150cc GY6 scooter lost top speed

[sc00ter]

Is there is a chance that the tank had/still has sludge in it? The fuel system goes as follows:
Fuel tank.
Petcock.
Fuel filter.
Carb.
Related jets.
Unless the tank has rust in it I doubt that anything slipped past the fuel filter and got into the carb. I bought a Zuma that sat for so long there was a hard sheen inside the fuel tank! I took it to work and flushed it out with a industrial parts cleaner. The smell was HORRIBLE but there was no permanent damage to the tank. I also had a pedal start moped that had a rusty tank and no matter what I did the rust would slowly clog everything in the fuel system. I ended up using that KREEM stuff to re-coat the inside of the tank. What a PITA! I hate that tank liner stuff.

The petcock is opened with vacuum pressure from the carb. I've seen those hoses get dry rotted and slowly break down on the outside AND inside. I've also seen this happen:

The scooter runs great until ridden. When routing the fuel lines one got slightly pinched when ridding it from the suspension compressing, then everything is fine when off the scooter, because the pinch point isn't being activated. VERY annoying but we eventually found the problem. I forgot to tank pictures of how the fuel lines were routed before pulling them off for replacement. One got routed by a pinch point and caused all sorts of dumb issues because we changed a bunch of stuff at the same time. I take pictures now before removing stuff!

Edit! Reading back through your thread I'm starting to run out of ideas. Re-check the float height in the carb? Still check for vapor lock. Petcocks are vacuum delivery rated. They aren't just on or off. The needle jet regulates the amount of fuel being delivered to the bowl in conjunction with the float, but the vacuum line ensures that the fuel flows with throttle position (I hope that made sense, I'm tired right now!). I only know one person with a scooter that runs clear lines and I recall the fuel line being partially filled during idle. The scooter currently doesn't run (he has a wife now) so I can't get a video for you but we found it kinda fascinating see what the fuel does in the fuel line according to throttle position. The first clear lines he purchased were not ethanol approved and eventually started breaking down. We found ethanol approved lines after that little fiasco.