Originally Posted by 60wag
The charcoal canister has a couple of check valves on it. The fuel vapor has to build enough pressure to push a check valve open before it can flow into the charcoal. The pressure required to open the valve is supposed to be pretty low but it exceeds what you can produce by blowing on it with your mouth. I think the valves get sticky over time and the cracking pressure can go up.
I would check the FSM for a test procedure for those valves. My 40 manual just says you should be able to blow air into the tank port, as well as the purge port. Then it says you can clean the charcoal by capping off both of those and blowing air into the port that vents the carb float chamber into the can at 40 psi.
There are no pressure specs for testing the check valves.