650R Carb Air Cut Off Valve?

6 replies to this topic
  • Dutch

Posted November 08, 2001 - 02:35 PM


My 2000 is a 49 state bike, is this on mine? - Is this something riders are modifying? What are you doing and what are the results? :)

  • skywalker951

Posted November 08, 2001 - 03:40 PM


Dutch, I don't know if the 650 has a shutoff value, the bigger question is if you have a shut off valve (that would be a bonus).

  • barnumpro

Posted November 10, 2001 - 08:42 PM


Im not shur what it is that you are refering to a cut off valve but i do know a little about the XR 650 haveing raced this bike for the last two years I cantell you that you need to change the jetting and use the honda power up kit if you have a aftermarket pipe the jet kit from honda will help with a 175 main jet needle 4th clip from top but if you would like to realy make your bike run better start better then you will need a EDELBROCK carb kit I know I won 3# 1 plates with the carb.

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  • RedThunder

Posted November 10, 2001 - 09:15 PM


I wouldn't touch it. It serves a specific purpose in your carb, messing with it will do absolutely nothing at all in the way of performance gains.

  • Dutch

Posted November 11, 2001 - 02:30 AM


Thanks for the replys guys. I have the good needle and the jetting mods done to my carb. What I am trying to figure out is how to get the big XR to idle better. The way it works now is if the idle speed adjustment is just set perfect the motor returns to idle normally. What I'm trying to do is set my idle speed up a little. When that is done the motor won't return to idle right away, almost like a vacuum leak. When I see the carb has a valve called an air cut off, and some of you have modified it, perhaps that is my solution.

  • qadsan

Posted November 11, 2001 - 08:01 AM



The "ACV" (Air Cutoff Valve) is the diaphram driven device on the carburator that injects bypass air into the carburator's slow circuit
when manifold vacuum is low. Low manifold vacuum occurs at throttle positions greater than about 25%. The ACV affects the rate as to how fast the engine throttles down. If your engine doesn't throttle down quickly enough for you and you're unhappy with it, then you 'might' want to consider disabling your ACV.

You can simply disable the ACV by cutting the little metal tip off the diaphram. You can also reverse this mod by installing a new diaphram if you wish to do so. A few people that have done this mod have reported excellent results with no other ill affcts by disabling the ACV, and then there's one guy who says his bike ran poorly after doing this mod. If my bike didn't throttle down quickly enough, I'd certainly try the mod. Buying a new diaphram to put it back to stock can't be all that expensive.

Page 5-10 of the Honda service manual has a good picture of the ACV and diaphram for your reference.

  • RedThunder

Posted November 11, 2001 - 08:48 AM


Are a lot of people really having problems with the idle hanging? On my bike if I smack the throttle, it revs nice and quick, then settles right back into a normal idle. Strange....

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