RE: OS 50SX tuning/glowplug problems..
Interesting--- I suppose you could use the stock muffler to see if the Pitts is doing anything, can't imagine the muffler is the problem though...
What fuel are you using? Also, this may sound silly but are you sure you have the glow-washer on there?
I run OS-8 in my OS-50SX, no problems---