pull the gas line where it enters the carbs and point hose into a container.
go to your gas petcock and turn it to "prime".... gas should flow out the hose and into the container until tank is empty. When the tank is empty, put a little gas in the tank, slosh the bike around and then dump the contents out using the method described above. repeat until you feel most of the crap is out of the tank.
when you remove the gas line from the carbs, there is a little filter on the end of the hose that you will need to remove for gas to flow freely.
You might consider removing that small filter altogether and replace with a small plastic inline filter instead.
