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!

if you are right which i think is definatly a possibility that the plug is just cooling too fast than attempt to run the airplane upside down so the engine head is standing straight up and see if it runs any better. If it run you know you have a problem with the plug and need something a little hotter.

You make it sound as though this engine is brand new and not broken in? Some brand new engines on thier first tank of fuel will do this but usually clear up after the first tank.

Also I've had some bad experiences with mufflers other than what came on the engine simular to yours.

Also was this muffler made for the 91 or the 61?

I'm just trying to get some detail into the picture where maybe some others will jump in here?

good luck,

Jim

Thanks for the replies so far!!! Keep them coming!

I did run two tanks of gas through the engine (upright) with the stock muffler with no problems for a "break-in period".

The Jtec muffler I have for it was purchased by the builder, not me. I have no way of knowing if the Jtec muffler is made specifically for the OS .91 FX. I'm going to try running the engine in the plane upright as fast'n'low suggests and see what happens. I will then also try putting the stock muffler back on it to see what happens with it.

I have always owed a OS engine, I have owed several OS .46, .61, and .91 FX engines. This is the first one to give me major trouble!!! Thanks for the suggestions!!!

The operating problems you're experiencing can very well be due to the fact that you're running the engine inverted.

Inverting any model engine can make low-speed and midrange running problematic. With the cylinder head pointing down, the glow plug can become like a sump and become over-cooled at low throttle settings. All of the fuel that comes into the engine just doesn't get burned off at low throttle settings, and it all doesn't get expelled in the exhaust. The liquid can build up and "put out the fire".

Unfortunately, two people with identical enignes can have greatly-different inverted running characteristics. Ganging of tolerances, type of fuel, specific mounting of the engine, fuel system, propeller, muffler, and how they're set can make large variances in how the engines run.

Use the best plug you can find. We use the O.S. #8 plug, which we believe is the best R/C plug you can get. Many times, engine running problems have been solved with a new #8 glow plug.

Difficulty with inverted running has been with us since people started running engines. The only sure cure is a onboard glow plug battery that powers the glow plug at mid to low throttle settings.

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Try the "pinch test" at idle and do it the moment the engine shows the first sign of stumbling. Chances are it'll come back to life..release the line and wait for the next stumble. If you can keep it running by pinching and releasing it's a sure sign you're way too rich at idle especially with an inverted engine. Near enough isn't good enough for them

Try the stock muffler! I had an O.S. 1.60 that would not run on a Jtec muffler. Maybe another brand would work. My engine would run on the ground with a long reach Fox plug just fine. With an O.S #8 it would overload and die at midrange sometimes. I have never had an engine give me so many dead stick landings. My new Ultra Sport 1000 still lives, but looks like it has hail damage from the trees it landed in :{ It was a muffler problem. Good luck!

I've had great results on inverted two strokes using the OS Type F or the YS plug. Both intended for four strokes but have worked well for me in Magnum XLS52A, XLS91A, Supertigre 2300.... all inverted. Althoughl unlikely with a new engine, a bad main (rear) bearing can also cause some of the same symptoms.

Good luck,
mike