Low power/ top speed.. How to fix?

[rivalutionist]

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