72 Johnson 25hp warm up and start to missfire

ditchen

Seaman Apprentice
Joined
Mar 18, 2009
Messages
47
I just have to share this in hopes it may help someone in the future. A little long winded, sorry about that.

I obtained a 1 one owner 72 25hp Johnson that spent its whole life on a pontoon boat. They would use it to putt around this small lake and the motor hardly ever got over idle and did not get too many hours put on it.

This thing had a problem when it warmed up it would start to miss fire, almost sounding like it dropping a cylinder form a fouled plug. They would let it cool off and it would run ok for about an 1hr to 1.5hrs and start doing it again. They had it in the marina new coil packs, points, plugs, wires, carb, head gasket, the list just went on and on.

They through the towel lower in when the lower unit frozed and cracked the housing. They bought a new outboard and I got this one cheap. 135psi a hole, pressure test excellent, I went through every thing again, very reliable outboard, starts right up and would get ya back to the dock. Just when it heated up it would start to miss fire and drop a cylinder (appeared to be both cylinders), but would run WOT. Swapped the coil packs again with a good parts motor, still same problem. I was at my wits end and I knew it was ignition. So whats left; the flywheel. Swapped the flywheel.

WHA LA, it was fixed. The motor sounded noticeable smoother and I ran around the lake with it all day at all different speeds no problems. So just to see if that was it or not, I put the old one back on and ran it in the test tank; it was right back to not running right. Change it back to the replacement, back to running like new.

I am guessing maybe a poor magnet, the flywheel physically looked and measure out good.

Any who, I just wanted to share this.
 
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