How to Fix Popping Carburetors in Corvairs and Any Other Car
There is a difference in a popping carburetor and a backfire carburetor. The latter is a more serious condition, louder and sounds like a firecracker. Popping is less serious and less loud and sounds like a pop gun.
Many times, a car will be running fine with no issues upon acceleration etc. Then, out of nowhere, you start to hear some sporadic popping from the engine, most likely the carburetors. It may only happen in certain low gears or upon acceleration causing you to lose power.
Basically, what's happening is that the mixture is being fired in the intake rather than in the cylinder. There are some common causes: An intake valve not closing properly, an exhaust valve not opening properly, and a given spark plug firing out of sequence (cross fire).
Intake valve: Intake doesn't really close and the flame in the cylinder backfires into the intake, which set it off (that's the pop)
Exhaust valve: Exhaust is not fully exhausted and when the intake opens, exhaust left in the cylinder backfires into the intake (that's the backfire).
Crossfire: A spark, intended for a different cylinder, is produced in a cylinder with an open intake valve. The spark backfires into the intake.
The easy attempts to fix include: checking the distributor cap and wires and trying a new set, do a leakdown and compression check (last resort), check for weak or broken valve springs can work at lower speeds but can't keep up at higher rpm, disconnect the vacuum advance hose on the distributor and see if that corrects it.
A sticking spring or valve may be the issue, so add a quart of Mystery Marvel Oil, been around since 1923, to the gas tank and oil (split the amount of one quart, 50\50) and drive. This works, at least it solved my popping in the 1966 Corvair. Rev it in N to see if it pops, if it does...pull one plugwire at a time and rev until it stops.. that's the bad cyl / spring / valve.
If Compression test shows good... then you need to change the intake valve spring on that cylinder. Also, check your spark plugs for damage and cracks, replace if needed. Since water in the tank can cause a variety of problems, add some gas treatment to remove it.