Kitty Cat Snowmobiles

Well after constantly fighting with my 72 Kitty Kat, I gave in and found a 1980, had a locked up engine, but I had a 97 Engine sitting on the shelf. I installed it, cleaned the carb that came on the 80 engine, It started 2nd pull wouldnt idle, Even sat the idle high, still nothing. My son took it out, got about 5 good laps before it fouled the plug and wouldnt restart. I set the pilot air screw to 1 turn out as per the service manual. The carb was easy to clean, one of the easiest ive ever done, I just dont know what else could be going on

Thanks