There is an easy, less professional solution. Remove your petcock, drain your gas. Plug up the gas tank hole with something, fill it up with Behre Concrete etcher/rust remover (available at home depot $10) then just let it sit for several hours, maybe a day if its real bad. Drain, then flush it out with your drained gas several times untill it is running COMPLETELY clear. there your done. Removing petcock it very important, it is the only way you can be sure to get all the crap out, and this stuff will really crud up your petcock, and carb (I learned the hard way) but I have a clean tank now.