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"Air Circuit Valve" missing in my carb? What effect will it have?

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I took my carbs down to clean them out today, this is on an '86 XR600R, twin carbs. I'm missing a little spring in the primary carb (left one). It's shown on the exploded diagram, in the middle of the box #20. It fits between the cover and rubber diaphragm. The whole valve is labled as "Valve Set, Air Circuit" on the fiche. If I understand it right, this valve is there to close the air on the pilot circuit to richen the mixture while decelerating to stop the bike from popping. Is that right?

What effect will it have on the bike with this spring gone, and everything assembled as normal? I've been having intermittent problems with the bike starving for fuel when running it hard, but I've always found trash in the carbs when it starts doing that. I clean the bowls and jets out and it's fine again. I did find some old nasty looking crud above the slides and behind the valve in question today. Hopefully that will solve the issue with the trash, but other than that it runs ok. Idles fine and runs great when the jets are clean, though it did tend to pop a little on decel.

Should I worry about getting that spring, or just leave it as is? I would buy the spring, but to get it, it looks like I would have to buy the valve, cover, and spring together as a set for $48! I've seen a pair of used carbs sell for that on eBay... lol If I leave the spring out, would it make any difference if I took the diaphragm out too?

Again, the spring in question is in the middle of the #20 box.


Edited by keithluneau

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If I remember correctly, that valve is basically a controlled air leak for high vacuum conditions. Without the spring it's an air leak all the time. Usually it will not cause too much of a problem.

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Yes it will make a difference taking the diaphram out, next time you park it next to another XR it might get pregnant.

No, don't take it out, fix it or you will start a downhill slide with the bike repair and next thing you know when one exaust valve gets bad you'll just be taking that set of rockers off because you don't want to spend $48.

You didn't think repairing a motorcycle would be free when you got it did you?

If you are not going to do it proper, at least be resourcefull and not hack things up. It is after all just a lightweight spring, go to a hobby shop, local hardware, or retail hardware store, they should have something very close to what you need. The old small town hardware stores are a diamond in the ruff for that type of stuff, maybe less than fifty cents for a spring like that.

If I didn't have the money, I'd go sell some plasma a few times and then spend the $48 on a new part, I care that much:) (I don't have a pet)

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I have no idea what effect it has on a twin carb RFVC, but I do know that some opt to disable the ACV on the 650R...

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If I remember correctly, that valve is basically a controlled air leak for high vacuum conditions. Without the spring it's an air leak all the time. Usually it will not cause too much of a problem.

That might explain why it's been running in a way that makes me think it's lean... :ride:

I have no idea what effect it has on a twin carb RFVC, but I do know that some opt to disable the ACV on the 650R...

I've just done a lot of searching and reading, and it seems that most people disable them by either plugging the hole that leads to to the backside of the diaphragm, or cutting a length of plastic tube to fit inside the spring. Both of them would work the opposite of having no spring in it at all...

I might dig around and find a spring that will go in there, and plug the hole. Or just cut a plastic tube to fit behind it. Not sure how hard it would be to get the length right though. The bike is already back together, and the carbs have to be split to get to the valve again, so I'll ride it a little and see what it does after cleaning them, maybe it'll be ok. If not I'll tear it apart again...

An no, I didn't think these bikes would be free. I'm out of cash after just having rebuilt the top end. You might find this amusing too, both of the sub rockers on the exhaust valves were worn through the chrome facing. Such a shame, if it had only be one of them I could have just taken it out and saved myself about $48. Damn. Silly me went and changed them both. Ironically they were parts sitting in a local guy's shed. He donated them to the cause, cost was $0. So there, I fixed it properly, and it WAS free! :thumbsup:

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Ok, next question... How critical is it that the spring tension or "weight" be exactly what Honda intended? I read that they were set up to open at about 19 psi vacuum in the carb. I dug in some parts boxes and found two springs that look like they might fit. I have no idea how strong the stock spring was though.

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I read that they were set up to open at about 19 psi vacuum in the carb..

There you go, thats your answer.

If you have a hand vac pump like a Mighty Vac with a gauge then put in a spring, start the bike and get it warmed up, let it idle and pull a vaccum on it with the hand pump until you notice a change in the idle. When the change comes close to 19in you got the rite tension.

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I pulled the carbs off again, and found one spring out of the pile I gathered that fit the valve and the cover right. I cut it a little shorter, as it was quite long, and put it back together. I don't know if it's 100% right or not, but it sure is running a lot better, so I'll leave it alone for now. I've got to order some jets soon to fatten it up a bit, I'll see if I can't get the whole valve set then. I just hate to pay $48 to get one little spring. I wish it was a separate part number... lol

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