i am running an os91fx 2stroke with 12x8 prop and ultra thrust rossi muffler... on the ground the engine is fine, revs 12900... once it gets into air and I do a declined pass at WOT the engine starts puffing little bits of smoke out the exhaust... any ideas what can cause this? it doesn't sound well after this then once I get to lower revs it cuts and I do a deadstick landing... engine then is fine on ground again, compression is still high too... and the head is not hot at all...