Hi: I started up my dad's OS FS-.91 4-stroke for the first time this season, and it will not make any power when the fuel tank vent tube is connected to the muffler pressure fitting. when i take the vent tube off, the engine runs normally. i checked the vent tube and both the soft tubing and the brass tube into the tank are clear of obstructions. i removed the muffler and the pressure fitting is also clear of obstructions. any ideas? i've tried messing with the needle valve setting but it did not help. thanks.

Mabye your needle valve is open way to much. When the pressure is connected, more fuel goes in. When it isn't, the suction is all it needs. Try with the pressure valve on and leaning it out.