1999 Mercury 60hp 2 stroke will not reach/stay full power

[Leadhead143]

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 min then suddenly lost RPM. It didn’t seem to be running rough but at full throttle it seems like it’s running at ¼ to ½ throttle and struggling a little. 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 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. 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 are full. Revs fine out of the water on muffs.

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. Anybody familiar/experienced with this?

 

[racerone]

Perhaps a timing light to find the cylinder that is loosing spark if that is the issue..

 

[Leadhead143]

Perhaps a timing light to find the cylinder that is loosing spark if that is the issue..

I used an inline spark tester under load on the top cyl and it was showing spark.. The other two holes the tester was too close to getting very wet the way water was kicking up so didn't do them in the water. So I did all three cylinders about 30 mins after pulling it out of the water and it shows spark on all. Couldnt tell if the spark was weak though. Also tested by grounding out the plugs and in the bright sun, not so easy to tell the quality of the spark. The fact that it is somewhat random in nature and seems to run fine for a little while when the engine is overnight cold does point me toward electrical and not fuel.

 

[Leadhead143]

Bump.. still looking for help.

 

[jbuote]

Hey there!

Not a pro here, but I'd look at post #2 again..
Take a timing light (inductive), clip it to one of the plug wires, and run it on the lake..
Watch the timing light and if the flashing stops or has dark spots then that cylinder isn't firing correctly at temp..

Then do the same on the other cylinders...
Could be an ignition problem when it gets warm...
Only way to tell, is to test it while warm.

In-line spark tester (bulb type) is pretty much useless...

To attempt to check it on muffs or in a barrel, use and open air adjustable spark tester.
Not sure what the open air gap should be for that engine, but I "THINK" about 3/8" gap should get a good idea anyway..
Should be a nice blue spark with a crisp "SNAP!!"...

The gurus, like racerone, can tell you the correct open air gap if not 3/8".

Hope it helps!