Modern Vespa

So I have a 2001 Vespa ET4 150 with 1000 miles. Ran pretty will when I bought it, little bit of stuttering and slow acceleration off the line but not bad. This it started to show some problems so I cleaned the carb and the float bowl gasket was shot so used some liquid gasket to get by until i found one. Turns out nobody makes parts or gaskets for the Walbro carb anymore so I went to scooterwesy and bought a brand new Kehien 2600a carb. Was supposed to be a perfect drop it but that was not the case for me. Trouble off the bat with starting and running. Stranded my girlfriend half a dozen times on her way to university. So I did a full work up, new spark plug, new battery, new belt and rollers, new petcock and fuel line, new airflter. Did an EVAP system delete and replaced all the vac lines. Still refuses to run, will only start and stay running with 1/4+ throttle applied and runs terribly, sound like it chugs and blows a touch of black smoke(I'll attach a video) will not idle at all. Out of curiosity I threw in the old Walbro carb and of course starts right up and runs pretty dang good (still a bit slow and stutter off the line) once up to speed 15MPH+ it runs and accelerates wonderfully. Also it's not a manifold problem, I sprayed it with carb cleaner and no idle variations. So what could be the problem with the new carb? Are the jets a different/wrong size? Thanks for any help!

https://youtube.com/shorts/gdp8yp_a8ig?si=LDe_p6ePMwTnqGyV

the line on the carb that is usually attached to the evap, the one coming off the top left just under the diaphragm lid. where's that go to?

have you tried running with out that?

Are you sure the intake manifold is OK? No cracks?

Yes that is the bowl vent I think, it used to go to the EVAP canister thing, I just took a section of the vac hose and put a makeshift filter on it to keep out dust/insects whatever could get in there. No change with it on or off

Yes fairly certain, sprayed it with carb cleaner to see if the idle would fluctuate and nothing happened. I'm going to replace it and the gasket with a new one when I get the valve cover gasket to do a valve check/adjustment just to be sure.

So I took the new Keihin carb apart to make sure that the jets were all open and seated tight, diaphragm was good and everything checked out. So I'm wondering what could cause the old original Walbro carb to run and this one to not being they should be the same? My gut says it could be a problem with the float valve? The jets are the same in both.

Doesn't sound like a float valve problem. It sounds like the carburetor is out of adjustment.

I've tried the mixture screw from 1/2 turns out to 5 turns out, nothing that range wants to run. Maybe I just got a bad carb? A shame scooterwest won't take a carb on return

The carb in the video works fine, it's just out of adjustment. If you have a bad carb now, it's because the person who thought liquid gasket was a good idea took this one apart. You can poke at it for as long as you want, but the best course of action to get your bike running likely would be to find a qualified Vespa or Piaggio technician who has worked on these bikes. And tell them everything you did, not just to the carb, but to the entire bike. It will save them a lot of time diagnosing the issue(s).