is it worse when the engine is warmed up or cooler, like ti was just started a minute ago..
something is messing with your fuel/air mixture
check your main jet - that will cut your engine down if its clogged and you mash the throttle
if its not that, you may have air leaking into your engine from a point after the carb..
so check the intake boot, the reed cage bolts..
but you should have put in a bigger main jet with your air filter.. if you put the filter in over the summer, and its acting up now that the weather is getting cooler, then you need a bigger main jet. youll need to increase your main jet size by 20 or 30 numbers, ie go up 2 or three sizes