times going to the coil? Is the resistor wire going to the coil/ignition still connected or did you bypass it? Have you tested for 12V at the coil when your issues happen?

Flooding is a carb/fuel pressure issue, but you should easily smell it. Maybe your choke is adjusted too tight or not working. Have you checked its operation when cold/hot?

Is your battery fully charged? How old? Does it spin the engine over quickly?


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## nuggets (Apr 27, 2011)

Pontiac Jim giving me a grocery list 🤣 I'll try and figure out all that I can as the days go by.

Spark plugs are fine. No wet, fouled, carbon buildup, or any signs of detonation. 

Unknown on the compression. PO said it was rebuilt with stock internals. I run 89 ethonal free when I can find it. Other than that I use 93. If I run at 87 octane, I tend to diesel on hot days after shut off, I live in Florida. Which of course probably means my compression is higher than stock. 

I did replace the timing chain and gears when I had the water pump off a few months ago. No issues at all until I had the heads switched out. 

Plug wires are not touching or crossed, as much as I can avoid it. There are some that are close together just because of their location on the rotor cap. The engine has never had a grounding strap to the body of the car, ever since Ive had it. I know there is normally one connecting the back of the passenger side head to the body/frame, but it was never there so I never put on back. Only ground is to the battery. 

Tested 12 volts to the coil. Kinda hard to test when the issue happens since it only happens under heave throttle while driving. I guess I can run a meter into the car and look at it while driving. Resister wire it still attached actually. Never thought about bypassing it until now. Kinda forgot it was there. 

Battery is a year old. Have not had it tested but it cranks the engine quickly and appears to be under full power.


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## nuggets (Apr 27, 2011)

I will say this, even though it is Pertronix ignition, it is attached to the original distributor body. So there is a chance the vacuum can is bad, etc. I have a spare that was never used I can switch out to see.


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## PontiacJim (Dec 29, 2012)

nuggets said:


> I will say this, even though it is Pertronix ignition, it is attached to the original distributor body. So there is a chance the vacuum can is bad, etc. I have a spare that was never used I can switch out to see.


The Pertronix is a conversion then? Depending on age, there were issues with the early versions. The problem was mainly from those leaving the ignition on when the car was not running - it damaged the module. You typically want 12V going to them as well. The resistor wire can drop voltage too low and they don't work well. The later conversions are improved and eliminated the problems with the earlier conversions.

Bypass the resistor wire by running a jumper wire. Make a simple jumper wire using alligator clips on the ends and connect to the battery post and then to the Pos + side of the coil. Hook it up once the engine is running and take it for a drive. You will have to unhook the jumper when time to shut the engine off or it will continue to run.

Sounds as you still have high compression. 87 octane and engine dieselling is cause for engine damage real fast - piston/rod & main bearings. The '67 400 "067" heads are closed chamber and can tolerate a little more compression than open chamber heads. I would not use anything less than 93 octane and probably should be using an octane booster and why the timing is sensitive - it does not like the lower octane which is also aggravated by summer/Florida heat.

The other thing that comes to mind is that it could be that the secondary air valve flap is popping open too quickly - which would be about in the RPM range you are having issues. If the secondaries open too quickly, the carb could be sucking in more air than fuel just for that instant prior to the flow of gas catching up. There needs to be a transition to the opening of the big secondaries and the air flap is what does that.

Just for fun, wire the secondary air flap closed and take it for a spin. See if it still pops through the carb. There are 2 things that control the air flap opening - the plastic dashpot/vacuum pull-off on the side of the carb and the adjustable spring on the secondary air flap found at the end of the flap's shaft at the top/outside of the carb. Tiny flat bladed screw to adjust flap tension and there is an allen set screw underneath that locks the spring in place. You may want to tighten the spring 1/4 turn (flat bladed screw) to slow the opening time/rate down to provide a little more time for fuel to be drawn into the secondaries before the secondary flap pops open to draw in more air. But, make sure the choke pull-off vacuum can is working and/or is hooked up to the air flap as this too has bearing on the air flap opening time.


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## nuggets (Apr 27, 2011)

Update: replaced the vacuum pod on the distributor. While doing so I found the transmission vacuum shift line was cracked in a few places. Could be potential causing these issues since there was a vacuum leak. Will test drive once I get a new one in.


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