The large black idle screw should be used to set the engine idle speed when the motor is at operating temperature. Set it to 1200 RPM and leave it alone. The motor will idle slower when cold unless the choke is on, which sets a higher idle speed. Use a can of Sea Foam in your gasoline to clean the carbs (Use at the recommended ratio). Get Sea Foam at auto parts store or Wal Mart. If that doesn´t help, you need to (1) Check for vacuum leaks at the carb-to-engine boot, (2) Clean carbs manually. Sea Foam really helped my 1983 Nighthawk 650 & 1983 Shadow 500. Good luck