I use denatured alcohol. Is this a metal tank? If you add nuts or gravel as some may suggest, I would be concerned about promoting rust inside the tank. Might be a good time to seal the tank while you're at it?
https://www.amazon.com/KREEM-Gas-Tan...em+tank+sealer
More reading on this;
http://www.onlycubcadets.net/forum/s...882#post340882
