Sounds like a fuel supply problem to me. I would drain the carbs (with the petcock shut off) and see what and how much comes out. It is possible that you have a stuck float or a stuck float needle. If you remove the float bowls to check this out, be careful that everything is free and in position before you reinstall the bowls, especially if the bike is on the kickstand. The pin that the bowl swivels on can move out of the holes it lives in and then nothing works....!
