RE: Fox .15 slant plug
Mine has the last carb they sold with it - MKX, I think? It is identical to the carb on my Schneurele ported .15BB. I think the lackluster performance is due to the head shim still being in place. With the figures you got it must have about the highest power/weight ratio available in such a small engine.
I have the stock muffler and this engine is the most deafening little cuss I've ever owned (well, my Cox .15 TD might have beaten it, but that was a long time ago). It's not just a simple expansion chamber either - it has two concentric chambers with perforated holes between them. I haven't dared to try it without the muffler, so maybe it actually does something, and it would be even worse without it. I am really spoiled by OS mufflers, and for that reason alone I haven't decided if I'll use this on a plane. Jim