From the service manual ...
The engine has something called Intake Air Control which supposedly stablisises idling speed. You need to take this out of the loop first before setting the 'underlying' idle speed using the bypass air screw (which is hidden under a plastic cap - get back to me if you need a piccie).
Engine warmed up and throttle closed. Using the bypass screw raise revs to >1000 for >10 secs. This will set the IAC system to a fixed value, and set off the caution buzzer - 0.5 sec tone every 3 seconds, for 5 mins. Set the idle speed during this time to 625 plus minu 25 rpm using the bypass bleed screw. Cancel this IAC mode by opening the throttle else use the warm up lever. Check idle stabilises at desired value. (If it doesn't then the manual suggests the IAC is at fault).
Not tried this myself - Service manual (covering DF90/115/140) bought recently from Brownspoint Marine! - looks a comprehensive book.
Twenty years in the aircraft industry, five years boating, and I still can't spell propellor ...