Pitts mufflers can also be the problem, as they have two outlets they create less pressure inside the muffler and hence less fuel tank pressure.. most guys i know of running them get something with a flat end (just a peice of solid 6mm rod) and belt one outlet with a hammer to squash closed one of the outlets, to still have the look but the same pressure as a normal single outlet muffler. Also check ya glow plug, i had a similar problem in my OS 46ax and was cured after changing the plug and retuning it.