Throttle valve

I have a 1998 wr 400. I have dual sported my wr. On my last dual sport ride, when reaching our off road destination, the bike did not want to come down from idle. I adjusted the idle speed and the bike came down a little. I rode the bike back about 29miles and the bike was doing the same thing. We took the carb apart only to find the throttle valve broken. I have ordered a new one and will be replacing it in next couple of days. My question is how concerned should I be with the fragments of the throttle valve that have vanished? The bike runs and no foreign noise is present.


I would imagine a little. I am assuming the pieces were large? Could they be sucked into the motor? Are they small enough for that? Also, is the material of the carb the same as the cylinder? I assume not, thus I am assuming the carb is a softer/lighter material.....

I would only concern myself with the pieces possibly blocking the carb and maybe getting into the cylinder?

Sorry I am not much more help, I am not that mechanically savvy... Ask Clarke


From the carb, the pieces went into the cylinder via the intake valves. From there into your cylinder. They may have gone out through your exhaust valves. If you are energetic, you can remove the valve cover to at least take a peek. Then you may want to take off the (valve) head to inspect your cylinder walls and valve seats. Something happened to my bike also, but I have no clue as to what. My cylinder wall is damaged enough to piss me off. I am blowing off REALLY fixing it until winter. I will have to research the IMS big bore kit.


99 WR, all YZ mods, de-octopused, OEM YZ tank and seat, jetting by Clark.

[This message has been edited by Kevin in New Hampshire (edited 06-01-2000).]

If the broken piece(s) was big enough then it wouldn't be able to go past the intake valve and enter the cylinder. Remove the carb and look down into the intake port, you should see three passages with the valves at the end of each. Do you see any pieces of broken carb slide in there resting against the valve head? If so remove it. If you don't see anything... when you get the carb fixed ride it and see... if it seems normal then you got lucky and the piece(s) went out through the exhaust. Just to be on the safe side, check in the airboot between the filter and carb, I doubt you'll find anything there but it's better to be safe than sorry.

I do not know if the frame drain plug is identical to the engine drain plug

No, the frame drain plg is an 8mm (thread size, not bolt head size) and the engine plug is 10mm or 12mm, can't remember which...

You don't have to worry about the carb slide pieces going in here, the cylinder is sealed off from the bottom end by the rings.


'99 WR400

'92 GSXR 7/11

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I want to thank everyone for there reply. I went to my dealership to pick up the new throttle valve today (109.95)and they told me the same thing. The pieces have more then likely passed through the cylinder and out the exhaust pipe considering I rode the bike back from when the trouble began (29miles).Call me stupid but I did not want to walk. The throttle slide seems to be a widespread problem with WR's and YZ's. I am going to write yamaha about this problem.EV

Thanks again for the info. The new throttle valve is in and the bike is running fine. Does any body know of an after market throttle valve that is available? I think somebody needs to make one. I will advise on the results of my letter to Yamaha. I feel they will tell me its the carbs manufactures problem. :)


How do you do a YZ426 piston on a 99WR? The 00 YZ wrist pin is a larger diameter and will not fit through the 99WR rod small end?..

Have you any info on the IMS 426 as to how well it is working out? anybody out there in forum land using the IMS 426 on a 99WR or YZ?? If yes how about some feedback.


[This message has been edited by Clark Mason (edited 05-31-2000).]


I am clueless on the cylinder swap. Some guy on a 520KTM 4 stoker told me it was a straight forward swap. Now I know this is not the case.


Eric V,

Does your WR have a screen type spark arrestor? If it does, check it and the tailpipe for pieces of the throttle valve. They may not have escaped. If the pieces are big enough you could mail them to Yamaha for consideration... The Keihin manual calls this part the "vacuum release plate"

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