I have a Nissan 15 HP outboard.

The mechanic turned it up with the Nissan tuneup kit and parts.

It seems to run ok, but stalls in idle.... meaning I need to change gears at
a
non-idle speed to keep it running.

Any thoughts on what might be happening?

He adjusted the carb idle setting to low, its a simple fix and he shouldnt mind
fixing it for you quickly at no charge.