Throttle won't twist. '90 Mariner 15 hp

[Jack Spratt]

1990 manual start 15 hp. New to me, unknown history. Tried to twist and has a lot of resistance. Removed the cover at the 90 degree elbow and the cables are free, and free to move where they terminate on the double sheave pulley(?) on the side of the carb. The sheaves just don't want to turn. The link between the back of the motor and some mid point doesn't want to move. Does the engine have to be running to advance the throttle?

I've looked for an original repair manual but can only find general ones.

Thanks for any help.