takes 3 things to make an internal combustion engine run - fuel and air, spark at the right time, and compression. One or more is missing or poor.
1. Try spraying a little Berrymans carb spray, like pssst pssst, into the carb. Try to start it. If it starts right up and dies (or keeps going) then you have a fuel / carb problem.
2. Get a spark tester and see if you have spark while it is not starting.
3. Get a compression tester and see what the engine compression is. If it has a compression release in the engine maybe 50 - 60 PSI. If no compression release then 110 PSI+ . Low compression can be several things.
Post your results when you can.
Also if you have a fuel shut off lever, leave it on for four or five minutes before you try to start it. I turn mine off and let the engine run out of fuel when I will not be using it for awhile.