This is a 1998 prarie 400 2 wheel drive and I juse purchased ityesterday, It will only run when the choke is on and when I turn the choke off it dies. The previous owner said it has been sitting a while, when I start it up it sounds like the motar is knocking alittle but when I rev it up to a constint rpm the knocking goes away, but I don't know if it will go away when it warms up? Thanks for any input.

 

Sounds to me like the carb is clogged up Newer gas causes a lot of problems when it sits, If you are handy drain the fuel remove and clean the carb(all of it some jets are smaller than a paperclip and will cause what you describe) I'd then change the oil and see what happens from there, The knocking could be bad news or noise from bad fuel or fuel contaminated oil

 

Change the gas and clean the carb, if you do not know how to change clean the carb take it off and get your dealer to do it,it will not cost much. Here is a link on how to clean a carb. Disassembly and Cleaning of a Mikuni Round slide Carburetor