hmmm, I wouldn't think 4 cycle fuel would matter. I would bet the only thing differant is that the '4 cycle' fuel doens't have any caster and is all synthetic. I've heard some ppl don't like caster in there 4 strokers, cause it gums up the valves. Have u played with ur mix valves? U might wanna give that a try. See if u can beg a tank of fuel of someone else to see if the problem goes away. My guess is u just need to retune the engine. Sounds rich.
-John

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Oh, also have u checked ur clunk? My engine was chocking to death today at 1/3 throttle after I misjudged hight on a shrub coming in on a simulated dead stroke. THe clunk line was stuck and kincked.