Make sure your clunk is not stuck at the front of the tank. Disconnect your fuel and vent lines, insert a brass tube into one line. Pinch one line and blow into the other, then pinch and connect the other line to the brass tube. Let it sit for 10 minutes or so, then unplug the lines. You should have pressure still in the tank. If not you've got a leak somewhere. Also check the carb to engine seal. It may be leaking as well.