So I will start by saying I know next to nothing about outboard motors. Having gotten that out of the way, I came into possession of a 1983 Mariner 25 CM 695 which is made by Yamaha. From what I can tell compression is good, carb is clean and the motor runs. My issue, and this may not even be one is that at idle, the motor seems like it is missing every now and then. Again, my uneducated guess as to what’s happening but it seems like the spark plug doesn’t fire or ignite for one rotation and then when the next rotation comes around if fires twice as hard(maybe twice the fuel mix in the cylinder) and makes the motor jump a bit. This miss or jump is at regular intervals but still happens frequently. Is there an issue here or is this just how old outboards run at idle? On another note, the motor has a sticker that says use NGK B5HS spark plugs. Since it has Champion equivalents in it when I got it, I went and got a set of NGK’s. Looking at both plugs next to each other, the Champion’s look like they would extend further into the cylinder than the NGK’s. Will this make any difference?
