GMC truck backfire

Q: Greg, my 1974 GMC truck has started to stall. It starts up right away, but also backfires and scares the daylight out of our dog and us in the cab. I have replaced the distributor cap, condenser, coil and spark plugs, checked the ignition wires and also the pressure from the fuel pump. It is all OK. Do you think it could have anything to do with the timing chain? The truck runs fine otherwise. — Ron and Anita, Kelowna, B.C., Canada

A: My dad had a ‘76 Impala with a 350 engine, and it did the same thing your GMC is doing. The problem turned out to be a tiny fuel filter in the fuel inlet of the carburetor (not the filter in the gas line leading to the carb) that was completely clogged, resulting in the car shutting off when driving. It would start right back up, and backfire a bit. Check to see if your carb has a little filter in it. I fixed it in 10 minutes.

If this isn’t the problem it could be a short somewhere in the wiring. You’ll have to check all wires and such, because it sounds like your ignition system is “shutting down” (stalling) and allowing unburned gas to lie where it shouldn’t.

Next, check your carburetor for a rich condition. Lastly, check your timing chain.