I have a 1984 s10 with a 372 small block chevy in it and it runs fine tell you get on it then it backfires through the cub the timeing is good is it the valves or the cam??

This is most likely due to over fueling. You carb mixture is to rich or the carb needs to be rebuilt.

Hello, I would check the lobes on the camshaft. Remove the valve covers and start the engine. Watch every rocker arm and make sure they are all going up and down the same distance as each other. A badly worn cam lobe will show as only traveling up and down a little bit .

sounds like the catalydic converter is plugged/partial blocked- replace

You can tell by the intake ports. If there's 2 close in the middle then it's for a small block chevy. If they're evenly spaced then more than likely it's for a big block.

You want to check your timing for accuracy and then check all plug wires for burned or cracked ends. Also check the single coil tower where the coilwire connects for very small cracks. Then check the inside of the disrtibutor cap for the same small cracks. Throughout this process I'm certain you'll find the culprit. Thanks, Dana

I also have an '84 s10 which was recently converted to a Small Block v8. I had the same problems, only when I took my foot off of the accelerator and let the motor ride the gears down. Your backfiring may be coming from a slip in the motor timing belt.

make sure that the timing advance mechanism in the distributor is working freely.

Yes those are the next step, especially the coil.

possible ignition coil pack breaking down...unit cools off them fires plugs again...fuel pressure is a concern also ..possible failing fuel pump

first change plugs wires dist cap and rotor this tune up would be a good start