I have an OS 46AX. I have about 2.5 gallons of fuel through it. I originally had it mounted, upright, open, in a Super Sportster, and broke it in per manual and had 2 gallons of 10% Powermaster Synthetic/Castor blend through it running a 11x6 prop and Fox R/C Long Glow Plug with idle bar. It ran beautifully with awesome power and had a great transition without hesitation once I adjusted the low and high needles appropriately. I NEVER had a dead stick from first break in flight to last flight. However, a radio malfunction crashed out my sportster.

It was firewall deep in the very soft ground. I got it out and tore the motor down, washed it with carb cleaner, oiled then reassembled with great care to keep the cylinder sleeve and piston in the same already broken in orientation.

Now I have installed it inverted in a Venus 40 (engine in cowl but large front opening and inverted). I still run the 11x6 prop, but had already purchased a gallon of powermaster 15% nitro. I run the same Fox R/C Long Glow Plug with idle bar. The engine runs great on the ground. I had to richen both high and low needles for the higher nitro fuel. Again no hesitation and great top end on ground.

However, when flying, if I takeoff and fly at full throttle for some agressive aerobatics for the first 4 mins of flight then throttle back to less than 50% the engine quits instantly. And this is during level flight. It runs WOT through VERY agrresive aerobatics without a hiccup.

I know quiting usually means overheat/lean condition, so I richened it up and the motor sputtered and coughed and never reached peak RPM. I am concerned it runs hot as the oil residue on the wing of the sportster used to be more a pink color with the same brand fuel and now is a darker maroon to brown.

I tuned and tuned, and still haven't gotten it right and not being able to reproduce the quitting on the ground (I ran WOT for 6 mins then throttled back and it idled perfectly) is really frustrating me.

The problem I am having in pinpointing a cause as so many variables have changed since it ran so reliably.

Is something not assembled correctly or is there a bad gasket with small vacuum leak?
Is the inverted mount causing fuel delivery issues?
Is the cowling causing overheating? (The opening cut in the front is wider and taller than the motor head casting)
Is the 15% and cowl contributing to overheating/lean?
Is the motor unloading and turning a lot more RPM in the air and then running too hot and lean?
Is fuel or oil pooling on the glow plug due to the inverted mounting causing it to cool with anything less than WOT?

Since I can run it WOT on the ground and throttle back without stalling, it has to be related increased RPM when unloaded.

I appreciate any and all help.

My plan is to install a new Fox R/C Long glow plug, go back to the 10% fuel to eliminate that variable and 10% should run cooler and re-tune. If I still have the problem, I will try an A3 or #8 OS plug, and then take the cowling off.