Kehin FCR Pumper jetting

I just installed a kehin pumper and yosh rs-3 exhaust on my 97 xr600 and it makes awsome power from bottom to mid but at high rpm it almost acts like it hits a rev limiter. I pulled the plug and it looks pretty normal. not white but not black either. This may seem basic to some of you but this is the first big thumper I have owned. Do they even have a rev limiter? I never felt this with the stock carb. My gut says try a leaner main. I have a 170 main 50 pilot and the clip is 3rd from top. Any help from the carb gods? Thanks.....


First, I'm sure you know, XR's don't rev out like 2-strokes. However, if you didn't experience the problem with the stock carb, then it probably is the main jet. However, I would guess you are too lean on the main, no gas, no go. I would try a richer main, before I went leaner, just for your motor's sake, and for your wallet's sake.

Furthermore, I have a Mikuni Pumper on my XR400. The pumper allowed my engine to rev up quicker than ever before, a good thing, but with the low RPM nature of XR's, you rev out quicker. So, maybe that's it.

Is your power falling flat on it's face after the midrange, or are you just running out of RPM's? I think I would still enrichen your main, and maybe even your needle to see if it's better. If it is worse, then you KNOW you need to go leaner. Good luck.

ok here is the deal i went up to 175 and raised the needle one it was worse. i wnt to 168 same went all the way to 152 main and still wont rev at wot. the cool thing is i can feel it when i go from 3/4 to wot so i know its in the main jet but how far lean can i go? this does not seem normal. what do you think? i put the stocker back on and it runs smooth but i like the pumper power.

i put a thumper racing 660 big bore kit on my 96' 600 ported and polished head. i am using a 72 pilot and a 178 main in the stock carb. it wouldn't hurt to take another jump to ?178-180 on the main because the carb is bigger and is a flat slide so it is moving alot more air. the needle would not necessarily need messed with because it covers mid range only(i would leave in the mid)if this does not work i will buy the carb off you!!

Are you using 39mm or 41mm FCR?

There is also Main Air Jet in this carbs and flat top end is one of the symptoms of not using correct MAJ.

Search for "Jetting Qs" in Yamaha WR list (6 months ago) and you will find a lot of info for FCR jetting.

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