I have a '96 Merc. 50 2cy that I troll all day with.
I have to give it gas to get up to 1.8mph for trolling on Lake Erie for walleyes, and steelhead.
What I did was down size the prop pitch. They usually run with a 13 pitch, and I run a 11 pitch. It idle/trolls at about 700rpm, and I have to get it up to about 900 to go 1.8 (more against the wind). I guess I'm lucky, my 50 runs smooth and quiet.
You loose about 2 mph on the high end, but I'm not a speed demon anyways. With the waves on Erie your lucky to run anything above 20mph without busting a gut, or your boat.
Top speed on mine with the 11 pitch is 29 or 30mph.
I always open it up when going back to the dock to clear out any carbon build up, and try to run real gas (no 10% ethanol) 100% of the time + Staybil.
Works for me...