Kitty Cat Snowmobiles

hello , i have a 77 kc , I can get it to start with a new plug give it throttle and it bogs a bit it will idle fine for about 15 min then die , ive cleaned the carb, replaced the pilot screw, adjusted the float many times over , but no matter what i do it will foul the plug after about 10 to 20 mins the plug gets a black dry carbon coating . would this be a carb problem or ignition problem?

no ideas , any ideas, come on guys help please

what are you mixing your gas at...24?-1.what kind of oil name brand? are you use solid top spark plugs( the little cap on top of the spark plug can be removed)?check your exhaust not plugged up.