The crankcase vent is clogged or pinched off.
There is a nipple on the cylinder head cover above the spark plug that has a hose attached to it that goes to the oil tank. This hose vents the tank to the engine. There is a nipple on the rear of the engine below the carb that has a hose attached to it that is routed to a bottle that is plugged into the tube that connects the carb to the air box. One or both of these hoses connected to the engine is obstructed in some way.