Motorcycle Repair: carb

Question
when u try to start it it fills the crankcase with gas

Answer
It sounds like the needle valve is not seating due to a
bad needle valve or needle valve seat. It could also be a faulty float.
The fuel is getting into the intake and finding its way to the crankcase
either through the piston rings or the valves.
It sounds like its time to get the carbs cleaned and rebuilt.
Also many of the petcocks have diaphragms in them and they normally only
let fuel enter the carbs when a vacuum is applied to the diaphragms.
If the diaphragms get bad, then fuel can run all the time sometimes causing
the fuel to enter into the crankcase.
I am a Carburetors Specialist and I Rebuild Carburetors for a Living.
My Website is:   www.viragocarb.com
I have alternate Contact Info there in case you need further Help...  Joe
I would need more specifics, like model make and year and the brand of
carburetors.