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Muffler plug/Airbox lid removal???

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I am a proud owner of a 2001 WR426 and was wandering what I can expect realistically in performance if I were to remove my muffler plug and airbox cover? Do I have to do any other modifications if I remove these?

This site rocks!!! Thanks to everyone for your information on this subject.


(Highland, Illinois)

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If your bike is the US version, you'll want to cut the throttle stop down to 23 or iis it 24mm's. Your throttle is only opening to appx. 5/8ths throttle.With all of these mods you may have to tweak your jetting.

There is also the option of going to YZ timing (instruction in the tech section) and the latest fad of cutting the gray wire. The "cut" is supposed to change the mapping/ignition curve and improve mid range performance and eliminate flat spots and surging at constant speed....I think?

Most questions you'll have in the beginning of your ownership of the blue beast, can be found by using the search function.

There is also:


yz style seat and tank

aftermarket pipe

dumping the POS 739 tires

52T rear sprocket or 13T front

steering damper

panoram computer

:):D :D :D :D




99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP, 48PJ, 175MJ at 500-1000' Thanks James Dean!

[This message has been edited by Bill (edited 06-29-2001).]

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Yamaha has hidden alot of unused ponies in your motor. If your a mechanic, do the YZ cam timing. The insert is a must go. I went the (cheaper) aftermarket performance insert route, most are less than 80 scoots and help get the noise to a reasonable level. Removing the airbox lid is free. Some guys cut the throttle stop and others buy the YZ part. If your real carefull, cut it down, but if you don't make it right you could damage the carb. For what it costs I would but the YZ part. The gray wire is simple as well and there are plenty of posts on that. I wouldn't cut it though. I pushed the male pin out of the connector with a jewlers screwdriver, then folded back on the harness and taped it up real tight. That will enhance the midrange. If you run tighter rails lift the fork tubes up 5mm and change to a 13t countershaft sprocket. You may have to richen the jetting. Tons of posts about that as well. Find someone with a similar set up and ask them there jetting specs. This is the trickiest adjustment though. None of these mods will hurt the bottom end and will yield a much more responsive motor. good luck.



'00 WR 400

Western Fab Insert, YZ timed, airbox lid removed, YZ throttle stop, Michelin S12's, UFO YZ plastic,Mobile 1.

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