Steve, what Barry states is correct. Also, have him ask his mechanic to pull up the service bulletin # 97-17 from Mercruiser. It outlines what is happening and specifically states solutions. Some suggestions to minimize run on:
1. Idle engine 1 to 1-1/2 minutes before turning key off.
2. Be sure engine idle mixture, idle RPM and ignition timing are correct. On engines with Thunderbolt IV ignition, setting idle RPM as low as possible can help stop run on.
3. Try a different brand of regular fuel or try some premium fuel.
4. Increase idle mixture by 1/8 turn 'richer.'
5. Go to next colder heat range spark plugs.
6. V6 and V8 only: Install a 140*(60*C) thermostat.
If you need a copy of the bulletin I could fax it to you. Let me know.