If there is a venting problem here's another possibility. My dealer's mechanic said there is a ball check valve in the vent tubing just beneath the carbs. He said occasionally the ball can get stuck if you've ever overfilled the tank and some residual gas has dried and gummed up this valve. He recommended the fix was to just follow the vent tube down from the tank to the plastic piece between and under the carbs, pull it off and throw it away. You may have to reroute the vent tube so that it drains onto the ground not onto the back of the motor.
Haven't had to try this so I can't vouch for it. For what it's worth.
Regards,
Rich
