I just bought a 1987 70hp evinrude on a boat. First time out on it it ran perfect for several miles and most of the day. Then while cruising at speed back towards the boat ramp it seems to have lost a cylinder. I throttled back to an idle. It seemed to be running on all three at idle. With the throttle part way open it would go from 2 to three cylinders back and forth. The finally stayed on all three. Got back on a plane and ran fine. Closer to the boat launch problem back. Would losing one cylinder keep it from planing? Looked at plugs seems to be fine, all three look the same. So coils we're fairly cheap so I put new ones on. Next time out, higher elevation problem is much worse. Still goes between two and three cylinders but stays more with the two? Please help before I start throwing more new parts at it..thanks
