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.

First off, put the engine upright on a test stand and see how it runs. If it will run properly "all day" on the test stand, then the problem's in the installation.

Vibration-induced fuel foaming is very, very common. That means that the engine's vibrations agitate the fuel in the tank so that air gets mixed into it. This effectively causes the engine to go lean.

Inverted running usually causes problems with idle and low-end acceleration, but throttle-down from full to mid should not cause the engine to quit. We'd look for another cause.

Since engine is cowled, we'd wonder how well it's being cooled. You need to make sure that the air flowing into the cowl actually flows THROUGH the fins of the cylinder. A large opening in the front and a large one to the rear don't insure airflow through the engine's cooling fins. You may need to make baffles to insure proper cooling airflow.

It has 3 full tanks through it on the ground as setup, perfectly running. It isn't until I take it in the air that I have the isue with quitting.

I have gotten 5 and 10% fuel, a 10.5x6, and 10x7 props, a tachometer and a #8, A3 and A5 glow plug to try out.

Hopefully it'll run cooler with those options and solve the problem.

I should have mentioned it is 92 degrees in Houston and the first full tank went through it without the quitting, but all subsequent flights suffered the quitting.

Hi Bill I'm having a problem with the OS 46AX mufflers. I have lost 5 in the last year or so, have done what some sugested to loosen the muffler shaft and tighten only with two fingers, and then thighten the nut including adding lock tite (blue) and still loosing them, I have adjusted the mixtures to run rich to keep it runnung cool. Bill is there a problem with these mufflers. I have robed all my spare motor's mufflers new still in the box the mufflers. and here locally there are no more to be had, other then buying a new motor.

roltech