RE: OS 46fx Glow plug problems
You noted this is an OS 46FX (not AX). If so...... the engine was made to turn - not to lug real big props.
I suggest putting the engine back on the test stand - install a fresh plug in it (any one you have is fine) put the stock muffler on it, install a 9x6 prop (yes, a 9x6) - run it with that prop, full throttle, set about 700-800 rpm down from peak rpm (Slightly rich) for at least 20 minutes.
Next, keep it on the test stand - install the pipe (as you have it set now). Install a 9x7 prop. Run it with that. Set the high end about 500 rpm down. Check for throttle response - adjust the low speed mixture if needed. The engine should behave well at this point, and should be turning somewhere around 16,500 rpm. Maybe a bit more.
Next, dont touch the pipe - move up to a 9x8 prop. See if the engine runs the same. If you find the engine gets hot, or misbehaves, or does not transition well, then the pipe is set too short for this particular load. My guess is it will run ok with the pipe set as is. In which case, also try a 10x6.
With the BENCH stuff completed, then install the engine into the aircraft. Fly it with 9x7 to start with. Then move to the 9x8 after it has some air time on it. Always set a piped engine 500-600 rpm rich of peak before launch - when the engine unloads on the air, it will stage deeper into the pipe, and it will need more fuel as the rpm increases a bit.
Of note, the distance you cut the header is not really relavent. The measurement for a tuned pipe is the distance from the glow plug, along the centerline of the header, to the "fat" part, or weld in the pipe.
A pipe works like a governor. It will try to force the engine to an rpm. If an engine is under-propped, it will quickly and easily reach the tuned rpm point - and settle in there. Generally will not exceed that point. If the engine is overloaded, it will still try to get there.... if the pipe is too short (rpm set too high) and the engine can not get there..... it will get hot, not needle well, will not transition well, and in short order this will destroy the engine.
Switching to a Jett wouldn't solve your problem here. If the pipe is set too short or you over-prop the engine - its going to become a $250 paper weight in about 30 minutes. Doesnt make a difference if it is an OS, Jett, Rossi, Magnum or SuperTiger.... all 2c engines respond the same, and the pipe physics are basically the same.
(the only reason a Jett would help is the jett-stream muffler - there is no tuning involved, but you still have to prop the engine within the desired rpm band)
If you do indeed want to turn a 9x10 prop with the engine, it is possible..... but you have to tune the pipe VERY long. Pipe length for a prop that size would probably be about 15.5". And, do not expect any substantial rpm from it. Getting up to 14K from that size prop on the 46FX is asking a ton.
Bob