If you suspect it's crudded up, you have to pull the tank. It's probably more
intimidating sounding than it really is though. There's very little to it.
Remove the small screw from the fuel cock handle, pull off the rubber grommit
around the fuel cock, remove bolts at the top of tank, lift tank up gently while
making sure the fuel cock is coming back through the hole. You should be able to
lift the tank up enough, getting the fuel cock rod out of the body with the fuel
line still attached. Tip the tank toward the back and then disconnect the fuel
line and let it hang. This is obviously best done with a pretty much empty tank.
Typically you would put your fuel cock on reserve and open the carb drain bolt
to let all the gas out. Since your reserve is suspected as not working, you'll
have more gas in the tank than you'd like and may need another method.
With the tank out, the fuel cock can be unscrewed and examined. Check the
screens for crud. The white screen filters do come off but can difficult.
If you're going through the effort of taking out the tank and cleaning
the fuel cock, clean your tank really well! Take a wire brush and get all the
paint off to the top around the fill hole area (that's what flakes into tanks in
the first place).