Motorcycle Repair: 1979 honda cm400t, vacuum gauge, rusty tank

Question
Okay, so I asked before about my 2.5 questions.  I replaced the 2-1 exhaust with an old, busted OEM exhaust.  I have some power, I can get to 60 mph.  :-D  I also get 24 mpg :-/  I also lose power around 5500 rpm in 5th gear.  My right carburetor leaks like a sieve.  So now my next question - I am having a hard time finding parts for my 1979 VB-21C keihin carburetor.   Can I swap in a later carburetor, like a 1980 VB-22B or VB-24C?  Is the only difference the jets?  I guess there's also an accelerator circuit.  Do I need parts for the carbs?  I have assumed the problem with gas mileage is a set of 30y/o floats, but it could also be improper jets (the bike starts perfectly though).  I have assumed the leaking is the float in the right carb (the one that leaks).  Is it possible I just need to adjust the carbs with a vacuum gauge?  Any suggestions.  Thanks a lot, btw.

Answer
Well, you don't need any parts to fix what you describe other than possibly needles and seats and the right jets which should be available from Honda. The leaking carb is caused by the needle not shutting off the fuel.  You need to take the carbs apart and replace the needle and seats.  Check to see if there isn't a fine rust power in the bowls. If there is, you probably have a rusty tank and need to seal it or put a filter in the line.

The bad milage is because of the leaking float so once that's cleared up, I'm guessing the rest will resolve itself.  

Regards
Rich