Modern Vespa

Hello all,

I have an S50 4v/4T with an upgraded malossi variator and I can only reach 28mph flat ground (my speedo matches GPS, at least that's working normal). Once in a blue moon, it "opens" up and I can hear the RPM go up which I then reach 35mph on flat ground. I cant figure out why it does that sometimes. I recently opened up the carb and, yes it was dirty, but surprisingly both jets were clean. I was able to see the holes clearly. I cleaned everything out with carb cleaner and assembled it all back. It did not make any difference in speed.
Is 28mph about all I can expect from a 240lbs fat ass? Ofcourse when I go downhill it will reach 40mph easily.
Note: The pilot jet is 38. I believe stock is 35?
Also worth mentioning, it starts up immediately and idles perfect. No issues when I'm cruising at 28mph either. Just wish It can always "open" up and go 35+ mph

Well a little update. I accidentally found out that my needle jet #4 is not moving up into the diaphragm, thus now allowing the carb to fully open up the main jet channel.
When reassembling the carb, I accidentally put the long spring #3 on the wrong opposite side and even though the idle was horrible, once I got some speed by peddling, it was easily hitting 40mph.
Once I put the spring #3 in the correct location, it idled great but never goes into the main jet channel, thus my top speed of 28mph.

So my question to everyone, why isn't my diaphragm #6 working properly to open up the needle jet? Is my carb still clogged up somewhere like the internal veins that I can't see? I'm going to dip the whole carb into ultrasonic soon. Any ideas?

How?

what main jet? what air filter? Does the autochoke work? What color is the plug? how is the vacuum hose?

I put spring #3 below item #6 by accident. Please refer to the diagram from my previous post. The autochoke works perfectly, air filter is clean, I checked vacuum hoses and replaced a couple but most are good. I believe my issue is somewhere inside my carb.

Please note: the diagram is not exactly 100% identical to my Vespa's keihin carb. It's close enough to get an idea of what I'm talking about.

I completely took apart my Keihin NCV20 AR8 carb and ultrasonic cleaned it. Once I'm done reassembling everything, I'll give it a test ride.
Pics of cleaning results:

Nice!

SUCCESS!!! test rode and I'm easily hitting top speed of 40mph now! Ever since I bought this s50, (used and unknown), it has never gone faster than 25-28mph.
The needle jet was getting stuck inside the corroded main jet. Ultrasonic cleaned it all up.