1985 Mercury 35hp Surging under load

[Ian Short]

Hello, I have a Mercury 35hp that has stumped me for 6 months. Sometimes it will run for 30 mins before surging at about 4000 rpm and up when accelerating, sometimes it will do it instantly. It must be under load as it will run perfectly in a test tank. When having the issue, it seems as if one cylinder cuts in and out as you're driving on a plane. Occasionally, the entire motor will stall, but it always starts back up. I have cleaned and rebuilt the carburetor, replaced the fuel lines, rebuilt the fuel pump, and tested the fuel tank and vent. I've replaced the stator, the switch box, the ignition coils, the plugs, and the wires. Everything in the ignition system both DVAs to spec and OMS to spec. But the number 2 ( bottom ) cylinder spark plug looks very clean compared to the upper cylinder plug. I boroscoped the pistons and see no signs of steam cleaning or water intrusion. I've split the powerhead and cleaned all carbon buildup, replaced every gasket on the engine, and replaced the reed valves and upper and lower seals. Compression is 125 on both cylinders, hot or cold. Cylinder leak down is 5% on both cylinders. I'm unsure on what else to consider, any ideas would be appreciated.

 

[Ian Short]

Just for reference, this did turn out to be a small crack in the water jacket expanding when water pressure and heat do their thing; a new powerhead fixed the issue.