Mikuni HSR42 will not idle

[ekucolonel17]

So for a quick update. I put a new manifold seal on, new seals on the head side. I then put the cv back on so it would idle and I checked for leaks spraying with wd40. Put the mikuni back on and same thing as before. Small bump of throttle and it’ll start, shoot to 4000ish, start to die off, another bump of throttle and then it will hang idle at 4000 until I shut it off.

To who asked about the cables: I routed around like the mikuni manual says and extended them to give correct free play. I’ve also tested without cables connected at all.

[Iron Mike]

Air leak from somewhere other than manifold seals. Does the HSR have a vac port on the body somewhere that’s exposed?

[rocketmangb]

Vacuum port open ?
Used carb ?
Now there is a seal on the slide I guess it could leak

[sungod]

Time to pull the carb and start over.Inspect and clean every passage, jet and check float level.You have eliminated air leaks, cables and fuel delivery(cv works). Use only Mikuni factory parts for the hsr42.I suggest a close inspection of the throttle shaft and slide to make sure the slide is not binding. Make sure the spring is returning the slide from full open to idle.With the idle screw backed off the slide should be closed.

[ekucolonel17]

So, I finally figured out what was causing the high idle. I found a crack in the top cap by one of the screws. I’ll have to find a new one somewhere online. Just another reason to be careful when buying used. Thanks for the suggestions along the way.

[rocketmangb]

Didnt think of that !