Here are my findings as promised in my post # 86

As Wayne has indicated in his post # 106, after completely disassembling the engine, we did not find anything mechanical that might cause sudden flameouts. After seeing the annular reservoir formed by the .030" deep by .250" long undercut on the outside diameter of the main jet, and the 2 main jet inlet holes, I discounted the theory that fuel flow restriction caused by misalignment of the main jet inlet holes and the fuel inlet nipple, was the cause of sudden flameouts on this particular engine.

The carb venturi diameter of .312" impressed me as being very large for a sport .47 displacement engine. This is the same size as the venturi used on .60 size engines of a few years back. While this size venturi would give great power at the high rpms turned on a 9" prop, it could certainly affect fuel draw when a 11" or 12" prop was used for aerobatic flying. I therefore made up a venturi restrictor with a .281" inside diameter, the approx diameter of the venturi on my old OS 45 FSR.. I also opened up the muffler pressure tap from .062" to .070".

After thinking about the possible reduction of muffler supplied fuel pressure caused by resriction of the rear mounted main needle valve, I decided to mount the needle on the carb replacing the acorn nut and plastic plug in the end of the main jet. With these mods in place I mounted the engine on my test model which normally has the OS 45 FSR engine installed. The fuel tank is particularly well isolated from vibration and fuel feed has never been a problem in this model.

Last weekend's weather provided a great opportunity to check out the engine. Details are as follows; temp. 28 F, 8" powdered snow; Master Airscrew 11x6 prop; home brew fuel with 5% nitro, 20% oil, 5% acetone; Fox long idlebar 1.5 volt plug; mixture set backed off approx 300 rpm rich from peak .

First takeoff- engine quit abruptly at about 50'. The exact same problem as encountered several times last fall with the engine in the owner's trainer.

I changed the gloplug to a new OS F four stroke plug, reset the mixture as above and took off. Engine ran beautifully during many take offs and landings and aerobatics.

Changed to a new Fox Miracle plug, reset mixture, another engine failure at 50' altitude on T/O.

Changed to an old used Fox idlebar long plug, reset mixture, took off and the engine ran beautifully, equally well as with the OS F plug.

Changed back to original Fox idlebar plug, engine again quit at about 50' altitude.

I removed the GMS and installed my OS 45 FSR and installed the owner's Fox idlebar plug which was giving the engine cuts. I noticed that when removing the igniter current that there was a greater than normal rpm drop but the engine tuned up ok. I did many circuits and aerobatics during 4 flights and suffered no engine cuts. I used the same fuel and prop and the weather was identical to the first day's except that the temperature was 30 F.

My conclusion- Gloplug selection even amongst plugs of the same brand, is very critical with this particular engine, and may well be with many GMS engines . What is particularly disconcerting is the fact that the symptoms exactly match those of an interrupted fuel flow.