What I did to chase down my vacuum leaks was by spraying carb cleaner around the carburetor mounting surfaces, vacuum lines, fittings, along the manifold gaskets, and very importantly, the throttle shaft on the carbs. The throttle shafts on my first set of carbs leaked and it meant I could not balance the carbs and the idle would not settle below, say, 1200 rpm. The bushings are common wear points and wear out of round (oval) creating air leaks. The idle would only come down with the throttle adjusting screw backed out fully on the front carb, using the rear carb to control idle. Basically this means they are grossly unbalanced at idle and the mixture is haywire because you've really shut down the front carb by closing the throttle screw.
You will need to verify you have no vacuum leaks before attempting the synchronize/balance procedure. The rpms will bog if the carb clean finds a vacuum leak. Some people say it raises but mine bogged and I could have stalled the engine had I kept it on the leak. Either way, you will likely hear a change in engine rpm.
Looking at the amount of turns you have on the mixture screws, that sounds like the float height could be off. You really need to make sure you don't have vacuum leaks before fiddling with the float height.
Factory manual can be found here: http://xenons30.com/reference.html
Also, the instructional DVD from http://www.ztherapy.com/ is supposed to be very good. Bruce Palmer, proprietor of ZTherapy, is a regular contributor to this board, and will likely have advice for you if he sees this thread.
Aside from what I've said, read through the threads in this section, there is a wealth of information here. It's really how I become familiar with the carbs. The FSM helps to understand the carburetors as well.
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