I have a Yamaha 15 hp that is ten to fifteen years old. It was on the
inflatable dinghy of the trawler I recently acquired. The engine will
start in neutral using about half choke and it will rev up quite
nicely. When you throttle back however it will stall immediately
unless you can catch it with the choke and jockey back and forth.

By carefully playing choke vs throttle you can get it slow enough to
put in gear and it seems to develop decent power at higher revs
without any choke. It still won't idle back to lower speeds however.

Any ideas?