Small Engines (Lawn Mowers, etc.): fuel pressurizes with primer bulb, duckbill, smart person

Question
Hi Tom.  I have an 18" Craftsman chain saw (358.350180).  When I try to prime the saw, the fuel tank pressurizes. I have looked for a vent, but it makes sense that the tank would want to leak outward.  The duckbill in the tank cap is clear, but obviously, is designed to let air in, not out.  The saw won't run, and it leaks something awful with a pressurized fuel tank.  What am I missing???

Answer
I would check the inlet in the carb where the primer sends the fuel into the Carb. Sounds as if it may be clogged not sending the fuel into the carb. When you press the primer bulb it clears any air out of the line into the tank, when released it now has a vacuum in the line and draws the fuel back up and into the carb to prime it. Almost works on the same premises of how a smart person siphons fuel from a tank, you place the hose into the tank and sharply blow into the hose (not suck from it) as gravity comes into play to refill the line the force of the fuel refilling the line will force the fuel to begin a siphon (no fuel in the mouth). If you would hold your finger over the end of the hose after blowing in it would never refill the hose. Just the same as if your primer port was clogged. With out the fuel after you press the primer not being able to squirt into the carb, you are pressurizing your tank.   Thats why your fuel cap is designed to allow air in but not out, it needs that little pressure to force the fuel up the line and into the carb. Make sure also nothing is clogged in the hose and it's not pinched. ...........Tom