My friend's 1990 Buick Skylark with a 2.5 liter four cylinder engine and an automatic transmission seems to be backfiring out of the carburetor. We think it's the timing. How can I check it?
This 2.5-liter engine you're referring to does not have a carburetor, but utilizes a Throttle Body Injection Assembly to supply fuel. It also does not use a distributor. Instead the ignition involves a crankshaft position sensor, an ignition module, and a coil pack assembly known as the DIS (Direct Ignition System). The ignition timing is not adjustable on this engine.
I strongly suggest that you get the car to a good local shop so they can properly diagnose the problem. The most likely cause of popping through the throttle body on this particular engine would be in the Direct Ignition System, and it usually turns out to be a failed ignition wire or coil pack assembly. But please have it diagnosed before you go swapping parts.