Low power/ top speed.. How to fix?
 

Hi everybody, I encountered a problem I haven't seen yet in my brief time working on 50cc GY6 scooters and need some advice on diagnostics/repair.

I got an old project bike to start up fine on first kick, idle fine, and run fine from idle all the way up to WOT on the stand. It even looks/sounds like both the engine and rear wheel are reaching a good max RPM at WOT on the stand.

The problem is this power can't seem to be replicated when there's a load on the bike. When I try to ride it there is very weak acceleration and almost no hill climbing ability, and it struggles its way up to only 20mph on flat ground.

My two main guesses to investigate are weak compression or a problem in the CVT since the air/fuel seems to be fine based on easy starting and proper throttle operation while it runs on the center stand (also I've cleaned and tuned the carb and replaced spark plug). I'm also a bit hesitant to diagnose it as a compression problem because I thought an engine with poor compression would have more trouble starting and running at various throttle positions.

Has anyone ever encountered/fixed this problem due to poor compression, faulty CVT operation, or any other cause? Any and all advice is appreciated!
Thanks

Two things right off the bat. Have you checked the valve clearances? A 50cc GY6 only starts out with .002 inches of clearance out of the box. It doesn't take long for those to close up. Second, look for a cracked or broken vacuum line or intake maifold. It will start fine and run good on the centerstand with a wound like that but will run like crap on the road. Been there myself. These engines are bone simple, reliable and usually are hindered by something simple.

Thanks for your reply! Didn't find any leaks in the vacuum line or the intake manifold . The exhaust valve clearance was way too high (.013) so I set them both to .005. Still having the problem. I've got a full day of diagnostics ahead of me tomorrow.

Your bike will act exactly as you describe if there is low compression.
Anything at or below 120PSI with wide open throttle during a compression test is too low.
That would be the FIRST test I would do.
Then follow up with a vacuum test IF the compression is at or over 120PSI.
Your vacuum should read at 20inches.

The issue can then be in the CVT or ignition timing if not in either of the prior tests but I suspect compression.
You can get a good compression tester from Carlisle for about 40 bucks, and it is a MUST have tool. Even more important than a BFH.