After all the usual engine mods to your Kymco 50cc, you'll find that top speed hasn't changed much because speed is limited by the final gear ratio. The biggest speed increase will come from using a high-speed gear set. Chinese qmb139 gear sets are cheap and excellent, yielding about a 25% speed increase, but Kymco (as usual) has tossed a spanner in the works. The qmb139 gear set ALMOST works out of the box, but one gear doesn't fit. Normally, you'd be hooped at this point, but fortunately there's a cheap fix. Just buy a stock Chinese gear cluster ($29) for any Chinese scoot like a Lifan or Benzhou, extract the needed gear for your Kymco, and you're done. The gear press to dis-assemble/reassemble is found in any machine shop or transmission repair shop and it only takes a few minutes to dis-assemble/reassemble the cluster. Your Kymco 50cc with a 50mm BBK will now cruise at 70-80k/hr instead of 55-60k/hr!
In the photo, the Kymco gear to be re-used can't be re-used because Kymco made the bore diameter too small (17mm). You need to find a stock Chinese gear instead with a 19mm bore (Sorry I couldn't upload the rest of the photos - there's a file size limit which doesn't appear to work correctly)
