Trolling with a 1996 Mercruiser 3.0 Alpha One

I'm new to this site and apologize if this topic has already been covered.  I fish by trolling upstream in the St Clair river in Michigan which has a swift and variable current.  I can set my idle speed at anything between 500 and 1000 RPM by turning the screw in and out but once it is set I can't give it a little more power without the engine going into high speed mode (over 1000-1200 RPM).  There seems to be hump in the speed control that keeps me either in idle or high speed mode.  My question is if there is a way I can modify the speed control so that I can get smooth even RPM's anywhere between  500 and 1000 RPM by moving the throttle lever.  The obvious answer is to by a trolling motor but my Merc runs so well it would be great if I could just adjust the RPM's in the low range.  Thanks for the help.

First thing is to be sure there's no "slop" in the cable and all linkages (some carbs have these bent wire linkage that can get out of whack). I guess you probably did that already, so step two is to be sure that the carb is good and clean. The transition from the idle circuit to the speed circuit inside the carb is designed such that as the butterfly opens, it uncovers the idle vacuum port and exposes the Venturi to more air flow, drawing fuel from the main circuit and away from the idle circuit. The passages have to be clean clean clean for this to be smooth. Any air leaks, dirt, etc. will reduce the efficiency of this. Adjusting the idle up and down simply increases the air/fuel volume of the idle circuit and does nothing about the main.
There should be an accelerator pump that will add a bit of fuel as the carb opens up. Maybe that's the problem? But I would think that that would put it into a starvation situation and not want to run away.
Dunno man. hard to say from my keyboard.

Could the cable be sticky? I had something similar on a pontoon boat. I put new (used) cables on and took them apart and lubed them thoroughly. You can check for stickiness by having a helper operate the control in the area you see the problem. Look for that lack of smooth operation at the initial movement off of the idle screw. Also, the type of grease they use in the control can also begin to fail from moisture over the years....this can cause "stickiness". Controls can be intimidating, but you can clean and regrease to elliminate that problem. It is similar to older OMC tiller handles with the spring loaded friction clutch....the only way to fix it is complete cleaning, then fresh grease. Really might be within carb too as o2 suggests. He is a very good tech.