Motorcycle Repair: fuel cock, mike burgess, yamaha virago 750

Question
I have a 1985 yamaha virago 750 ,when ever I turn the fuel cock to prime I completely flood the engine so bad that the fuel comes out the exhaust.To fix this problem I have to undo the fuel line then start it up then plug the line back in.It also backfires on deceleration.any sugestions
would be appreciated. I had the carbs cleaned out but did not get them syncronized yet not sure if that is the problem
Thank you for your time.
Mike Burgess

Answer
Mike,
it sounds like the float levels are too high
or the float needle valves are leaking badly.
The floats need to be adjusted so they close
the float needle before the carb overflows.
The needles should completely stop
the flow of gas or they are no good.
Also, have you checked to see if the
gas tank has pressure in it? If the tank
vent is plugged the pressure could
build up and force gas through.
Syncronizing will not help this problem.
It only helps to smooth the idle and running.
Under normal conditions carbs should not
leak any gas into the engine even on "prime"