[Gaz64]

All of the above, and add an anti-dieseling solenoid to the carb, set the idle speed at whatever speed you want with the solenoid screw.
Then with the solenoid unplugged, set the idle speed with the carb speed screw to 100-150 less than with the solenoid connected.

The engine will run at the speed you choose, then the butterflys will close to the lower setting when the ignition is turned off.

Some engines are more prone than others to run-on.
Compression ratio, cam timing, chamber/piston shape are more influencing factors than base timing or fuel octane.

Too much base timing can cause slow cranking/starter kickback.