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Revs spiking whgile at constant speed

Discussion in 'Technical and Troubleshooting Torque' started by Cheeba, Nov 26, 2008.

  1. Hi all,

    I foolishly put some e10 in my bike the other day, have just pretty much drained that so filled it with a 95RON unleaded as I should have done all along. Sitting at a constant speed, say 80k's and the RPMs are spiking, sometimes as much as from 4k to 5k - there is nothing happening with the engine, no surges and the bike is sitting nicely at those constant without any variance. Any ideas what may be happening or if it is even something worth worrying about? I am only assuming that it may be fuel related.

    Cheers :)

  2. My crystal ball is broken and there are so many things you have not told us I give up.
    Lets start with what bike are you talking about then go on along the lines of general mechanical condition................

    Some of us may even want to know if you have a hot sister :p
  3. Please tell us more about your hot sister. Photos, vital statistics, price list etc.

    Also, if the RPM is fluctuating but the speed is remaining constant, it sounds like you might have issues with a slipping clutch.
  4. These statements do not compute. Do you mean that the rev counter is shooting up, but the engine sound like it is at a steady speed? Or is the engine suddenly revving, but the bike not picking up speed?

    The first is tacho cable, the second clutch.
  5. :LOL:

    Thanks Guys. I do have sister, but she lives in the UK - single with her own house so if any of you are rich and travel a lot internationally, PM me with pics so I can forward to her. She is quite selective though ;)

    The bike is a 650 Sachs Roadster and in pretty good condition. I am awaiting a part for the carb so in the interim it has two vacuum pipes going to one aperture at the moment but this has been the case for the past month with no noticeable ill-effects; it runs really well, picks up perfectly on acceleration in all gears etc and clutch is biting as it should, doesn't feel sloppy.

    So yes, the rev counter is spiking slightly and erratically but the engine isn't racing or slipping or accelerating - if I couldn't see the rev counter, then I would be unaware of an issue.

    Hopefully this is more informative.

    Oh, as an addendum - she does have a 9yr old boy :)

    (who is extremely cool)
  6. Battery terminal issue?
  7. Well then... if it's a cable drive to the rev counter, then it's most likely that the inner cable is about to snap/unwind. If it's an electrical wire to the counter, then it is most likely a bad connection somewhere, it's earthing where it shouldn't (insulation worn through and wire touching something?) or some other component is causing the spike. I suppose it's also theoretically possible the tacho itself is on the way out.
    You'll need to look at what kind of fittings go into the back of the instruments and trace it from there.
  8. Some bikes have swinging needles and flashing lights in the gauge cluster as an identifier for other problems, as detected by the ecu. See if your user manual mentions this.
  9. Bad Tacho earth connection, I reckon.
  10. Loz & Chris ...

  11. Electrical. the same thing happened to my old gs500. check your terminals and connections.
  12. Well it must have just been having a bad couple of days, or it loves the rain as it has been fine for the past few days now. Unless it is going to be one of those problems that are irritating intermittent head doers ;)
  13. It won't have gone away.
  14. I know, wishful thinking really. I'll get stuck in and have a proper look at the weekend as it was going nuts earlier then fine for the second half of the trip.
  15. Tachos are fed by 3 wires only. The earth wire is going to cause your problem.
  16. If it went away in the wet it sounds like a loose connector .
  17. make sure that earth is tightened
  18. Well. As improbable as it may sound after a bit of hypothesis testing I have got to the bottom of this problem.

    Fuel type.

    After repeatedly checking the connections and wiring and finding no visible problem; I had a lucid flash of enlightenment. I had been running the bike on 95 Premium and the time it had stopped I thought may have been when I accidentally put standard unleaded in so when I needed a refill - I put in normal unleaded and sure enough the tacho sat solidly after about 15kms into the new tank, not even the slightest twitch. I thought this was too good to be true and expected it to falter again the next day but it didn't - ran it on 95 again when the tank was empty and sure enough the tremors were back! Now running on unleaded again for a week and the problem is gone so in mythbuster's jargon - fuel type leading to tacho shakes; confirmed.
  19. If the problem is how I understand it (absolutely NO change in engine speed, only fluctuation in the indicated engine speed on tacho) then fuel type has precisely NOTHING to do with the problem.

    You have an intermittent electrical problem which is coming and going.
  20. I agree . Fuel has nothing to do with it, but you can believe whatever you like.