If the powervalve is blown it will leak fuel into the engine. It will be very rich when you start it after it has been shut down for a few minutes. A lot of the time you can hear fuel drip when it is shut down.
If you take it out of the carb. suck on the side with the big end to draw the valve in and put your tongue over the hole. It should hold. (if you do not have a tool to do so)