1977 Evinrude 20hp seems to have lost power

[HypnoCraft]

I just bought this motor, it seemed to work fine when shown, but I assumed I would have to do a little work on it to get it running the way I want it to. I was out on the lake and it was running fine. It was able to get a 10 foot rib onto plane with 2, 200 pound people on it plus the motor and 5 gallons of gas. we were able to get up to about 20mph (not even maxing the throttle). Once we had been going for a while as I was coming into shore and lowered the throttle it started to surge from low to high throttle, so I believe a cylinder was dropping. after this surge period I tried to go out with probably around 450 or 480 pounds of people on board and it wouldn't get onto plane at full throttle. I went home and looked it over a little bit. found that a spark plug was a bit loose, so I tightened that up.

I have tried doing a cylinder test a few times by pulling a plug while its running and seeing which cylinder is weakest. they both ran pretty similarly, but when I pulled a plug wire at a time and tried to start it the upper cylinder was having problems and wouldn't start when hot.

The spark plugs look as though they may be fouled because they look to have wet oil fouling, but these plugs look to be basically brand new because the person that sold it to me had just fixed it up with a new fuel pump, spark plugs, etc.

I am going to test the spark with a spark gap either this weekend or next week.

The model is 207B62S from 1977, or that what the serial plate says anyways.

So if anyone has any suggestions that would be great, and f anyone knows where I can find a manual for this engine that would be great as well because I have not been able to find one in my searches.

 

[jimmbo]

First recommendation is as always, a compression test. Check for good spark(can it jump a 7/16” Gap?), fuel is next. 500 lbs of people in a 10 ft boat sounds more than a little dangerous, as does a 20hp. Maybe you were actually lucky it didn’t plane.