Chainsaw Repair

Had an old saw 60cc come that is in pretty good shape. However there is so much old fuel goo in the tank that I doubt it will draw fuel, let alone start. You can stick a screw driver in the tank and scrape it off like soft tar. I have had gas in the tank for several days, all that has happened is the gas turns black, and the goo is still there. This a metal tank saw and I do not want to split the case to get inside the fuel tank. Any thoughts on some other solvent I might be able to put in to dissolve the varnish?

or some lacquer thinner work...