An easy way to tell if it's not venting is to run it with the gas cap loose. Most of the saw companies experimented with different ways to vent the tank. If the cap isn't vented they must have a hole in the top of the tank with a line coming out of it or a duckbill valve sticking into the hole.
I took a vented cap off a 150 put it on and it did not seem to help. I ordered a carb kit.