Troubleshooting a Backfiring Exhaust on a 350 Cubic Engineby John Stevens
Backfiring can cause serious damage to an engine if the problem is not quickly fixed. The problem can usually be traced to the carburetor or distributor. The carburetor delivers the air/fuel mixture to the engine, and the distributor ignites that mixture within the cylinders. Thankfully, because both of these components are on top of Chevrolet's 350 small-block engine, checking them is quite easy.
Backfiring is typically caused by an air/fuel mixture that is too lean, meaning the mixture has too much oxygen in it. First, check that the mixture screw(s) on the carburetor are adjusted properly. Most of the carburetors installed on the 350 use a single air/fuel mixture screw, while some carburetors use two. The general rule of thumb is to tighten the adjustment screw(s) into the carburetor with a screwdriver while the engine is idling until the engine speed starts to increase, then loosen the screw(s) half a turn. If the fuel is contaminated with water or dirt, this too will result in a lean air/fuel mixture. The fuel tank should therefore be drained if the fuel has been standing for more than three months. If the backfiring only occurs when the engine is cold, makes sure the carburetor's choke is not stuck in the open position.
If the carburetor is adjusted properly and the choke is functioning, the problem may stem from the distributor. Underneath the 350's distributor cap are nine metal contacts. One contact is for the coil wire, and the remaining eight are for the 350's eight cylinders. Electrical current enters the distributor from the coil at the top of the distributor cap and is then sent to each of the eight cylinder contacts individually. Backfiring can result if the electrical current is transferred to the wrong contact point at the wrong time. This problem is sometimes referred to as "leakage." Remove the distributor cap, and inspect the exterior and interior of the cap for cracks, which are usually the cause of leakage. If any cracks exist, replace the cap. If the cap is not damaged, make sure the spark plug wires are not mixed up on top of the distributor cap. Refer to the engine's specifications manual to determine the precise placement of each wire on the distributor cap.