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I recently purchased a 2002 KDX 200 with a FMF woods pipe. I ride in southeast Idaho from 5,000 to 9,000 feet.
Right now it was a BR9ES spark plug but the service manual and everyone here says to drop to a BR8ES.
However my brothers KDX also had the BR9ES... Is it just a coincidence that both the bikes we bought had the wrong spark plugs??
I'm just wondering if I will mess something up if I drop to the BR8ES.

What do you guys think?

I run both. The # is the heat range, try one of each. If there is a difference continue to use the one that you like best....

Stock plug is BR9ES. Most people switch to the slightly hotter BR8ES.

That's funny cause the service manual calls for BR8ES

My 220 called for a BR8ES.

Once mixed up after a top end and grabbed a BR9ES I had for a sled in my tool box. Kept cold-fouling plugs when riding to work. Would start right up and run great. Trails, no issue. But hop on the road and putt along and it'd foul within 1/2 mile every time. Of course, I pulled that first plug and kept buying 9's after that. Was so frustrated, ready to change crank seals thinking it was an issue like that. Finally realized my mistake, went back to 8's, and back to only changing a plug when testing jetting or doing a top end.

searle7 wrote:That's funny cause the service manual calls for BR8ES

Aha. You are correct, H series and 220 call for BR8. E series calls for B9. I seem to be easily confused these days.
Long story short, most everyone runs a BR8ES without any problem.

regardless of which one you use, it is the same as any other change you make in the intake/exhaust track....pick one, jet to it, and don't change.