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By jamesbowers
Hello all.

If you are reading this, then thanks for taking the time to try helping me out. Any advice or suggestions will be much appreciated.

A few weeks ago I picked up an old GY6 125cc running scooter for a few €€. It was riding ok, but was leaking oil through the 2 case halves. Took long to reach 80 km/h (49mph) on speedometer and did not go any faster. I decided to do a total rebuild on the engine incluiding bigger piston ( no case boring needed ) and bigger crankshaft. According to calculator, I should go from 125cc to 163 cc. After rebuild, I notice more power on take off but no increase in top speed and still takes long to reach the 49 mph. Still does not go faster than that

This is all that I installed:

SKF bearings and oil seals
58.5mm big bore kit
+3mm crankshaft
A10 cam
Blue racing electronic cdi box
Orange sparkplug lead + better spark plug
Free flow cone air filter direct to carburator
Re-jetted carb. Stock carb is pd24
Free flow exhaust
Belt 835/20/30. Stock was 842/20/30
Racing variator (Bigger than stock)
Racing cluth with red spring (2000rpm)
16/37 +25% Final gear combination. Stock was 13/40

I have tried different weight rollers ranging from 8g to 14g. I tried 12g sliders. I tried different jets in carb until I found the combination to not stutter or back fire. Valves are set to intake 0.3 and exhaust 0.4. Spark plug is right colour so there for not running to lean or excessive

It gets very quick to 60 km/h (37mph) but then takes long to reach the 49 mph. Rev counter is almost at 6000 rpm. Does not go higher than that. Red line starts at 10000 rpm and max is 12000 rpm

Back wheel rolls free by hand and tyres are at correct pressure

A friend of mine has a stock SYM Fiddler 125cc. Side by side he falls back on start but then easily overtakes me and leaves me behind. I tested it and it clocked a little over 105 km/h (65mph) on speedometer.

Any ideas where or why ? Any help or ideas are appreciated.

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By jdl357
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Don't think you'll get much response on this forum, but I have been tuning scooters for a hobby for few years. This forum has been so much help with anything gas scooter related-. It's website is Please check it out, you will find everything scooter related that you need. Good luck To view images REGISTER or LOGIN for full access.

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By jdl357
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First look I didn't see you said rpms didn't go past 6000, that's your problem. If that's as high as it revs there's something wrong in your CVT, varistor or belt.

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