Just put a big bore kit on a Yamaha Jog (50cc to 70cc) and am getting ~115psi compression. It will reach full rpm on the stand but on the road it won't pass mid throttle and makes a hollow/rattling sound. What could cause this?

  • Did you make the carb adjustments or modifications since you motor now requirires more fuel? – mikes Oct 3 '14 at 9:22
  • It previously had a big bore kit where the wrist pin clip came off and wrecked the piston and cylinder. It has a large muffler too, so has had a larger main jet put in. Have tried playing with the needle clip position with barely any change. It apparently ran very well before it died the first time – Jerry Oct 3 '14 at 12:52

Agree with @mikes ... your issue is lack of fuel. That "hollow/rattling" noise is pre-ignition/knocking caused by a lean fuel condition. If you keep trying to run your scooter like this, you will blow the piston out of it (if you haven't caused damage already). You'll probably need to rejet the carb and get more fuel into the system. The reason why it doesn't do this when on the stand is that the engine isn't under a tremendous load. It is probably doing it, but the noise is so faint you don't hear it. I would bet the manufacturer of the "big bore kit" has recommendations as to what size jets should be installed and how to do it.

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