Motorcycle Repair: spark plug issues, suzuki pe400, heat range

Question
i have got a suzuki pe400 two stroke,I'm having a bit of a problem with spark plug failure,it will run great for about ten to fifteen minutes then it starts to sputter and miss then quits,i take the plug out and it looks great its not fouled or burnt it actually looks very clean then when i test it for spark its a no go the spark refuses to jump the gap and instead it jumps off the insulator to the inside wall way up inside the plug,this same thing seems to happen to all the plugs i put in it,I'm using ngk b9es plugs any help or advice would be greatly appreciated thank you  John M.

Answer
John,
try changing the brand of gas and use 87 octane.
Then buy an NGK b8es plug. The heat range
you are using is a little cool for two strokes.
By using a b8es you are going to a little hotter
plug which might help keep it clean.
You might want to change your 2 stroke mix oil
as well. Some combination of these changes may help.