It is a 99 polaris 300 2 stroke and the carburetor is sending fuel back towards air filter, and sputters and will idle just will not run under a load.

 

Check compression first. If it's low(below 100psi) common problem for fuel to spray back towards the air filter if it can't compress the fuel/air/oil mixture into a fine mist for ignition. Also can be carb needle and seat could be worn allowing too much fuel in the cylinder. If compresion is over 100psi,you can see if a carb kit solves the problem.Shindy Carburetor Carb Rebuild Kit Polaris Xpress/Xplorer 300 1996-99 ATV 03-40 | eBay

 

A restricted exhaust can also cause those symptoms. Carbon in the exhaust port, tail pipe, etc..

 

If it is truly running in reverse that's a timing/ignition issue...I believe...