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Stiff throttle when engine is running

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So I recently acquired a 1994 XR600R. Throttle action is nice and smooth and easy when the engine is not running. But as soon as you start it up the throttle becomes stiff making low speed riding snatchy and quite fatiguing on the wrist. At first I assumed it was the cables or twist grip needing attention but I then noticed that it is only bad when the engine is running. Its as if the vacuum pulled by then engine is is causing the slide to stick in the bore. Never encountered this before and I'm not that familiar with carbs so I thought I'd seek counsel here before I open it up. Any suggestions? Here's a pic of the carb :

carb.jpg

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If the carb slide/ bore is worn vacuum can cause it to stick. Try heart_of_darkness' suggestion and see if that helps. Could just be cruddy inside, but probably not if it's only when running.

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I saw a fix for this somewhere- filing a groove somewhere I think, or drilling a hole, for when it's clean and not dinged and just worn out and you want to get some more life out of it.

Edited by XR650L_Dave

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I've opened the carb and it all looks really clean. The slide looks in good condition, shiny and no apparent wear or scoring. The slide is silky smooth to operate. The bike only has 9000 km on it so apart from cosmetic blemishes, it's all really tight. I've ordered new throttle cables just for peace of mind. The push cable was missing, is it really necessary and can I leave it off?

 

As you can see in the pictures another issue I have now is that the rubber boot has come away from the plastic flange that bolts to the intake port on the head. 

 

 

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If that is a stock choke plate you are going to want to replace/remove it while you have that thing off but I think the original has a hole in it so maybe aftermarket?

Strike that... the replacements have the hole. My plate is off :)

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that is the OE choke plate,they are noted for breaking off the lower part in pieces and sucking it into the combustion chamber..you can guess what the result is..

 

XR`S ONLY sells a replacement plate that will not nuke your top end..

 

B

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Thanks for the heads up brianhare. Looked on XRs Only but they want $47 shipping to send it to the UK!

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On the OE choke plate, is it the little sprung flap that comes off and ends up in the engine, or does the whole plate come loose?

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I'd recommend using the push cable.  A stuck open throttle is scary.  When installing new cables, lube them thoroughly.  Also disassemble and lube the throttle tube and housing.

Is that cracked boot causing an air leak?  If so, the boot should be replaced.  The air leak will cause poor engine running and is admitting unfiltered air.

The external spring which closes the throttle is fairly stiff from the factory.  You can ove the resting position of one end of the spring to reduce its tension.  But if your throttle is only stiff when the engine is running, that's not your problem.

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