Brand new petcock for all Yamaha Qt50s.  On, off and reserve positions.  Built-in fuel screen and o-ring.  You should run a separate inline fuel filter with any petcock as the built-in fuel screen can't trap really small particles.  Clear up fuel starvation problems with a more free flowing petcock or end that nasty gas leak with a new petcock.  Easy install.  Drain tank either by opening drain valve on carburetor or by removing fuel line from carburetor and draining into an appropriate container.  Detach fuel line from old petcock.  Unscrew old petcock and make sure none of it stayed in the fuel tank.  Old petcocks are known to disintegrate and break apart.  Screw in new petcock.  Do not use teflon tape or related products.  Do not over-tighten!  If your tank is old and rusty, over tightening petcock may damage your fuel tank.  Reconnect fuel line.  Add gas and GO!  

Do not over tighten!  If you do, it will deform o-ring and leak. Hand tighten and then snug up with 22mm wrench or crescent wrench.