1986 Yamaha 25 HP intermittent miss on one cylinder

[Black Tail]

I have a 25 HP Yamaha on my Sea Nymph FM164, I seem to have a intermittent miss on one cylinder. I thought it was a cab loading problem from the slow acceleration (12 pitch prop), trimmed the motor and would do 25MPH and ran fine. Now it is not running right on one cylinder, the dark plug is from the top cylinder, the light one is on the bottom, there is only about three to five hours on this motor since last tune up, I mainly troll and don't use my big motor much. I had thought at first my prop pitch was off, but I can see it is in the ignition circuit some where. What parts could it be?

 

[Black Tail]

I did a compression test and both cylinders were right at 140

 

[gm280]

First thing to do is swap the spark coils and see if the problem follows the suspect coil. If it does, you found your problem. If it doesn't then see what it feeding the spark coils. JMHO

Oh and :welcome: aboard to iboats...