ORIGINAL: Cookieworks
It flew almost perfect until the muffler popped out of the cowl, then it shut off. That tells me that it was running to rich. However, when I leaned out the mixture and tightened the muffler, I couldn't get it past half throttle before it stalled. I've had this problem in the past, but I cannot remember what exactly I did to correct it.
If you're running muffler pressure (~3psi), then you lost that pressurization when the muffler fell off that resulted in the engine going very LEAN. Conventional venting (or Uniflow) wouldn't exhibit that kind of behavior, but muffler pressurized systems will.