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carb issues?

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1986 xr200r, sweet bike

 

  so ive had an issue almost since i got the bike, it idles very strangely, the carb is brand new. so what happens is you start the bike and the rpms are pretty high, mostly because its cold, then after it warms up enough the rpms come down and it almost immediately dies, the rpms shoot down too fast as if its out of gas or something. start it up again rev it a little to keep it from dieing right? i give it gas and the rpms shoot up and stay there for like 8sec then they come down as if its going to die give it gas and it does that again, stays at high rpms from 4-25 seconds, it really varies sometimes, some times it stays there and i just kill it...

 

   ive checked the gas flow and when the petcock, (also new) is on the "on" position the gas just drips out, but when on "reserve" the gas comes out in a consistent stream, that helped the problem a little, but still wont hold an idle... sometimes it holds one for a few seconds-possibly a minute, and you can hear it almost miss firing and trying to quit, but when you ride it it has all sorts of power, dosent really bog down at all, pretty snappy throttle otherwise.

 

ive got a few ideas, cracked manifold, manifold gasket, *incorrect carb float level*, incorrect carb tuning (although ive messed with it so many times and nothing really changes except how long the bike stays at high rpms), maybe a bad head gasket, incorrect timing.

Edited by Tubs26

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I think you already have answered your own question, probably in order of likelihood. I'd go after that carb boot with some carb spray or something safe for what will get wet, with it idling as low as it will and listen for an rpm change.

Another possibility may be exhaust leak, but in my experience, that usually causes more minor idle issues. Ymmv.

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I havent tried it, but someone on xt500.co suggested a coat of vasoline on the boots as a test.

Sounds pretty kinky.

Dried throttle shaft seals will ruin your day if it has a butterfly type valve, as on a CV, BST and some others.

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Have not done the throttle cable yet, how do I do that? It's a butterfly carb. Also I ordered a service manual and it came and it's very handy but I did a compression test and my results came out to 90-92psi dry and the manual says it's suppose to be at 199psi. I'll do a leak down test to see where I'm loosing my psi...

Also my decompression lever/switch is not connected and when the bike is running you can hear some sort of air escaping from that area. Maybe it's time for a rebuild...

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Sounds like the decomp is likely the cause of the compression issue. Thats the type that holds the valves open, right? Not like the two stroke, where decomp is its own valve on top of the head.

If its leaked for a long time, holding the ex valve slightly, i think it will need a valve job. Unless the leak is through the linkage or something.

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