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Over Reving

Discussion in 'Bling and Appearance' started by toe, Nov 15, 2007.

  1. Hi

    I just bought a 1979 XS 250 off ebay, have used it for 2 weeks now.

    For some reason it just starts to over rev up to 3000 rpm's.

    I only use it for work and travel maybe 3 k's max each way, so it proly just gets to normal operating temp.

    Would it be a dirty carby problem or something else?


  2. First thing I would check is the cable route from the bar to the carb, it could possibly caught or kinked.
  3. Wiil do but i dont think it will be that as it will do it after ive been siting at a set of lights for a minute or so.
  4. Is the throttle cable a 2 way one or does the carb rely on a spring return? I work on cars for a living, but do bikes have a idle up valve for cold mornings?
  5. Have you turned the choke off (if you have one)? Try a bit of WD-40 around the cable ends, levers, throttle, outside the carby...
  6. Sounds like you might have an air leak downstream of the carbs. Check any vacuum hoses (particularly any from the carbs to the vacuum fuel tap if it's got one), caps and plugs to make sure everything's there/intact.

    On a (nearly) thirty year old bike, it's quite possible that the rubber inlet manifolds have turned into something with all the flexibility and sealing properties of fossilised elephant scrotums (scrota? scrotii?). If that's the case, replacement is the only long-term solution, although a couple of turns of plastic duct tape might effect a temporary repair.

    And if you haven't already done so, change the oil and filter. It won't help the erratic idle but your camshaft will thank you. Old Jap engines like lots of clean, fresh oil.
  7. Checked all the above, made some minor adjustments to the idle and syncronization screw and now it wont start at all :(

    I also took the air filters off and could see the carby slides.. i thought they should move up then back down when working the throttle?
    Cause they didnt move at all.
  8. i had the opposite problem.
    i bought a CBR250RR and i think i bumped something when i was cleaning it because the stupid thing wouldnt idle.
    had a fiddle with the idling screw and it was idling at about 10k rpm :p
    didnt stay like that for too long though.
    to turn up the idling, from memory you twist it towards the front of the bike, turn to the rear to decrease it .... from memory

    hope it helps

  9. Oh. Not content, hey?? :)

    U have vacuum/diaphragm carbs - there is a largeish inverted silver plate-like device on the top of each carb, and the throttle cable connects to a lever on the side, correct?

    That is why you don't see anything move up and down - the slides only move when the engine is running...in response to manifold vacuum operating on a diaphragm inside the plate-like device. The diaphragm is attached to the top of the carb slide.

    If you have messed up the idle the choke will also possibly not work properly. If the throttle slides are open tooo far at idle the choke will be bypassed, if it is a plunger device.

    If the idle has been dropped back too far then the slides might not be open enough to let the engine start...you never adjust idle or synchronisation! with the engine off (unless you know what u r doing

    All the best

    Trevor G
  10. a mate has sorted it out for me.

    Fuel was leaking into the vacum hose via the fuel tap.

    It has a small hose running from the fuel tap the the inlet manifold.

    New chain and spockets, oil and filter, plugs, rear brake pads, tuneup, new fuel tap $400 includes labour :grin: :grin: :grin: :grin:

    ps & roadworthy