Do a complete check. Check compression first. Next,pull one plug wire at a time and see if any one cylinder missing makes no difference to the way it runs. If a cyl. is bad, pull the plug and hold it to ground while someone turns the engine over (unless you have a powerful coil, then use a clamp on tester) to test for spark. If you have spark, then the problem is probably in the head. Dirty valve head or something in the intake path. If everything looks good, then it MUST be carburetion, so follow the advice of the other post that says check all gaskets and seals. Maybe it is just sediment in a fuel or air jet.