Backfire upon acceleration

1968 440 stock with mild cam. What would be the most likely cause of a backfire upon hard acceleration from a 30 - 40 mph rolling start?

Is it a backfire out the exhaust or carb?

Could be the old Holley proverbial power valve.

Some additional info brother, carb, EFI, stock or aftermarket distributor, how fresh is the build, etc.? Has it just started to do this or been doing it?

the more info the more we can help

this is from BAD *** CARS website


First off, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".

"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang.

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.

Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.

Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump inthe carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it is probably the accelerator pump in the carb not working, plugged up or out of adjustment.

As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.

When and how it backfires or spits will give you an indication for where to look.

Also.....
If the engine sounds like exhaust is popping up through the carburetor, sometimes that is due to one or more exhaust valves not opening fully. This is usually due to a lobe on the cam that is going bad. The exhaust has a reduced path to escape and it tries to leave through the intake during the overlap phase.
OR....
The heads have worn out valve seats and the combustion or exhaust is trying to escape around the poor sealing valves.

68Moparmaniac said:

Some additional info brother, carb, EFI, stock or aftermarket distributor, how fresh is the build, etc.? Has it just started to do this or been doing it?

Click to expand...

OEM Carter
OEM stock dual point distributor from 1965 with lighter advance springs
.030 rebuild last spring
I have just recently started to push it because of new clutch break-in. I have been regularly driving it for only the past two months. I have not pushed it to WOT yet.
I believe it is a carb backfire.

Check point gap (dwell). Try a new condenser.

Doing it when cold, needs a little more choke. Think Simple first !

My thoughts would be the dual point distributor, now that you have given some specifics. When the points aren't set exactly right that will cause cutting out, spitting and backfiring.

This is happening with a hot engine. I will recheck dwell. I think a very important point I've left out is that this is only one backfire, only on hard acceleration.
The carb is freshly rebuilt.
This is my first experience with a motor that can't be timed to OEM specs, so it has been set to where it seemingly runs best, but again it has been basically babied so far because of clutch break-in.

Something you might want to check is, your distributor mechanical advance. If it's hanging up, when you throttle up, timing isn't getting the advance it needs. Cough, cough.