Page 1 of 2 12 LastLast
Results 1 to 16 of 28

Thread: Bike going slow then fast randomly?

  1. #1
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts

    Bike going slow then fast randomly?

    I bought a second hand bike on the weekend, Yamaha SR 125.

    I have a problem with it that i cant seem to solve myself. Basically sometimes it rides like a dog, and then sometimes it is really fast and i can accelerate quickly away and it seems quite powerful (like my mates SR). I have also had it ride on a dual carriage way like a dog then just kick in like someone is pushing me and it speeds up without any extra throttle, going fast again.
    It seems quite random and isnt just when its hot or cold. Any ideas?

    I have cleaned the carb, and bought a new plug, however both the new and old plug seem to have quite a weak spark as far as im concerned.

    The haynes manual says that timing could be a problem with weak acceleration but wouldnt that be a permanent thing unlike this?

  2. #2
    Now with added sobriety Rave's Avatar
    Join Date
    Jul 2003
    Location
    SE London
    Posts
    9,948
    Thanks
    501
    Thanked
    399 times in 255 posts
    Have you checked the fuel lines to the carb? Might have a blockage. It sounds like a fuel starvation problem; if it's not the fuel lines did you check that the float and float needle in the carb can move smoothly?

    Rich :¬)

  3. #3
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    Checked the fuel lines and the float moves well.

  4. #4
    Hexus.Jet TeePee's Avatar
    Join Date
    Jul 2003
    Location
    Gallup, NM
    Posts
    5,367
    Thanks
    131
    Thanked
    746 times in 442 posts
    Carb Icing? Some Silkolene Pro FST fuel additive would help, give it a try and see if it cures it. Long term a carb heater is the solution.

    Sticking fuel tap? Maybe it's gunged up with something like:

    Corrosion? Inside the tank. Have a look inside to see if the inside is rusty. Fixing this is possible with a rust treatment.

    If you suspect the weak spark is definitely the cause, first thing to try is a new battery.

  5. #5
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    I have two plugs, both with a weak spark. I also tested the voltage on the battery, it didnt come out like the manual says, it was
    12.4v with engine on, the book says 14?
    Could the bad spark produce these problems, and is it worth me getting a new coil / ht / regulator - rectifier to try and solve that?
    Bear in mind there is a problem with the lights that is also weird, i rev up and they dim. Bit backwards? I am yet to try that when its running well.

    If i take the fuel pipe off the carb the fuel flows out well, what is carb icing?

  6. #6
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    I doubt there is ice in the carb, as it isnt that cold here and when it take it apart its nice and wet, no ice.

  7. #7
    Hexus.Jet TeePee's Avatar
    Join Date
    Jul 2003
    Location
    Gallup, NM
    Posts
    5,367
    Thanks
    131
    Thanked
    746 times in 442 posts
    The low voltage does suggest the reg/rectifier is worn, but I doubt this would be the sole cause of your problems.

    Carb icing is caused by the temperature drop in the carbs caused by the fuel vapourising (Which can be as much as 30C below ambient) causing moisture in the air to freeze.

  8. #8
    Member
    Join Date
    Oct 2004
    Location
    Hatfield,Hertfordshire, England
    Posts
    61
    Thanks
    0
    Thanked
    0 times in 0 posts
    Have you checked the air filter? That can cause problems.

  9. #9
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    We retested the voltage and its fine. We have checked all the simple things like air filter, weve been at it for 3 hours today. It sounds choked like its the exhaust but sometimes becomes unchoked and revs nice.

  10. #10
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    There was some rattling coming from inside the exhaust but when we took it off it didnt seem to do it.
    I have a fuel filter and there is no muck in the carb.
    Coil has been changed by previous owner and is not the standard SR one from the looks of it. While we were fixing it, it just revved really high and nice one time which it does when it is working right. Then it went back to being bad. Its liek you have your hand over the exhaust. We loosened the bolts and it performed the same, im going to clean it later. Its not a carb problem im pretty certain. Spark is weak too.
    The voltage seems to be ok now, i measured it again and its like 13.8v

  11. #11
    Hexus.Jet TeePee's Avatar
    Join Date
    Jul 2003
    Location
    Gallup, NM
    Posts
    5,367
    Thanks
    131
    Thanked
    746 times in 442 posts
    Sticking power valve?

    Some 125's have power valves designed to restrict the exhaust at low RPM for smooth running, and open at higher revs. Don't know where it is or if you have one though.

  12. #12
    Senior Member
    Join Date
    Apr 2004
    Location
    Darlington, UK
    Posts
    245
    Thanks
    0
    Thanked
    0 times in 0 posts
    Im pretty sure thats only on 2 strokes, mines a 4.

  13. #13
    Hexus.Jet TeePee's Avatar
    Join Date
    Jul 2003
    Location
    Gallup, NM
    Posts
    5,367
    Thanks
    131
    Thanked
    746 times in 442 posts
    Indeed, it is only on two strokes. Not sure what an SR125 was!

  14. #14
    Oh no!I've re-dorkalated! Jiff Lemon's Avatar
    Join Date
    Jul 2003
    Location
    Sunny MK
    Posts
    2,504
    Thanks
    80
    Thanked
    44 times in 41 posts
    Sounds like an Air leak to me. Get yourself a peice of tubing and listen for Hissing around the inlet manfold/carb gaskets.

  15. #15
    Registered User
    Join Date
    May 2006
    Posts
    7
    Thanks
    0
    Thanked
    0 times in 0 posts
    Hi i have a sr125 and mine does the same as yours no power then loads of power i have done all the same things as you have any more help????????? Regards Dave

  16. #16
    Registered User
    Join Date
    May 2006
    Posts
    7
    Thanks
    0
    Thanked
    0 times in 0 posts

    sr 125

    i have the same bike and mine does the same any help anyone?????????

Page 1 of 2 12 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. need help with choosing a bike.
    By mt50 in forum Automotive
    Replies: 12
    Last Post: 27-08-2004, 04:12 PM
  2. New bike
    By ekundo in forum Automotive
    Replies: 3
    Last Post: 14-06-2004, 11:40 AM
  3. Metal children slow motorists
    By Basher in forum General Discussion
    Replies: 18
    Last Post: 23-07-2003, 03:51 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •