Here's a quick video showing the difference that installing a 47mm big bore kit (72cc) and A9 camshaft made on a 49cc Chinese four-stroke scooter. 0-30MPH acceleration is timed and wide open throttle cruising speeds are shown before and after the work. All modifications are listed at the end of the video.

Please note these two things. First, I am 300 pounds so acceleration will be much quicker with lighter riders and speed may also increase. Second, these are indicated speeds. I did not test with GPS, but I would expect this scooter to go at least 40-42MPH. There is always some error in stock speedometers, but this one appeared to be close.

47mm cylinder kits are commonly called 80cc kits by sellers. The larger 50mm+ kits are often advertised as 100cc. I prefer the smaller 47mm bores for reliability and longevity concerns. All modifications that increase horsepower, torque, RPM or speed are likely to have a negative affect on components due to additional stress. Larger bores may also have heavier pistons. They have been known to cause or hasten crankshaft big end bearing failures. I owned one with a brand new stock crank and 50mm BBK that ruined the connecting rod bearing and left me stranded at 2,003 miles. It seems that many people using 50mm and larger kits try to limit RPM with heavier roller weights to reduce their risk of failure.

If you are considering a big bore kit, I recommend going with a quality brand instead of the cheapest kit that you can find.

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