I had the same problem a few years ago - and the answer came from the chap who also serviced my Lawnmower.
The problem is only really with 'unleaded' fuel - and it is to do with water separation, so the fuel 'goes off'.
Leave it in the tank, and add a 'water evaporator' fluid - you can buy it from garages or Marinas, and the fuel will keep.
Not sure if it is best to add up front, or when you want to use it again - I put mine in at the start of the closed season - and have had no more problems.
I use the same stuff in my ex-military landrovet, which sits there all winter, and that has solved the same problem with her.
Hope this helps