G'day

Th A3 plug is at the hot end of the OS plug range. It is OK as a running in plug but I would not normally use one in an AX46 for flying. Hot plugs advance the timing of the engine. So does adding nitro to the fuel. 15% is quite a lot and will also be advancing the timing. The combination of the two is probably too much. A lean mixture also advances the timing. Eventually something has got to give and things will either get hot or the plug will get fried.

Generally speaking, two strokes don't like long exhaust pipes. They need to breathe to be able to get rid of burned fuel products and heat and a long pipe does not help this.

I suspect that you may be having some pre-iginiton problems when the engine is running at part throttle too with all those factors which advance the timing. Does the engine seem to make a crackling sound at part throttle? Some one once described it as the sound of frying bacon. If you do have this, you are not doing the engine any good. Pre-ignition means that the fuel is starting to burn too early and this puts unwanted stresses on the con rod and piston which can lead to premature failure.

If you must use 15% (I'd save the money and only use 10) then I would definitely suggest a No 8 plug. Then tune the engine for maximum revs and then back the needle out (richer) to drop the revs by between 300 and 500 rpm or a just noticeable amount (about 1/4 turn). I'd also use a fuel which contains a few percent (up to 5) castor oil. It is good insurance against a lean run.

I also find that unless you run an engine dry at the end of the day, fuel residue in the carby can block things up especially if the engine is left for a considerable time. Before I try to run and engine that has been out of action for a while, I remove the needle and blow some fuel through the carby in both directions to ensure there are no blockages. Fuel residue in the crank case can also attract water which can lead to rusty bearings.