There is a product called "Seafoam" that is great for dissolving fuel deposits. I used it on an ancient Mercury OB that I had for a while, which didn't like to start after sitting idle for a couple of weeks. The first time I used it, I sprayed it into the carburetor, removed the spark plugs and sprayed it into the cylinders, and let it sit for 24 hours.
It took quite a few pulls to get it going the next day, but it fired up and ran more smoothly than it ever had. After that, I used to give it a couple of squirts whenever I was going to be away for a while, or putting it away for the winter.
Sea Foam