It has a crankcase vent to relieve pressure from the case cause by piston blow by.
This vent needs to remain open, the excess pressure must have a way out.
What you can do is use a "T" in the vent line: leave one end open to atmosphere, and route the other one to the pump diaphragm on the carb. Place the T close to the crankcase, and use a fairly long tube to vent to atmosphere. That will ensure you get a good pump pulse.