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What does the Air Cutoff Valve do?

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I'm new to this style of carb...and I've seen a few discussions about the Air Cutoff Valve. What does it do? What are the typical symptoms of a bad one?

I have an '86 XR600R (dual carbs)...and it starts and runs fine for the most part...my only minor complaint is a bit of backfiring on decel. Sanding the joints and making sure everything fit well in my exhaust system seamed to help that a bit but it's still there. Could the Air Cutoff Valve effect this? It's an old bike...

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its really simple

it cuts off air to the pilot circuit when on hard hard decel or any decel for that matter

on other bikes and models its sometimes called and ENRICHENING valve

its purpose is to detract from popping on decel which is a sever lean condition caused by hard decels...most any 4stroke will do it to some extent

what you need to make sure is that the valve operates well

if not you will have bad idle issues or no idle at all and have to use constant throttle input, this in the case especially on the xr6, but others too.

basically it has 2 functions

1 help idle and 2 enrich decel to avoid popping and to a certain extent backfiring...

cheers

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Good description Elsalvator. Just to add to the above that the reason this air cutoff valve was installed was to permit using a leaner PJ to lower the emissions. Its definitely not needed and many people just block it off and properly tune the PJ circuit.

Edited by D0T-C0M
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Good description Elsalvator. Just to add to the above that the reason this air cutoff valve was installed was to permit using a leaner PJ to lower the emissions. Its definitely not needed and many people just block it off and properly tune the PJ circuit.

3rd function EMISSIONS, I completely agree, having said this guys only have real success blocking it on the bigger xr650r carb I have noticed and also, it should be taken into consideration that blocking it off is done correctly only 1 WAY.

lastly some folks say that it throws off their compression braking and is hard to get used to...but better in closed circuit competition

for every day usem get a new one, make sure the passages are clean towards the intake barrel and no pits in the diaphragm and you are good to go.

cheers

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So when the air cutoff valve doesn't work right I should get popping on decel right? I have that..although not terribly excessive. I don't have any starting or idle problems though. My idle could be a tad smoother...but it's not bad especially after the pig warms up to normal op temp.

I'm guessing this may me the ACV is leaking and allowing air into the circuit when it shouldn't which is creating the popping due to a leak condition right?

I have yet to feel the need to tear into these carbs yet...the bike just starts and rund fine enough that I haven't seen any reason to. I looks like the ACV could be removed from outside with tearingthem apart though... Do both carbs have one?

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if you jet to eliminate any popping or any decel pops you will invariably be rich...somewhere most likey too fat on the pilot circuit...

what I have learned is that ALL 4 strokes will pop some amount, some more than others but its no reason to go ape wire over some pops

the ac vale will work correctly when:

the diaphragm has no holes or tears in it(much like a cv carb diahpragm)

the spring isnt broken

the oring on the top is complete and seals well

the passaged to the carb bell are clean

if this is all there then look elsewehere for your issues

an ACV kit can be bought on ebay, k and s is a good kit and cheap, partsnmore also carries them etc...

hope this helps

Edited by elsalvadorXR6

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yes, a little...

 

a good way to see if you have a bad acv, is at idle will the engine die on you if you do not have your hand on the throttle? if it does every so often and its not tight valves or other engine issues you know that you diaphragm has holes in it...

 

its an easy cheap fix with the k and s kit on ebay.

 

cheers

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a good way to see if you have a bad acv, is at idle will the engine die on you if you do not have your handicon1.png on the throttle?

 

My nephew's 150F won't idle on it's own without either the idle screw turned fully CW or the throttle held open a bit.  I know the pilot jet is clean (just cleaned it), but I didn't bother checking the ACV or cleaning the associated passages.  The bike also runs a little rough when cracking open the throttle.  Would a bad/clogged ACV cause this as well?

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The ACV is good to have.  I have seen so many posts over the years discussing this poor misunderstood valve.  Many say disable it, it is purely for emissions, etc.  The sole reason is for fuel enrichening at closed throttle decel to quell the lean popping in the exhaust.  If you disable it and try to eliminate popping by jetting you will be way overly rich on your idle jet and have many other issues such as hard restarts and poor idle etc.  I would think it actually is counter to emissions due to the fact that it introduces more fuel yet when the engine is not really in a position to burn it properly.  It is merely a very good way to get a lean enough to be functional idle mixture and magically rich enough decel to not pop.  Win, win I think.

 

If the valve fails it does not affect idle mixture, it just merely does not activate and you get decell popping.  I know this because mine did fail and I could definitely hear the popping but all else was fine.  It was on the inside of the left carb and hard to get at so I was reluctant to just replace it.  I finally did, bought a diagphram online and stuck it in.  Decell popping wasn't totally eliminated, but very much improved. 

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Davek I respectfully to disagree with your comment that by eliminating the air cutoff valve would lead to a overly rich pilot circuit.  To eliminate the air cutoff valve you would not only increase the pilot jet but also the needle would have to be recalibrated.   Meeting strict emission control laws requires engine manufacturers to run at 14.7 AFR which gives the best emissions.  At that AFR the engine is running lean. If emissions were not an issue and manufacturers tuned their engines for max power performance than engines would be tuned anywhere from 12.5 to 13.8 for maximum power.  At those AFRs you'll not have any problems with idling and starting but if you only changed the pilot jet to compensate for the removal of the air cutoff valve to cure decel popping than for sure you could run into the problems you mentioned.   Too many people go this route without touching the needle and run into problems.  The FCRMX carb that I use is properly tuned for max power , doesn't pop and decel and starts with no problem and it doesn't have a air cutoff valve. 

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D0T, I can't agree more.  A carb can be set up to work well without the ACV, but that rarely happens.  Someone will read on some hairbrained post that you get 30 more horsepower by eliminating this or modifying that and slapping on a K&N sticker and that is ALL they do, and then bitch when it doesn't work.  It takes perseverance and thought and knowledge of theory to do it right and most people will lack one or more of those traits.  And many carbs, like mine do not have a full range of adjustments on air jets, needles, accelerator pump travel and leak, etc.

 

Bikes have to meet some kind of emission standards these  days and can't be fine tuned to the exact mixture for all altitudes and situations so some "cheating" is required.  I have found on my bike, the closer I get to ideal jetting, the finickier it gets, or maybe the the more intolerant I get.  I change may mainjets with the seasons since I can feel the difference in the way it runs.  Part of that too is from experience gained and knowing what it can do and being intolerant of it not doing that.

 

I one time reset my carbs back to near stock configuration with jetting corrected to my altitude to get a baseline and see if I really was improving things.  The results were eye opening in the fact that the bike ran and started very well, was nearly unflappable in any instance.  The downside was that it was so docile and not nearly as fun, throttle response was lethargic, hard to loft the wheel at will, no surprises at all.  In other words boring.  I can understand why people jettison CV carbs for the same reason, I did on my KLX.

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Depending on how the ACV is disabled, determines what, if any jetting changes need to be made. The only down side to ACV is if it malfunctions, you could have a hard to set fuel screw. Though that tells me the bike needs servicing and not hacking.

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