Idle speed or something stuck

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    Ok so the z750 is back on the road and now has a working starter motor.

    Today I rode it the 20 miles or so into London to work, all was good until about a mile from work and the revs went mad, they were sitting at 4k when the throttle was closed. I limped to work by some means, and hone again (which was painful in traffic).

    So what is the issue:

    Accelerator stuck on?

    Cable has somehow overstretched?

    Maybe air getting in?

    Something else?

    Thoughts: should I try adjusting r


    Adjusting revs? Although throttle moves a good centimeter before revs change! Has cable stretched or needs lubricating or adjusting.

    Any thoughts welcome, tomorrow I’m going to try turning the screw above the choke or the assumption it may be the idle speed screw!


    Hi Andy

    If your not sure how to attack this and what all the screws do hang fire. You need to systematically go through everything one step at a time.

    If I remember correctly you are running a B1 with BS38 carbs. Have you got the airbox fitted?

    From the symptoms you are describing I very much doubt it is the cables. I also doubt that adjusting the tickover screw will solve the problem either issue either. The screw you are referring to is one of the mixture screws and again is unlikely to be your problem so don’t touch it yet.

    The first thing to do is make sure the carbs are fitted correctly. Check the cables are routed and adjusted properly. The throttle should turn pretty freely and snap shut when released without hesitation. When shut it should be on the stops and there should be no tension on either cable. There will be a very small amount of play in each cable, app 5 degrees of nothing when you turn the throttle just using your finger and thumb.

    As you have the bike running the next thing to check is for air leaks on the carbs AND exhausts. Although not quite as critical if the exhausts are leaking you will hear and even feel it. The most important area for leaks is at the head. Sort this out if there is a problem.

    Next check for air leaks on the carbs. This is quite simple to do. Start the engine and let her tick over. Put your hand on each carb and with mild force try to rock them up and down. If the revs alter then you have an air leak on the rubbers, most likely to be the carb to head rubbers. If this happens check the rubbers are screwed tight to the head and that the clamps are tight enough. Do NOT overtighten the clamps as this can make them go oval and subsequently make the problem worse.

    Don’t worry about any change to the running while doing this as we can address that later. Let me know how you get on and we can then move on to the next stage.




    Will give the carbs a play with, I defo have a leaking exhaust, was thinking at some point to replace that.
    One thing I did notice tonight was that the petrol cap wasn’t closed properly when I had topped up yesterday, was covered by tank bag so didn’t notice it yesterday. Could that be causing the issue?
    I closed it properly and then popped out on bike for 20 mins and no issue with 4k revs etc.
    Report more next week

    • This reply was modified 1 month, 3 weeks ago by Kaptainkwak.

    God knows what happened to formatting above, and you cant edit 🙁


    I can and did lol. Not sure what happened either, I only know basics.

    Regarding the fuel cap I doubt that is the problem. The cap is there for only 2 reasons, to stop fuel slopping out and sh*t getting in. It does not seal 100% so as when the petrol is used the space is replaced with air.

    It is sounding heat related. As the engine and other parts warm up they expand. When this happens existing gaps can increase and new gaps can appear. Sort out the leaking exhaust and check all the carb rubber connections. Also do the carb rubber test as this will check for leaks and perished rubbers.



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