I think you mean that it turns over, but will not start. Draining the tank is a start, but you will have to flush out the entire fuel system, or it probably will not run. You will need to flush out the fuel lines, change the fuel filter, if it has a carburetor, that will have to be removed, disassembled and cleaned too. If you have fuel injection, all the lines to the injectors will need to be flushed out, and maybe try to clean the injectors. You have a lot of work ahead of you.