Help with Mariner Outboard

[kokerj]

I bought a 1986 Potter 19 that came with a mariner 5hp 2-stroke outboard. I do not know if the motor is of the same vintage of the boat. Does anyone know if there is an easy way to tell if this is a pre or post 1990 motor? There are no Mercury markings anywhere, in fact the motor was made in Japan. The cover is grey.

Also, I have been able to get it running, though the idle seems high. I adjust the idle down and it seems fine, but then if I run up to higher rpms, the engine dies after trying to come back to idle. I know that the motor has not been run in the past 5 or so years. The engine was fogged prior to storage, and the plug is clean. Any suggestions to get this motor running reliably?

Thanks in advance!