Sean,

I assume you have the 4.3 GM engine. Based on this need to verify the distributor cap does not have any cracks and is clean. Check a few spark plugs and make sure they are gapped correctly. Depending on the hours i would check the fuel pressure from the regulator. The pressure from the regulator to the EPR should only be 2.5 to 4 lbs. Should it read above 6, the engine is flooded. If it runs fine before backfiring, valves and timing should be good. Need serial number if you have different engine.