it could be a prob with the low end needle. the low affects the high on these carbs. One thing i do if i have alot of troubles, i start from scratch (both needles out 1 1/4 turn) usually is a bit rich, but close enough to run without damage (ex. being too lean). But if no matter what u do, it still is tempermental, suspect an air leak of some sort. check all gaskets and crank seals.
PS
Also check that lever thingy under the diaphram. I cant remember what its called exactly. Aviatior and the other guys can give u better advice than i can. http://www.geistware.com/rcmodeling/...ine_tune_1.htm (this is also a very good site)