jetting advice needed/ cuts out at top

Guys or Dougie or Clark,

At top rpm, she now cuts out....studders.

I have a canadian 2000 model with the following:

stock timimg

Air box lid removed.

No throttle restriction.

New fmf power core IV-2 pipe.

I went from 42 pilot to 45 pilot.

Main from 165 to 175

Pilot screw from 1 2/8 to 1 1/2.

I'am at sea level and air temp was 70.

I tried the new pipe with the original jetting and it did not cut out. However, it certainly did not have the same amount of acceleration.

After modifying the carb, it cuts out at top. The rest of the powerband from bottom to near top is good.

Is it overjetted? Maybe I should try the 168 main jet and work it from there? Or perhaps I go down one clip (3) on the needle?

What do you think?


Rev it up to where it cuts out and hit the kill switch, then check the plug to see if it's black. If it's black your jetting is too rich or it needs more air.

Are you sure your not hitting the rev limiter? It's not a 2-smoke!



86TT225, 98CR80, 99WR, WR timing, throttle stop trimmed, air box lid removed, White Bros head pipe, silencer and air filter. Odometer and headlight removed. Moose hand and mud guards.

Thanks Tom for singling me out in your quest for info. But I'm afraid you seriously misjudged my engine and jetting knowledge :)

While I'm no longer the mechanical moron I claimed to be in the early days of this site, I still am learning the theories of jetting and engine diagnostics.

James and Clark are your best bets here. Good luck.


Dougie, '99 WR400

Mods: YZ timing, Race Tech Suspension, FMF PC IV, FMF Hi FLo Moto, YZ seat, IMS 3.3 tank, One Industries Graphics

FWIW, if the throttle stop was too short on older models the bike would cut out at full throttle because the computer would get confused. Haven't heard of it happening on a 2000 and I certainly haven't tested it out on mine.

Sounds like you are jettied correctly and now just hitting the rev limiter. The rev limiter is a soft cut out it starts missing firing pulses and not a instant stop. So when you hit the rev limiter it feels and acts like its cuting out or missing on top. Maybe you never got to ON TOP before your jetting changes? Can you positively say what speed you were runing with each jetting configuraton you tried? With stock gearing on the US model you will hit the rev limiter at about 86--88 MPH. Remember missing by 1 mph either way can influence what you diagnose as the problem.. I doubt that a 175 MJ with WR timing would be too rich on top end if anything it may be too lean at sea level.


Guys and Clark,

Thanks for your advice. It made a lot of sense. I now believe I am hitting the rev limiter. Prior to the new pipe, I never tacked it out. Afterwards, I deliberately wanted to see the performance...naturally it cut out on top. Just to be sure I wasn't fooling myself about it, I rejetted the carb back to original (165 main, 42 pilot, 4 clip down, 1 2/8 screw adjustment). Again, it cut out at top!

Oh, one other thing, I have a different needle on the Canadian. I believe it runs a couple of clips richer than stock. It's last initial is a Q. Instead of richening it, I dropped it to the third clip. It seemed slightly cleaner on top than the second or fourth clip.

Bottom line...I now have a 45 pilot, 175 main, 3 clip and 1 1/2 screw adjustment. It seems to work well with the FMF IV-2, no throttle restriction and airbox lid removed.

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