I usually tilt the plane so the fuel runs to one of the back corners of the tank, and the clunk should go there as well to pump it out. The clunk will probably not reach all the way to the corner, so I usually rock it back and forth so the fuel goes to/from that back corner, and passes by the clunk. I can usually get all but a teaspoon or so out that way. I also drain or pump the fuel out of the carb/remote needle valve fuel line (if you have one).