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.