I have a 2016 Sym Symphony ST 125cc automatic scooter which I bought second-hand from eBay a few months back. Having never previously owned a scooter I decided to get it serviced by a Sym approved garage before using it. Since then it’s mostly been OK except that on some days I have trouble with it being very jerky at low speeds when using light throttle input and this can make it very difficult to control. I’m guessing that either the clutch or the belt is sometimes slipping but I’m not sure.

I took it back to the the garage and they looked at it again and could find nothing wrong. But on that particular day it was acting fine so I couldn’t demonstrate the issue to them. They did say they would give the transmission a quick clean though

Maybe it was a coincidence but the next week it seemed to be better than ever with no jerkiness at all. I thought that maybe the problem was cured but today after not having used it for a few days due to rain it was just as bad as before.

As a side note, I have also found that sometimes the bike is hard to start and needs several attempts yet other times it starts instantly. I always start in on the centre stand and have not noticed the back wheel spinning at idle revs or anything like that though.

Is there anything I can easily check to determine the cause of the problem?

  • Did they have to change or replace anything when they serviced it initially? Jun 26, 2019 at 16:35
  • I’m pretty sure the invoice had itemised charges for a spark plug, and oil and that was about it
    – James
    Jun 26, 2019 at 17:36
  • Do you notice any difference when the motor is warm vs cold? And have you checked the trans fluid levels? Jun 26, 2019 at 17:43
  • I haven’t really noticed it being better or worse depending on whether it’s been running for a while or not. I don’t actually know how to check the transmission fluid but I would hope the garage would have done this.
    – James
    Jun 26, 2019 at 18:15
  • I'm sure they would, but it's always worth double checking simple things. Sometimes I'll change shops forget to reinstall the oil drain plug... That's all. Jun 27, 2019 at 15:30

2 Answers 2


This should have some of the information you could need. Sym service manual

Page 14 has transmission fluid checking

  • I'll see if I can get some new oil first - to check the level I need to drain it anyway so might as well change it in case they used the wrong grade or something.
    – James
    Jun 27, 2019 at 16:10
  • Totally. There's also gear oil at the rear I think so you can check/replace that too. I'm not familiar with scooters specifically, but I know that generally the smaller and lighter the machine gets, the twitchier it reacts to small problems. Jun 27, 2019 at 17:10

Just an update but the problem was eventually resolved. I took it to a garage and they cleaned parts of the transmission which reduced the slipping. Then once the scooter had been ridden for a while the issue disappeared completely - I guess the clutch surfaces just needed to wear down a bit more to remove whatever contaminants were present.

The starting issues were unrelated and were due to a vacuum leak

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