Draining the fuel is definitely a good start, and B-12 Chemtool Fuel Injector Cleaner (part #s 0116 and 0101) is great at dissolving gum in the gas tank and fuel lines.  Your best bet is to buy the gallon size (that's the 0101) B-12 and add the most of it to the tank, saving a little for when you refill with gas.  You will then want to  move the mower back and forth a bit to get the  mixture sloshing around a little.  Let it sit overnight and then drain the mixture back out. When you refill the tank with gas, add B-12 Chemtool Fuel Injector Cleaner at a ratio of 1 oz. per gallon of fuel to complete the cleaning process.   Hope that helps.  Let us know if you have any additional questions.