float adjustment?

Did you look at this?

http://motorbicycling.com/f39/motorized-bicycle-carburetor-install-rebuild-302.html
sort of starts on the float at permalink #10.
I've never seen a book on just where you should adjust the float. So I usually adjust it so the float will sit sort of level when held upside down. If you need more fuel in the bowl the let the float set lower when up side down and opposite for less.
it won't hurt to read the whole post either.
Try to get the fork tongs on the float parallel best you can and adjust the float level at the fulcrum.
These carbs are very simple and when you do get it dialed in they work pretty good. My opinion.
Could be you need a one size larger or one size smaller main jet if you added a freer flowing exhaust. If you try a larger jet and it gets worse then you know you need to go smaller.
Running better with the air cleaner off means you need more air and less fuel( can't do much about adding more air) so I'd guess you need to try a smaller main jet.
You probably need to only mess with the main jet size, leave the rest alone.
It's best to change only one thing at a time and test the results, think about what was done and the results, then decide what to try next. Making many changes at once will make you scratch your a$$ and head wondering what you just did.

Yea maybe. In that case if you have to slowly run it up on rpms then it runs fine at wot. That would seem like to me that the needle needs adjusting to richen up the lower end of the throttle.
The needle affects idle to 3/4 throttle, main jet affects wot. They both kind of inner mix a little.
I can't be there to see or hear what it's doing so for me it can be a hard thing to try and tell you which way to go on the tune.
I try to get the wot setting first, or the main jet right. Then work on the needle for the lower settings. A lot of times if you get the wot right you don't need to do much to the needle setting if it's in the stock #2 slot. Not all needles are the same either so that can also make you pull your hair out. If you clean the air filter, wash it out in warn soap and water(foam type) and install it with out oiling it that seems to work good. The blow back from the way these engines work will naturally oil the filter.
Oh yea at lot will depend on the fuel/oil mix as well when new they seem to like the oil rich mixes 25 to1 etc. Once broke in they will run the best on 32 to1 or 40 to1. No need for premium fuel the cheap regular works best, I use the syn. type oil too so that is why I use the 40 to1 ratio. I also run a spark plug gap of 0.017 to 0.020" and a stock cheap china spark plug, or a champion I don't remember the number of that off hand but I have it listed in the stickys some where.