try - use some WD40 to cleanup and lube the Air Regulator valve. It’s responsible for cold start high idle and over time can get stuck, due to carbon buildup, in the high idle or partial high idle position. It’s just a rotating disc that opens and closes air flow. See below for diagram of Air Regulator if unsure of location. BTW - when engine is misperforming in high idle, pinch closed the Air Reg hose that connects to the balance tube - if idle drops may have found the issue?