Answers: It sounds like you hold dirty carbs. Has it been a while since the engine have run? If it runs fine over 1000 rpm I doubt you have ignition problems. Check compression, looking for at smallest 90 psi(preferrably 110) and all cylinders in 10 psi. If that checks, fuel is next surrounded by line. Ignition is the most difficult to check and the most expensive to fix. Always check fuel systems first. A angelic way to verify carb issues is to put some gas surrounded by a spray bottle and spray into individual carbs while trying to run it to see if the engine picks up. If you spray all the carbs and zilch changes, next start looking at ignition.
I would double check the spark plugs, especially if this happened once you put modern plugs in. Make sure they're gap right. If you have a carburator, I would product sure air intake filter is verbs, or try running it with nouns filter/flame arrestor off. I would also check fuel/water separator if you enjoy one, maybe you own some water within the fuel.
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