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CB250 acts weird at low revs

Discussion in 'Maintenance and Servicing' at netrider.net.au started by dexter, Jan 26, 2016.

  1. I have had my first ride on a recently purchased 2003 CB250 and it seems to hunt, pull and disengage at low revs. Is this something I need to adjust in the carbie?

  2. Welcome to the site Dexter, when you get a chance post an introduction thread for yourself in the Welcome lounge it is the custom around here.

    Regarding your issue:

    What are the idle Revs?
    What are the Revs when you experience these issues?

    Maybe inspect the spark plugs to see if you are getting a clean burn as a diagnostic option.
  3. It is a single carb bike so should be reasonably easy to tune carb but before you do you need to make sure everything else is right, Timing plug gap etc.
  5. Dexter, Engines have a range of revs within which they make good power and torque.

    Above this range, they make a lot of noise and do not deliver much, if any, more power and the amount of torque may even be reduced. They might go bang and stop working altogether at this end of the spectrum. That's what the red line is for. Wo give you an idea of when enough is enough. Stop there, or even before it if you care about your engine.

    At the low end of the rev range, engines do not make much power, but might still have some torque. People who ride their engines into this low range are said to be "lugging" their engine and as throttle openings can deliver more fuel than the engine can burn, engines often will baulk and this is amplified by "chain snatch" in which the engine might misfire and the slack in the chain is jerked. This is not good for your driveline or your engine. Getting down into this chug zone as a matter of habit is also high wear territory.

    Most engines idle somewhere between 700 and 1200 revs. Attempting to ride your bike at an idle engine speed is not a good idea. They don't make much power at this speed. Without defining a minimum revs, as this can vary quite a lot from bike to bike, the lowest revs you should operate your bike at is one which will allow your engine to appear to be operating smoothly with your bike able to accelerate cleanly when you open the throttle. You should always have the impression that your motor is spinning freely, and not "working" way behind the input of the throttle. Different bikes vary greatly in how well they operate at really low revs, but motorcycle engines usually operate best between a third and half way to that red line. Again this is most variable across the range of motorcycle engines.

    I'm guessing a bit here, but expect that a change in how you operate the engine will have a greater effect than a carburettor adjustment. Anyway, before you go looking for a mechanical issue in explanation for what you have observed, ensure that you have explored the possibility that your throttle technique may be at fault.