first pull the plugs and read them, meaning what color they are. Then have the carbs synced and everything should be fine. to know if it really is running on one cylinder, pull one plug at a time and see if it doesn't drop RPM when ya pull a plug, that one isn't firing, it could be to much or not enough fuel for combusiton. Maby it is Idleing really low, less than 1 grand, what year is it? As far as jetting goes, thats what the stock off the production line is set to, depending on your area, also make sure that the fuel doesn't have water in it, and that your using 2 cycle oil for snowmobiles. and that your using the right plugs.