strange problem

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    Hi guys,

    I am having an issue with my stock M bike.
    It starts up and runs fine for about 5 minutes. However, once the engine is at temperature it begins to idle strangely then it either dies or the revs stick at 5000rpm.
    Any ideas what is going on?


    Hi Jett,

    just my thoughts:

    1) You are aware of how to use the choke correctly ? (Silly question, sorry for that ;))

    2) Engine off, ignition cover removed: Check the timing advancer of the ignition. Does it move freely approx 15° when turned by hand ? Turn it from stop to stop. Does it stick in any of these positions ?

    3) Check tightness of intake boots (false air intake?). Turn engine on and let it idle. Spray some “starter spray” or just simple brake cleaner spray onto the intake boots (between engine and carbs). Are the revs still constant or are they increasing rapidly ?
    This test will work at any engine condition: running on choke or not, engine cold or hot, doesn’t matter.

    4) Have you tried to run the bike with the fuelcock in “PRI” position ?

    5) Have you tried to adjust the “idle screw” ? Does that help or is it getting even more worse ?

    6) Is your Twin in regular use or may there be an issue with the carbs ?
    Dirt, debris, resin ?
    It doesn’t sound like a typical carb issue to me.

    7) But it might be an carb issue !
    If there is resin in the carbs (typical after long time of “parked standby”), the idle system is the first victim.
    Your bike may start with the help of the choke to enrich the fuel/air mixture. Idle system is blocked, so idle speed is set to run on main system (5.000rpm). As long as the engine is cold and you run it with choke it may work a kind of “correctly”.
    As engine warms up and the choke is disabled, the drama as you described it starts. May run at high revs, but stalls below a certain “rev number” and can’t be rescued by pulling the throttle cable beyond that point. That would be typical for a blocked idle system.
    Might be the culprit ?

    Looking forward to your answers.

    Cheers, Michael


    Hi Jet

    I had a similar problem which turned out tof be an air leak on the carburettor side. Check the condition of the rubbers on both sides of the carbs but particularly the carb to head rubbers. They do perish and the cracks can open up and leak. An easy way to see if this is the issue is to push the carbs towards the engine when it is running and see if the revs change/stabilise.

    As far as the mixture screws are concerned on the CSR carbs the is a plug covering up the screw. If that is in place then the factory settings should still be there and therefore remove them from suspicion.

    I replaced my carb to head rubbers and the problem was solved.

    There is also the possibility of the choke not doing its job properly.

    Let us know how you get on


    Thanks so much guys. I will work through these solutions and let you know how i go.


    Michael, would it make a difference if i just ran the bike with the petcock on the PRIME position?



    Hi Jett,

    to set to PRIME eliminates any fuel flow issues related to the vacuum line and membrane in the petcock.

    It is easy to do and if nothing changes, then one of the possible culprits is excluded and we are coming closer to the real cause for that issue.

    Cheers, Michael

    PS: Don’t park the bike with petcock in PRI position.
    Always set back to either ON or RES.

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