sparkplugs going bad

i have a 99 yz400f and iv been riding alot lately and after i ride and come home my sparkplugs have just stopped working and theyre brandnew. this has happend several times thats why im saying sparkplugs as in plural. what would cause the sparkplugs to stop sparking. theyre not fouled or burnt they look perfect except they dont spark.

also my bike has been cutting out in the upper RPM range if that helps too

And if you replace them, they do spark?

What specific spark plug do you use?

yes when i replace them they spark and the bike runs fine but after i ride and and the bike sits for a few days then they wont spark. the sparkplugs i use are the NGK cr9e which is the one specified for the bike

That's part of the problem right there. The bike calls for CR8E's. CR9E's are too cold to keep from fouling.

Ok but the problem isn't that it's fouling. Something with the electrical is causing the plug internally to fail

try a cr8ek it is a little more expensive but the spark is muche better due to the dual ground posts, alot of KTM bikes call for this specific plug, however i found that it works great in my 426.

makes for easier start also :busted:

could the lower heat range plug be whats causing it to cut out in the upper rpm range?

Indirectly, yes. The colder plug will foul, after which it will misfire.

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