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XR 250 won't idle

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Hi Guys.

Got a question for the Keihen experts.

I'm running std XR250 carb on a 93 XR250L project bike, slightly modded. The stock XRL carb is no good because the plastic bushing that supports the accel pump linkage fell off and there is no repair aside from custom machining.

So I bought a used XR carb and re-jetted for my engine. I should add that I rebuilt this engine and it has about 300 miles on it since the rebuild which included bore, new piston/ring, valve job, new timing chain/tensioner, and a new clutch.

I've worked out the jetting perfect for my bike and altitude - -122/38. The bike runs well, crisp throttle response, no lean surge or bog. Plug looks great. But I'm having this really irritating issue.

Damn thing won't idle. If I turn the idle knob up to where it will idle, the idle will suddenly jump to, I guess about 2k rpm, then slowly go back down. Then the bike will die. There is no sweet spot for the idle adjustment. It's either too high or too low.

Also, when the idle is adjusted as above, gunning the throttle and letting off, the engine won't wind down all the way. It takes a couple seconds to get back to idle. Like the slide isn't returning to the closed position.

Here's where it gets interesting. If I back off the idle knob all the way, and I gun it and let off, the engine immediately slows down.. no run on or whatever -- just as it should.

I've been working with this carb I got from ebay (87XR250R) and I'm beginning to think that the problem is a mechanical problem with the carb. Like the slide is worn out possibly. I've thoroughly investigated the air leak issue although I may have missed something.

Anybody ever run into this? Could it be cables? Float level? I'm seriously out of ideas and I notice a new carb like this from servicehonda is about $400 for either the XR or XRL.

This ain't my first rodeo with air/fuel ratios and used carbs. I've successfully jetted a number of Holleys on cars, as well as dirt and street bike carbs for myself and others.

But this thing is driving me crazy. :thumbsup:

Calling all Keihen experts!!

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Take the fuel screw out and make sure the tiny washer and o ring are BOTH in there.

Make sure the o ring between the intake manifold and head is in good shape.

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Thanks! The tiny washer is missing. I had it apart last night. I thought it just had the screw, spring, and o-ring.

I see where service honda sells the needlejet set for that carb which includes the washer.

Thanks and I'll try that.

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Well SH wants $9 s&h to ship me something that will fit in a matchbox. And might not fix the problem.

What about the flat slide Mikuni carb offered by XRs only? Is that a good option?

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I' believe I'd spend $30 on the new needle assembly before I'd spend $200 on another carb.

There are other places to buy besides SH. Try PowerSportsPro and Ron Ayers. There's another too that I found a reference for off this forum but I can't think of it now.

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Thanks for the input.

I guess I'm thinking new Mikuni just out of frustration with this carb.

I'm bidding on another Keihen XR250R carb on ebay. Won't hurt to have spares of little bits and pieces if I can win it.

I'll have a look at those other vendors you mentioned. One other one I've used is www.bikebandit.com

I'll follow up when I get the washer and see if that works.

Thanks again!

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I had the same problem with my stock XR - cleaned out the pilot jet (actually replaced it) and viola' no problems.

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