I know the standard sparkplug for a 400ex is DPR8Z, but I read that a DPR9Z will allow my engine to run cooler....is this true? And yes I do tend to ride at high rpm's.

Thanks,
Mark

 

You aren't going to hurt anything by trying a cooler plug (although you might with a hotter plug). Plugs are cheap, so give it a try. Ride it a while, then check for fouling. If it is fouling, it is to cold.

Some good tech info about plugs-----> http://www.ngksparkplugs.com/

My 300EX always ran better with a half heat range cooler plug.

 

thanks man