I recently became the owner of a 1969 Evinrude Sportsman 155. It has the Buick "Nailhead" V6. We were told that the boat was lacking power and the thought was that it needed a valve or piston job. After sitting for about a year, it started right up in the driveway with the hose attached. Right away we could hear a tapping noise and suspected a valve issue. Upon further inspection, we found that the exhaust manifold had a small hole and when patched with high temp JB Weld, the noise went away and the engine for its age was quiet. We took it to the lake and began our first test run to see how it handled. The boat powered right up and went on plane and we were shooting across the water. Unfortunately, the fuel tank even though it was reading full, was empty. We ran it completely out of gas. Luckily a friend was with us and towed us in. The very next day, I took it to the gas station and filled up with 18 gallons of gas. We took it back to the lake to continue our test and for some reason, the boat would not go above 2800 rpm and would not go back on plane. We looked at the fuel filter and there was very little gas in the filter. More investigation found that the fuel pump was bad and the fuel filter had dirt in it. We changed the fuel pump and filter restoring the flow of gas to the engine. Once again we connected the hose and started it in the driveway. The engine ran strong, smooth and quiet. We could push the throttle open full and the tach pegged out at 6000 rpm. We decided to go back to the lake and test. We backed it in, ran at no wake for about 5 to 10 minutes, once the no wake zone was cleared, we throttled up but it would not go above 3000 rpm. After several up and downs on the throttle, back to neutral shifting forward and reverse and once again attempting the throttle up again, still no higher than 3000 rpm. Placing back in neutral, we did a throttle up again and it went up to 6000 rpm. Returning home, I figured that it was not an engine issue but maybe a Sterndrive problem since we had not changed any fluids yet. I did an upper and lower gear case fluid replacement. Both upper and lower fluids were a yellowish milky consistency. I replaced with Type 'C' lubricant and trecked back to the lake. Unfortunately, it was the same result. No more than 3000 rpm were achieved under load. In neutral however, full rpms. I am at a loss, can anyone offer me any assistance as to why the boat would go up on plane and run fast until we ran out of fuel and now I can't get above 3000 rpm when in gear.