Tillotson 212 - Mikuni 22 mm or 24 mm Carb?

Glad I could help. Try a 20 Pilot Jet and report back. Make sure to make adjustments when the engine is fully warmed up.

You are running a torque converter so adjustments to the unit may be all you need to gain speed. I only have experience with Hilliard clutches so someone else is going to have to help you with that portion of things. Your main jet may be dead on. The only way to find out is to keep fiddling and swapping.
osu1978 - thanks again. I really can't believe that a brand new genuine Mikuni carb would have an issue like that. Then again, what am I saying, they probably cheapened up the springs to save 2 cents a carb w/o testing the new spring. I get it!

I've been playing around with the torque converter setup, also. TheDullCarbide gave me a lot of great information on things to do with the TAV. I went with white driver pulley springs, changed the spring hole on the driven pulley to the first hole and also installed the correct size belt (by my calculation, 1/2" shorter) and changed to a 50 tooth rear sprocket from a 38 tooth sprocket. With the driver unit engaging later, the driven pulley shifting later and the other changes, this little mini bike really scoots! I've probably picked up 2,000 plus engine RPMs from where I started. I now have the engine running where the cam generates power. I have some other things that I could tweak, such as lighter driver unit weights, 60 tooth rear sprocket (definitely improve launch, but probably cut top speed a little), etc... However, this thing moves along pretty good now! Finally, actually fun to ride.

I really appreciate all the advice and comments from members of this forum! Helped me a ton and I didn't have to reinvent the wheel or spend time and money trying things that might or might not work. I was getting a little frustrated with a couple of suppliers/vendors when I started this process. One kept giving me awful advice! They constantly pointed me in the wrong direction. They gave me all these things to do/change and what results they got when they did it. Nope! I truly hate it when a supplier/vendor acts like an expert, gives you a fix and it isn't one or sells you parts that they are using on the exact same mini bike and they don't even come close to fitting! If you don't know, just say that. Ok, I'm off my soapbox.
I just ordered one. I bought the TCT carb to compare against the stock. I am not a fan of the Mikunis and prefer the stock look. I am also going to try to keep the stock muffler if it fits the frame. If not I am thinking a header with hot dog muffler.
Continue to work on this mini bike. I've been tuning the Mikuni carb. and changing garter springs and weights on the driver TAV unit, Chinese. Currently running white garter springs with 146 gram weights and new genuine Comet belt. I've run into a problem lately where the tach shows 5500 RPMs during hard acceleration and then falls off to 4500 RPMs at full acceleration, full throttle. Definitely dropped off top end speed on GPS. Can't find anything obvious. Any ideas where to start looking for problem? Thanks in advance for ideas.
UPDATE - Think that I found the problem. I had put on a new genuine Comet belt, apparently this belt is somehow not the same as the belt it replaced. I'm investigating that. Also, removed and cleaned both the driven and driver TAV units. Put everything back together with the old belt and everything is running back to normal with a 5500 RPM top end.