1976 All Other Makes Recreational Vehicles yamaha gs300 snowmobile

I know this site if for cars, but it's the best thing I know of for mechanical questions so.I have a 1976 Yamaha GS300 snowmobile (one cylinder, 2 stroke). It sat for about seven years (my uncle owned it and gave it to me because he didn't use it). When I tried to start it two years ago it would run off starting fluid, but not the gas. I took apart and cleaned the carburetor and it ran fairly well for about a day, and then wouldn't start. I cleaned the carburetor again and again it ran about a day, then wouldn't start. I got tired of working on it, and put it back into the barn. Now this year I am trying to start it again. I bought a used carburetor that I was told was good, and put it in. Now it wont fire. The gas is pumped, the gas tank washed out, new fuel filter, I get a good spark, but it wont fire. I tried putting starting fluid in the carb and putting a few drops of gas in the spark plug hole, didn't help. Any ideas?