1993 Sportsman 350 help

So I picked up a 93 sportsman model W938039, (I was told it was a 400, but motor says 350)(2 stroke) I picked it up fairly cheap and will be fixing it up for my wife to ride with me. They said it sat for 3 years. I cleaned out the gas tank, fuel lines, filter and Carburetor (3 times), I also checked the exhaust for blockage. Here is my issue…..After cleaning everything out I can get the engine to purr and rev up nicely. When i take it for the test drive it will accelerate and then when I go to coast it starts popping (assuming from back pressure) very loudly but then runs rough and dies. After this happens I cannot get it to start back up. The exhaust would pop so much that it would blow the exhaust off the engine hole. Any help on this would be very appreciated. It has done this 2 times after i get everything cleaned out.

Thanks in advance.