Normal. I bought my first boat a few months ago with a merc 900. Same deal. I replaced the shift and throttle cables, lubed the slides for both in the engine, and disassembel and lubed the mercontrol (1978). It all works and feels like new, the shifting stopped grinding going into gear, the throttle and idle adjust work perfectly.....but as you said...when you move the throttle lever forward it isn't actually the throttle that is moving. First the gear select moves...then the throttle kicks in via cams inside. It moves one cable, then the other. The long throw before acceleration is how it's designed.. Leaving just a short area near full lever stroke for throttle. Yes...I think it could be more linear... But after a full rebuild and inspection...that's just how it is.
