90 hp evinrude rpmbs dropping intermittently

I have a friend that is very

I have a friend that is very frustrated and I am trying to assist as his bills keep adding up with no solution.It's a 1989 Evinrude 90 hp that typically runs 40 mph at 5200 rpm's.Recently it has been dropping to 4200 while running and sometimes stays there and other times jumps right back to 5200 before doing it again later in the day.It will do this off and on each time he goes out and always down to 4200 area.Compression is excellent and they tried a used powerpack in place of his and there was no change.Plugs also look good.The dealer says the coils are ok and I wondered if a particular coil could act like this when heated up as one possibility.Any suggestions on what else to try or look for would be appreciated.

sounds like a carb...id say r

sounds like a carb...id say rebuild all of them and see where it goes from there..it could also be a faulty tach??

"Alan.... Regardless of the pr

"Alan.... Regardless of the problem, always check the compression first, should be approx 100+ psi and even on all cylinders. Then with the spark plugs removed, check the spark, should jump a 7/16" gap on all cylinders with a strong blue flame.

If the drop and rise in rpms is a sudden instantanous change, the problem would be most likely ignition. However if the drop and rise in rpms is a slow gradual change, then that would indicate a fuel problem..... air leak in the fuel supply somewhere, a fuel restriction etc.

It could be something a small as a faulty spark plug. Do they all look alike when you remove and check them? If one appears different, make a note of which cylinder it came from.

The spark plugs should be Champion QL77JC4 or L77JC4 plugs with the gap set to .040 .

Let us know what you find and we'll go from there.

Joe "

" Thank you for the replies.As

" Thank you for the replies.As I mentioned in my original post,my friend is very frustrated and as a result he is considering storing the boat for winter and not repairing until early spring and therefore I may not be able to report back at this time.I would like to thank you for the feedback that I have passed on to him and hopefully will give him insight when he gets back to it.
Alan L. "

" Have a 94-88HP Johnson, carb

" Have a 94-88HP Johnson, carbs rebuilt, new plugs, good spark, timings OK. No load,will run up to 5000RPM, under load can only get about 2000RPM. Help would be greatly appreciated. Thanks "

" Alan, I have a 35 hp mercury

" Alan, I have a 35 hp mercury that started to act the way you have described. It would run wot and suddenly fall back to around 3000 rpm.All of a sudden it would open up again and run wot. As time went by, it would happen more often and the event would last for a longer period of time. Eventually, it got to the point that it would only run around 3000 rpm at wot.However, during the entire time it was doing this the engine would idle fine and rev up in neutral. I found a used switchbox and replaced it. It works fine now. Perhaps that used pack you tried had a problem of it's own.Are you sure the pack was good? "