These engines have a fixed idle, there is no adjustment. Since it is an older engine, you have either had it a long time or bought it used. There could be mechanical problems or possibly the fuel system is in need of repair.
The starting setting for the main needle is 2 turns open, the regulator screw should be set flush with the housing. Make sure that the tank is pressurizing correctly.

Throwing props is a sign that the engine is detonating. Is the prop driver face worn smooth? If so, it MUST be replaced before going further as it will NEVER hold a prop correctly.