Motorcycle Repair: 1981 cb650c carb problems, lean mixture, air leak

Question
Rich,
after 27 years riding this cb650, bike would only run with a little choke to keep from stalling.  I rebuilt the carbs,cleaning them meticulously, and now have a different problem. After warm-up [which is considerable], when you give it throttle bike continues to rev to approx. 4000 rpm, and will eventually slowly rev back down. Throttle plates are not sticking, they snap shut fast.
The only parts I did not replace were those 2 rubber "t" fittings between #1 & # 2 carbs, and # 3 & # 4 carbs.

Any Ideas of what is going on? [By the way, the slides move smoothly also].

Thanks
Rick

Answer
Well, it sounds like the carbs are a little lean.  It could be the float setting.  I can't remember anymore if those carbs floats were adjustable or fixed.  The lean mixture is the cause of the cold blooded nature.  The hanging at 4000 is probably the timing advance unit.  Sometimes the springs get tired and let it stay in advance too long.  If you twist the advance, does it snap closed when you release it or does it kind of lazily return to the retard position.  If it doesn't return smartly, that's probably the problem.  You could also have an air leak somewhere that could cause the lean mixture and allow it to hang up.  


Regards
Rich