I have a new Top Flight P40 with a inverted mounted new OS .91 FX engine, with OS #8 plug. To fit everything within the cowl, I replaced the stock muffler with a JTec pitts style muffler. I'm running the engine with the cowl off currently.

Engine runs perfectly fine with glow driver on the plug, both high speed and idle. I adjust my needle valve to peak RPM, bring it back down to idle, remove the glow driver, and about 5 seconds later, engine begins to sputter, and quits. I have tried 2 other brand new OS #8 plugs, I have tried a plug with a idle bar, I have tried a MC9 plug, OS #3 plug . All with the same exact results.

My thought is that I need a "hotter" plug like a Fox red hot plug, since maybe the fuel is not burning fast enough with the engine being inverted and is getting the plug too wet??????? Any thoughts, or suggestions are welcome!