Pipe Jetting

I'm am just about to purchase a FMF Power Core IV and think my bike carb will need adjusting, I am an engineer for the Ministry of Defence however I don't know if i'm prepared to start messing with the carb after spending £4000 on my WR400F 2000model!

Would anyone have any idea of what needs to be done or have any pics of their experiences with this situation!

Your responce would be greatly appreciated

Yours Sincearly

Tom Ireland

The pumper carb was intimidating to me the first time I tried to jet my bike. It's not that bad at all.

Put the pipe on, warm it up, ride around some and pull the plug. You may have to richen it up, but you can also look at the current post "To the honorable Dr.J etc, etc.

There is some excellent baseline jetting info there.

Good luck



97 KDX220, 86 TTR225, 99 WR400f, WR timing, throttle stop trimmed, air box lid removed, White Bros head pipe, silencer and air filter. De-octopussed. Works frame guards and Thumper Rad Guards, Scotts steering damper. Odometer and headlight removed. Moose hand and mud guards. YZ stock tank, IMS seat and number plate. Renthal Jimmy Button "highs" and Renthal Soft half waffle grips. AMA, SETRA, Happy Ramblers MXC.

Look at the rear of the manual for stock jetting and options. Find the pilot screw location (on carb, bottom front in recessed tube). Turn it counterclock to richen. Turn slowly, listening for smoothest idle and the least engine noise. Listen with idle way down while fine tuning, then turn the idle way up and fine tune, blip throttle and feel for hesitation a few times during normal idle. Give it a test spin, let it bog and feel for any hesitation, surging, or sudden on/off action. If you can't adjust it with the screw, post your jetting, symptoms, and any other info you can think of, someone will advice specific jets for the UK I'm sure.

Next, the main jet. the Hex shaped brass piece right under that 17mm cover at the bottom of the carb. Loosen the carb, tilt and undo the cover. I'm not telling you to change it, but here it is. Under that same cover and towards the front is the pilot jet.

Under the top cover and a holding bolt, is the needle :) It's a bitch to get to without t-handle allen screws.

For now, just play with the pilot screw for your next few rides. If it starts to backfire, pop, surge, etc, you'll need to make further adjustments.

I've got a UK '00 WR with an FMF Power Core IV-2 on the standard header and the airbox snorkel removed. With WR cam timing I used a #168 Main Jet. With YZ cam timing I've gone to a #170. Pilot screw is wound about 2 to 2 1/4 turns out. This setting seems to be about right, although I might try a #45 Pilot Air Jet.

If you follow the instructions in the manual, fitting the Main Jet really isn't that difficult.

Take time to read a few more of these posts. Some of these guys have forgotten more about carbs than I'll ever know, but I ended up with these settings from info on this site. There is also some jetting info on fmfracing.com.



Nottingham, England


'00 WR400F, YZ Timing, FMF Power Core IV2

[This message has been edited by David W (edited 03-27-2001).]

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