150cc GY6 scooter lost top speed

[Battercake]

I've ended up with three carbs: The original one from the factory, plus two I bought online trying to fix the problem. Going back to where this all began, the diaphragm in the original carb went bad back in January. I swapped out the carb with a new one, which made it run again but things were never 100% after that. I don't remember what all transpired after that, but I ended up buying a third carb and at some point started swapping parts around trying to make it run right. Somehow I ended up with the original carb back in the scooter with the diaphragm out of another carb. But last night I took a notion to swap things out again. The first carb I tried wouldn't work at all. It just filled with fuel and smoked out the tailpipe with every cycle trying to start. So I tried the other one and suddenly this thing started running better than it has since January! These are all the cheap $35 carbs like the one that came from the factory (but with working IAM screws unlike the original). I might consider upgrading the carb now?

[sc00ter]

Are all 3 carbs the same size (inlet size, like 20mm for example) and same jet sizes? Even if you order a "factory" carb it never hurts to check the jetting is the same as the original carb. Even aftermarket carbs can have different jetting even though they are "the same" carbs. Production runs can very for many reasons.

Another fun thing we've run into. My friend and I were getting so flustered tuning a Yamaha Zuma 2-stroke. Luckily my build was close enough to his build at the time and we'd swap my carb over and his scoot ran pretty good. Put his new carb on and it ran weird. Put his new carb on my Zuma and it ran weird. The main jet, pilot jet, needle size and clip height all set the same but his new carb just ran weird. After breaking it down it turns out his carb had a 4-stroke atomizer in it, and mine had the correct 2-stroke atomizer! Ordered the correct atomizer and his scoot ran great. Now we verify what atomizer is in a new carb.

The only reason you'd consider a "performance" carb is if you want more fuel, that will equal more power. BUT, if you do a bigger carb you'll need to provide more air, and that means a different air filter. Then you have to tune it correctly so everything gets along and it runs good. Tuning Matters website has a good How-To on their page that goes in steps.

Another thing to consider, get a brand name carb. A Dellotro or Mikuni are the 2 main players and sometimes do runs under other names, like Stage6 and Polini.

If you are happy with stock then keep it stock, just fine tune everything and make it perfect. The bonus is it will stay reliable. That's what I'm doing with my Buddy 125.If I want something faster I'll just buy a bigger scoot. Keeway Sixties 300i is a dream scoot for me.

[Battercake]

So yesterday it wasn't running its best again, not quite as bad as last week, but it definitely felt like my fuel supply was restricted and I was having trouble going any faster than 47. But on the way home yesterday evening it fell like something loosened up and I was right back to running 54 or 55 with no trouble, even up to 64 down a hill! Perhaps I had a trash go through the jet or something?

[Battercake]

Since I've replaced the fuel line and filter with transparent ones, I can see very clearly that there's not nearly enough fuel in the line when it's running slow. It's mostly air. I don't know what's causing it because the entire thing is lower than the bottom of the fuel tank.

[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.