Drain the tank through the petcock and a short length of fuel hose into a suitable container. Some fuel will remain because of the tunnel but as long as the bike remains on the side stand this isn't an issue. Place a catch can under the petcock because some fuel is going to spill when you unbolt the petcock. Unbolt and gently peel off the gasket. I'd bet you got paint flakes and the like clogging the screens which can be washed with some kerosene.
