1979 Backfiring through carb

[Chriskillface]

1979 Toyota pickup
148,xxx miles
2wd, 20r, 2bbl carb


Ok so, I bought the truck, had a blown conrod, so i swapped motors with a 20r i previously had from an old project, ran well never had a problem with the motor. I have timing set to factory specs, carb adjusted properly, and every time i get on the gas to hard it back fires thourgh the carb. also, it will just backfire through the carb just at cruising speed. I fellow toyota enthusiast suggested the timing chain might have jumped a tooth, but im not sure now to tell if the cam is set dead center without having to rip apart the whole timing cover. I was thinking if the zero is straight up, shouldnt the timing mark on the harmonic balancer be set to zero? In anycase, its not.

[theJC]

Did you ever resolve this problem? Please inform

[email protected]

[fourwd1]

A backfire occurs when there is an imbalance in the air to fuel ratio. If the fuel mixture is too lean you may have a backfire in the intake, or too rich you may get a backfire out of the exhaust system.

Backfiring can also occur with a sudden drop in fuel pressure. This may be due to a faulty fuel pump or a plugged fuel filter. Correcting problems in the fuel system usually resolves these issues.

Incorrect ignition timing to the spark plugs is another cause of backfire. Timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner.

[MagicRat]

Originally Posted by fourwd1

A backfire occurs when there is an imbalance in the air to fuel ratio. If the fuel mixture is too lean you may have a backfire in the intake, or too rich you may get a backfire out of the exhaust system.

Backfiring can also occur with a sudden drop in fuel pressure. This may be due to a faulty fuel pump or a plugged fuel filter. Correcting problems in the fuel system usually resolves these issues.

Incorrect ignition timing to the spark plugs is another cause of backfire. Timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner.

I agree with these explanations, excpet, perhaps for the last one. An ignition so far advanced would not allow the engine to run... which this engine actually is.

Also, a leaking intake valve can cause an intake backfire, as well as the incorrect valve timing, as stated in the OP.