I just resurrected a Toro S200 snow Blower that had been sitting for some time. I put a new diaphram and gasket as well as fuel lines on it. It starts and runs fine.

However, when it sits for a while the gas disappears from the tank. No leak. It seems the fuel is siphoning from the tank into the carb and overflowing out the air intake.

Would this be a faulty needle valve, or is there something else I'm overlooking? Is there a check valve?