Helping a friend with a 1999 Mercury Marine 60 HP 2 stroke outboard (3 cyl). Took it out 3 weeks back, idled out for about 5-7 mins, ran right at full speed for about 3 to 5 mins then suddenly lost RPM. It didn’t seem to be running rough but at full throttle it seems like it’s running at 1/2 throttle. Limped around the rest of the afternoon. We took it out the following weekend and it ran perfect all day. Last weekend it ran perfect for about 3-5 mins again after idling 5 mins out to the channel then back to the same symptoms of no power at ½ to full throttle. It idles fine, doesn’t seem to be missing. I pumped the bulb with throttle at full about 10 times. One time the engine briefly picked up speed. The other pumps had no effect. We parked the boat and let it cool for about 20 mins and it still would not produce any upper RPM.
We changed and gapped the plugs, checked the compression (110, 119, 120), took fuel pump apart, ( was totally clean inside, no rips, diaphragms and gaskets all looked ok) Used a thermal temp gauge under load and all are cylinders are warm and close to the same temp, inline spark tester shows all cylinders are firing out of water. Under load was only able to check top cylinder, the others were a bit low with water coming back up on tester) We also pulled the fuel line from the engine and squeezing the bulb produces a strong stream and it freely leaks when pressing in the end of the line with a screwdriver. Fuel filter is free flowing. All carb bowls leak out a stream of fuel when cracking up the drains on them.. Revs fine out of the water.
Seems that if the engine is stone cold it will run fine for 3-5 mins each time out. Then you can feel it stumble and the RPM’s drop and it falls off plane to half speed. Anybody familiar/experienced with this?
We changed and gapped the plugs, checked the compression (110, 119, 120), took fuel pump apart, ( was totally clean inside, no rips, diaphragms and gaskets all looked ok) Used a thermal temp gauge under load and all are cylinders are warm and close to the same temp, inline spark tester shows all cylinders are firing out of water. Under load was only able to check top cylinder, the others were a bit low with water coming back up on tester) We also pulled the fuel line from the engine and squeezing the bulb produces a strong stream and it freely leaks when pressing in the end of the line with a screwdriver. Fuel filter is free flowing. All carb bowls leak out a stream of fuel when cracking up the drains on them.. Revs fine out of the water.
Seems that if the engine is stone cold it will run fine for 3-5 mins each time out. Then you can feel it stumble and the RPM’s drop and it falls off plane to half speed. Anybody familiar/experienced with this?
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