Blackened Plug- YZ450

I was riding at the Badlands in Attica, IN this weekend. I have an 05 yz450 that I recently put a big gun ehxaust and header on without rejetting the carb. As soon as I put the pipe on I could tell a difference in the power and the bike seemed to run great so I rode without rejetting for 5-6 rides. This last weekend I was at at the Badlands and bike seemed to be riding fine but at one point I cut the engine to wait on a friend and it wouldnt start again. I had to push my bike outta that bitch. I removed the spark plug and it was straight black. I popped a new plug in there and she started right up and rode great for the rest of the weekend. Any ideas why the bike fouled the plug and what does a black plug mean? too rich or lean?

Black sooty plugs are from low quality spark, or from being too rich. It is not unusual to have an aftermarket exhaust, especially one with larger mid pipe and core sizes, richen the jetting. Common lore would have you believe it is always the other way round, but it ain't so.

Grays right.

But, I'd like to add that a similiar problem happened with my 426. It would run fine and sometimes it would foul a plug. I chased the jetting for a couple days and then I found a tight center intake valve. I reshimmed the valve and all the problems went away. You may want to take a few minutes and check your valves while your rejetting.

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