I recently bought my first boat, a 1985 190 well craft with an 170 Mercruiser engine. It sat for 3 1/2 years unused before I purchased it and has required a good bit of work to keep it on top of the water. Here is a run down of what has been done mechanically. Removed and cleaned fuel tank, replaced fuel lines (fill, vent, and delivery), replaced fuel pump, rebuilt carb., replaced plugs, wires, cap, rotor, points, condenser, and coil, unstuck distributor weights, set timing, and replaced water pump in lower unit and to top it off, until I bought this one, I knew nothing about boats. I've had it out several times working bugs out of idle and take-off issues, now that those are corrected, I'm losing about 25% power after 15-20 minutes of 1/2 to 3/4 throttle, it doesn't loss plane, but there is a noticeable difference in engine sound and speed. I noticed that the temperature normally runs at about 150 on the gauge and when temperature reaches about 170 that's when I have the power loss. If I leave the throttle in the same position, the temperature will come back down after about 5 minutes and the power returns. Also found that if I trim as far up as possible it doesn't reach the 170 mark and runs fine. What can be causing the loss of power and what is the relationship of the power loss to the temperature?
