99 2-stroke Mercury 20 Hp Black Oil Leak and Corroded Plug Wires

[scumfrogmillionaire]

Alrighty two-stroke experts, couple concerns/questions regarding my 99 20 hp Mercury:

1. Ran the motor in a tank for 30 minutes after replacing the pump and water jacket. Put in the garage and noticed a semi-large puddle of black oil coming from behind the prop (see picture). Took the prop off and there was a bunch of fishing line behind it. Pulled all the fishing line out fairly easily and put the prop back on. Checked the lower unit oil, and it looked like new (green, not watery or milky). Is the oil coming from the prop just unburned exhaust? Why is there so much? This is the first time running it since last year. There is no sign of damage to the prop shaft seal even though there was fishing line behind the prop.
2. Completely unrelated to #1, but the motor knocks a little bit during idle and sometimes shuts off. Other times it’ll idle for as long as you need it to. Compression test is not good (120, 110) so I feel that this may have something to do with it. Brand new plugs but I did notice that one spark plug wire was extremely green and corroded. Will cleaning this up or replacing this affect the performance? If not, I won’t bother.

Thanks for all of your help. I just spent 2 full days dealing with broken thermostat bolts and replacing the water jacket / thermostat housing on this. Combine that with replacing the pump, plugs, lower unit grease, etc. I haven’t even been on the boat this year and have about 20 hours into this thing.

 

[flyingscott]

Your compression is good. The oily spot under the motor is normal. If you want to know for sure do the sniff test. As for the knocking could be as simple as the idle mix adjustment.

 

[scumfrogmillionaire]

Thank you flyingscott