[100IH]

Does it idle smooth or does it sound like one is missing ? Short one at a time to see if one is dead by listening to the RPM drop off. This is called cylinder balance test. Takes 1 ear or 2 and a screwdriver. Where it backfires is important: tailpipe or carb. If guessing is the preferred diagnostic path way based on stories of others who don't understand diagnostic procedures : go back to original points and plate in the dist. In the ideal world, Your good list would tell what test you did and what were the results and NOT just say Carb, Dist, Valves move,. By the way, even smelling the tailpipe is good information. What you learn along the way is more important than just getting it to run better. Good luck. There is pretty much a test for everything that is necessary to run.