An old timer once told me in an effort to keep it simple, an engine only needs atomized fuel/air, spark, and compression to run. He said to start with the easiest to check/fix first. He said that he saved the carb for last because it is the least likely culprit and the most difficult to fix. Is your intake heat riser functioning properly? If it is stuck in the heat position you will have poor performance once the engine is warmed up. I would next look at all of the plugs to see if they all look the same. If one or two look bad then you may have a problem with those particular cylinders (IE rings, valves, valve clearance) and not a fuel delivery problem. Have you checked the condition of the points? Have you checked for vacuum leaks? Bottom line, start with these easy checks then move on to the more difficult checks if necessary.
