Hey there! I have some questions first, does it idle? When you release the throttle, does it return to proper idling RPM's?
Anyways, Check your idle screws, Little golden colored screws on the left side of the carburetor, Screw them in about... 1/4 of a turn, pull the cord, if it doesnt idle correctly, turn the screw(s) A little bit more, Do this until it idles correctly. When it does idle correctly, you should just be able to pull the cord without holding the throttle, Ive had the similar problem with a 1982 Ski-Doo Nordik 377, Fixed it, now it runs fine, if this does not fix the problem please tell me.