I recently acquired a 1992 Mercury 125hp engine from a private party as part of a complete used boat package. The engine has very low hours for it's age (just over 100 hours), and is very clean under the cowling, which seems to confirm it's low hours. When warmed up the engine runs well at idle and at plane speeds. However, when accelerating from an idle up to running speed the engine mis-fires, sputters, and surges. It also behaves this way in a narrow range of throttle settings I would call a fast idle, i.e. the range at which the hull pitch is really inefficient, nose up and plowing water. I've replaced the old fuel with fresh non-oxygenated regular gasoline, replaced the spark plugs, fuel-water separator filter, and in-line fuel filter without any noticeable improvement. I've been told this particular engine model runs on only two cylinders at lower throttle settings. The other two kick-in as you pour the coal to it. If that's true it makes sense. I'd say the problem I'm having happens around that kick-in point. Once you get above that point, the big dog really eats and you're flying in no time. I hope my explanation is clear, sorry it's so wordy.
