Hey livemusic
I have a 2005 Mercury 50 HP ELPTO 2 cycle. I got it last year Mid July and I believe that although the motor was maintained properly, it sat out a season. I had much troubles with it running rough and surging at WOT. I ran it with Quicksilver (Mercury) fuel additive which you can get at walmart. I had the dealer rebuild the carbs, replace the reed valves, all of the fuel hoses and primer bulb, experment with different carb jets, adjust timing, so forth. I had that boat back to the mechanic 5 times in 3 months and finally the snow started falling I put it to bed for the winter. I checked everything I could, compression, I rebuilt the fuel pump, experimented with different spark plugs, it really had me baffled. I was using Shell 87 octane with the Quicksilver (I think it's their #1) fuel additive.
Well anyway, I had all winter to think about it and looked into ethanol free which I can get athough it's a little inconvenient. So, I thought before going that route I would try premium 93 octane fuel. I have had the boat out 3 times so far and I've run the heck out of it and it's behaving quite well. I think I'm going check the timing before I take it out next which I think will be tomorrow, the timing may be a bit too far advanced. But the premium seems to be the solution. It is odd to my thinking that no one has suggested premium but there it is.
Make sure the stabol you use is specifically for ethanol treatment, not all stabol products are. The Mercury Quicksilver, as I said, is available at walmart. Regardless, use it always.
Good luck--Tom