The sender is a rheostat. The resistance varies with the position of the arm. Connect a ohmeter to the two sender wires (if there's only one wire, connect the meter to one wire and ground) and move the arm. The resistance should change smoothly with no areas of no contact. The usual range is from sometwhere around 1-5 ohms (full) to around 103-117 ohms (empty).

Acetone from home depot or any paint store will clean stuff up. I'd pour what's left of your gallon of acetone into the tank and run the fuel pump to flush the system, then add gas and run the pump again to flush the straight acetone from the lines. The mixture of gas and acetone will burn fine on fire up, and further flush the system as it circulates and burns out. Acetone is a component of most fuel injector tank added cleaners.

I dispose of the old gas by mixing in a little over time into the tank, and just burn it up, or if it's truly foul and sludgy, I pour it into sand in a steel container and burn it, or use it on ground hornet nests <img src="/forums/images/graemlins/smile.gif" alt="" />