I have an 89 Mercury 115hp outboard on my boat.  When it is out of the water with the earmuffs on it runs like a dream, starts right up, idles smooth and throttles up smoothly.  When it is in the water it still starts good, but it seems to idle a little slow, and has a dead spot in the throttle where it will die if you put it into gear and accellerate slowly. If you just put it in gear and slam it to about half throttle it does fine, and once past the dead spot you can throttle all the way back to idle and it does fine.  This is alright on open water, but is a problem and a little embarassing when your trying to put the boat in the water at a crowded ramp and you have to start the motor 4 or 5 times to get it out in the open so you can give it enough gas to get it to go.  Anyone got any ideas as to the cause and a possible solution.  I've considered putting it in the shop, but $80 an hour labor is rough to get told all I need is a $10 set of plugs or a $5 fuel filter or something simple. 

Well I finally found someone who seemed to have a little knowledge about this old Merc, an 88, not an 89 like the title says.  They told me it was a link and sync problem.  It's one of the straight 6's with 3 carbs, and the carbs are not all in sync with each other, one of them is opening too soon or not soon enough.  I ordered a repair manual and hope to have it corrected soon. 

Also on a seperate note, has anyone ever heard of a Mercury outboard running on 100:1 fuel mix?  I've been told all Mercs post 1964 run 50:1, but I had a mechanic I asked about the other problem tell me that I should be running it at 100:1. 

I have a 1988 , 80 hp with three carbs . I had a problem some what like you are explaining. It was a weak diaphram in the fuel pump. Running slow it would run the gas out but when you kicked it up to higher RPM's the pump worked fine. When you start it up check the ball see if it gets soft while idling.

I just don't see how those carbs links being out of syn would be OK after 1/2 way open. Seems like the linkage should stick or get hung up. But I'm just a back yard dude that works at it because I don't like the down time at a marinia. ( I'm been waiting inline now for almost three weeks to get mine in but I'm able to run it till they can get me in).

Just some thing else to check while you are waiting for the repair book.

Good Luck