Has anybody else fouled out a plug on their 400ex. I have gone through two now. This is strange. I have been riding for years and have never fouled a plug on a four stroke.
I have the LRD exaust system, a K&N, and I have the bike jetted to LRD's specs. Is this just an adjustment I have wrong on my bike or are others having the same problem?

 

Jetting is wrong most likely. For a 400 stock cam, bore, compression. Pipe, air box mod, K&N only I would go up one size from factory try 155, keep the stock 38 idle jet run the air/fuel three and half turns out. Needle on third clip. try that and if no luck tell us what you are running now... THey probably recomended a huge jet... Dyno jet recomended a 165 for my mods adn that was way too big. I still don't run that big with a cam and 425 kit.