Don't rule out the carb/fuel......Fuel can go bad in 6 months or less, and varnish up your float bowls, needles, jets, etc . Usually spraying in some carb cleaner won't remove old fuel buildup. I'd personally take that carb apart and eliminate that possibility. If your careful, you can save the gaskets and re-use but my guess is that you might want to soak the carb parts overnight in acarb cleaner and throw in a carb kit while your at it. And, like the others say, eliminate any vacuum leaks.