Tullahoma Tennessee! That place sure brings back lots of good memories. Spent many months there supporting Wind Tunnel activities at AEDC. About a year ago I purchased a Webra Speed 40 Pylon engine as a Q500 practice thrasher. The engine came with both a barrel type(TN) and a guillotine type carb. The guillotine carb didn't show much promise for a reliable idle. I didn't spent that much time working the low end carburation, but the muffler/pipe that came with it seemed inadequate to produce a pressure source good enough for tank pressurization. To me that check valve mentioned earlier sounds like a must.

Good luck

MaxQ