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GPX250 chain/sprocket/drive problem

Discussion in 'Technical and Troubleshooting Torque' started by Tenoq, Jun 5, 2005.

  1. Hey all - out on a ride today and towards the end of the day started to notice an odd problem with my GPX.

    The problem occurs (or is noticeable) at low speed, usually when accelerating from a stop. I get the feeling of a crunching/clicking feeling from the rear wheel as it accelerates (kinda feels like it's got a huge stone stuck in it, as it clicks faster as I increase speed). I don't think it's audible, but it is a very distinctive feeling.

    At first I thought the sprocket may be damaged, or their might be a dodgy link in the chain. Couldn't spot anything when I got home, tho, so unless it's a problem on the front sprocket, I'm not sure what it is. Any ideas?

    Oh, not sure if it's related, but if I crack the throttle open full, there is an odd vibration in the bike, that seems to happen at any speed/revs (more noticeable doing say, 70km/h in 5th). Related issue?

  2. Do you know how to check the wheel bearings?
  3. No idea! Any hints?
  4. The crunching noise makes me suspect the bearings. If it's really bad, you might be able to tell just by turning the wheel by hand and listening and feeling. Otherwise, With the back wheel off the ground, grab the top and bottom of the wheel and try to rock it sideways. Any movement at all (the barest discernable click) indicates a bearing problem.

    If in dount, take it to a mechanic. Any competent mechanic will check it and probably won't charge (except to fix the problem).

    Have you checked for tight spots in the chain? Check how far you can move the chain. Turn the back wheel a little bit. Repeat until whole chain has gone round. Any variation in the tightness of the chain might be down to the sprockets or the chain.

    Can you get to the front sprocket?. It doesn't sound like the problem (but it's 20 years since I had a bike with a chain...) but front sprocket problems can quickly become minor disasters.
    he vibration you feel when cracking the throttle open at 70 in fifth might just be the engine labouring in too high a gear.
  5. Turning by hand or running on the centre-stand doesn't seem to reveal the problem at all - it seems fairly smooth running. I have checked the play in the chain, and it's around 40mm (which is normal for the GPX). It doesn't really have any tight or loose spots that I can see.

    I also did go out and give the rear wheel some serious shoves to the left and right, but it's solid as a rock - no movement or clicking whatsoever. I'm thinking I might get it checked out anyway.

    Don't think I can get to the front sprocket easily.

    And... you might be right about being in too high a gear. But 70km/h in 5th is around 6500RPM, so it's not too low... and the vibration still seems to be present at higher RPM too. Perhaps it's always been there and I haven't noticed before. :)
  6. Check your tread for a lodged stone or nail/screw. The scraping as you roll over the stone could be what you describe.
  7. That was my first thought - that's what it felt like... but yeah, checked the tyre and it looks fine. Nothing obvious.

    Is there any chance this is an engine problem? Doesn't seem to occur in neutral, but just maybe...
  8. Speed related - but not engine rev related - rules out engine and transmission issues normally.

    I like moike's idea of bearings. Seems to fit the data. Try rolling the bike with a rider aboard(load the bearings).

    Check the brakes though - obstruction lodged in rotor (a loose screw?) or worn brake linings (talking down to metal on metal here - which would show up badly under braking). Should be able to check that out before pulling anything apart.
  9. I had an FZR that spat it's wheel bearings, and it did the same thing - cruched and clicked... Did you check for little tiny bits of metal - like filings all around the back wheel and sprocket?
    The other thing is the cush drive rubbers may have died. With the wheel on the ground, see if you can move the chain back and forth - any major play will indicate worn rubbers.
  10. Just tried that, it does move back and forth a little, I'd say the rear sprocket rotates 2-3mm. Is that a lot of play, or not worth worrying about?

    Didn't notice any metal or anything. Problem was a lot worse after the long ride on Sunday than it has been yesterday and today on my relatively short trips to work. Still noticeable, but if I hadn't felt it on Sunday, I would be ignoring it now... :p
  11. Im telling ya captain! Ze Gee-Peee-eX can't take it no more. :p
    Certainly gave her a workout on Sunday, likewise with my Fizzer. Although my fizzer is already thanking me for giving her a blat; idles smoother again. Seems like i just gotta get out there once in a while. :D
  12. Bah! My GPX was keeping up just fine with Alan's ZX10R. Just those damn straight bits where he seemed to get away! :p
  13. Rather embarassingly, my problem seems to have disappeared after giving the chain 'the mother of all cleans' last night. Will see how it goes.