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Air Cutoff Valve on '91 XR600R


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These bikes were bred for racing so I presume there is some benefit in leaving this feature intact. However I am curious as to the exact reason for this valve. My research shows that it may be to stop "popping" on high vacuum(going down steep hills?) or to reduce engine breaking. I can't believe(as someone on here suggested) that it is to make the normal idle mixture lean and therefore "clean"? Can anyone confirm this and do you think there is any benefit in removal...or disabling by blocking off the cap? And if the feature is disabled, do I then need to use a richer idle jet?

By the way I now have my machine running well after replacing all the electricals.(probably the Ricky Stator fixed my problem with the ignition)

The bike is a '91 XR600R with standard mods.(done about 12,000Kms)

My riding is serious off-road only although non competitive.

Interested in any comments.

And thanks to all for a great forum.🙂

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This explains the ACV pretty well, click on the 650R carb fix. Yea, I know it isn't a 600R carb but it is a pretty good write up.

http://borynack.com/XR650R/xr650r_caruretor.htm

The real reason is explained there. It is to pass smog. You see the popping is from unburned fuel burning in the exhaust. The system helps that fuel to burn in the cylinder. When you richen up the mixture, it kind of becomes redundant.

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Thanks..yes I have read this..but not satisfied by the explanation...being difficult I 'spose but that's me.

An important thing is I DONT race..and my riding is in steep twisty loose stuff...probably close to what you guys call "single track"?

Anyway...if there is too much engine breaking then this could be a disadvantage since the back wheel would be continually breaking loose when I'm going down a steep hill. Get it? Too much breaking ain't good.

As it is I have no complaint...but just wondering if others have done the mod(disabling the ACV) and what their thoughts are.

Thanks for your responses so far.

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These bikes were bred for racing so I presume there is some benefit in leaving this feature intact. And thanks to all for a great forum.🙂

If it works don't fix it. The ACV is only a problem when damaged. I've got bikes with them and without. I don't notice any difference in braking effect at all. XRs are not racers but they do get raced. Curiously all performance carbs for the 600/650 have no ACV. I have a Delortto pumper 40mm, Mikuni TM42 and had a TM 40 and Edelbrock pumper 42mm all for 600/650. On decel they all feel just like a stock carb.

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Any question worth posting on the internet deserves an answer that contradicts another. 🙂

My reply to another thread earlier this year:

According to 4strokes.com, the purpose of the ACV is to reduce the amount of air available to the pilot jet's emulsion holes at high manifold vacuum. That obviously will richen the mixture under deceleration, but is supposedly done to reduce popping, not cause it. I also interpret that to mean that disabling the ACV will richen the pilot circuit all the time.

Think of it as the deceleration enrichment that all auto manufacturers I know of have used in their EFI algorithms in some form or another for the last 30+ years. In their case, decel enrichment is done to improve throttle response and it's probably even more important in a carb without acceleration enrichment. It helps cover for that lean spike when the throttle is opened quickly and prevents stalling when the throttle is closed quickly. IMO, it was not put there due to a lack of purpose.

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I have my own theory. I think when a motor is in over-run(sucking in more air than is normal to expect) you basically kill the combustion process. This might even occur when "blipping" the throttle but it does mean the engine runs lean going down a long steep hill.

Bad for the valves? Too much braking? Why not give it some extra fuel to compensate.

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I agree with valvesrule and HT, I see no reason at all to disable it and many reasons to keep it intact. I don't believe it is an emision fix or a decel cushion. I am sure it's main function is to help eliminate the irritating decell poppping.

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