I would take the air cleaner off check to see if the accelerator pump is working. Basically look down the carb throat and open the throttle and look for the squirt of gas.

If you do get a squirt you at least know your carb has fuel in it. If not check for the output of the fuel pump.

I know that bad things can happen from gas sitting, but its possible / likely that the fuel in the tank and lines has not gummed, It's may have just gotten to a point where it does not vaporize easily and is harder to burn.

The most likely problem part is the carb. AS the fuel in the bowl evaporates and whats left combines with moisture you can get some nasty gunk.

The upside is once the bowl evaporates off there is no more fuel to react. The down side is whats left in the bottom may get nasty. Its worse on lawn mowers since the gravity feed tank keeps adding fuel to the bowl, keeping the reaction going.

Kevin