Jim Crenca

Mar 18, 2001
The shop manual for my 1994 KDX calls for a B9ES spark plug yet most of the posts refer to B8ES. Do the newer bikes use a different plug? Also, is there ever a need to change heat ranges due to engine mods (like pipe, silencer, airbox, or RB Designs carb mods)?


Grand Data Poohbah
Apr 30, 2000
95 and newer service manual calls for the BR8ES.

Anyone know off hand the big diff between the BR8ES and the B8ES. (I know R=resistor plug) but what's the impact of using a NON-resistor plug when the bike asks for one ?

(I have lots of the BE8S, as that is what I feed my RM regularly :p )


Jun 5, 2000
The resistor plug is intended to minimize EMI (Electromagnetic Interference). The more sophisticated the bike's control module, the more you have to be concerned about EMI. If I owned a something like and '01 YZF250, I'd be careful about sticking with a resistor plug, but IMHO, the KDX is not that complicated. I have put on many miles using B8ES plugs without any ill effect. I've seen one or two posts by KDX owner's reporting a high-rpm miss when using a non-resistor plug, but I have never experienced this myself. I currently run a BR7ES - since if you're buying a new plug and a resistor plug costs the same as a non-resistor - why not make the owner's manual happy? If, however, a case of new B7ES plugs fell from the sky, I would not hesitate to use them.

joe woj

Oct 29, 1999
ive never run a resistor plug in my bike have never had any problems, the higher the number the "colder" the plug, the lower the number the "hotter" the plug, im pretty sure i read somewhere that its a diffrance of about 30 degrees in the cly per each step, so i run the 8 in the summer, and then switch to the 7 when temps drop below 65, to keep some heat in the engine, this always seems to work well for me.:cool:


Oct 14, 1999
I've noticed some troubles with a 'non-R' plug.

My DiscMan kind of whines past about 4k r's, my radar detector gives off some false Z-ray detections. My portable HDTV does ok..but the NTSC unit I had earlier had some raster squiggling issues.

My MP3 player worked great...well, up to my endo in the mud last weekend. It doesn't work at all, now. ;)

Keep in mind that a change in heat range of plug also correspondingly raises/lowers engine temperature. Up to 50º! That will explain why your bike starts gurgling and puking out the overflow on that uphill after you change from an 8 to a 7.


Sep 5, 2000
I tried running the B9ES in my '92 KDX200 per the manual. However, it would foul a plug after about 20 miles of woods riding. I switched to the B8ES and have never had a problem since.
Top Bottom