RCW, most likely your Yamaha had too hot a plug in it, back when dirt bikes had two spark plug holes some guys put a hot plug in the extra hole thinking that if the plug they had the ignition wire connected to fouled they would simply move the spark plug wire to the hot one that was not fouled. Well, guess what, the hot plug ran just as hot whether the spark plug wire was connected to it or not and lots of pistons got burned and caved in from this, the very same thing can happen to a glow engine from too hot a plug, overloading or too much nitro.