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Erratic idle 1997 XR400R

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I broke down and bought a decent condition four stroke to follow my son around on. My beloved 1987 CR500 was not working for a chase bike lol, and I was tired of working my old RM250, I thought hey, an old XR400R would be perfect and nearly zero maintenance, so I found an old one on CL, gave it a 5 min test ride and off I went. After about two rides it started a weird idle issue. I am having a really hard time figuring out the issue, any ideas would help??? My sons xr50 is ready but dad is failing...

Bike started a bit hard, and would idle really high after you let off throttle from 1/4 throttle softly. If you popped the throttle it would then drop down and idle so low that if I wasn't moving in gear pushing the engine over it would die. I suspected an air leak or dirty carb, so I ordered a rebuild kit and rebuilt the carb. Same issue, no change. So I replaced the oring on the manifold and same issue, then I noticed a crack in the manifold and thought woohoo I found the issue!! I ordered a new manifold and an OEM oring. I put it all together and same thing! I can hear an air leak (suction) but cannot figure out where in the world it's coming from. I checked the carb side of the manifold as well, and dissasmbled it to make sure everything was seated right, exact same issue. 

Rebuilt carb, new manifold, new oring, still sucking air and idling so erratic that is dangerous to ride, completely unpredictable. One turn it will idle high and push through and the next it will fall on its face and nearly die.

Any of you four stroke guru's have suggestions? 

 

Thanks, 

J.D. 

 

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If you actually have an air leak, you can find it by spraying carb cleaner all over the carb area. When it dies, you found it.

...but more likely, you have a lean condition due to one or more:  a corroded pilot jet passage, a bad fuel screw oring, a poorly adjusted fuel screw, a worn out emulsion tube, or a worn out slide and throttle body.

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