Like FLYING MOUNTIE said, your most likely too lean or not getting proper airflow to/from the engine and it's getting hot. My 2500 did this. My setup: lots of cooling inlets/outlets, Perry carb, OS F plug, and Omega 15%. Runs stronger than most newer motors, swings a Mezjlik 18X8 @ 8900rpm. Keep it cool and lubed and it will run well for you.