Re: 1974 Johnson 15 hp runs on 1 cyl at low speed

[Don]

So, the spark is arcing between the rubber boot and the plug porcelain, is this correct? And, this is a brand new coil? Seems like we have heard this same story recently….Is this a new OEM coil, or aftermarket? In any event, I would go with Mumbles’ advice and replace the plug boot/terminal assembly. I would definitely use the OEM piece Mumbles showed us, even though it is a pain to install, don’t try some other style. How about the plugs? What kind are they? I would replace them as well, gapped at .030".
Unfortunately, you got the first year that OMC produced these style engines, and there were many ignition/plug fouling issues. There are about four different update kits that are supposed to alleviate this condition, but the effectiveness is questionable. This ignition system is kind of a hybrid system between a conventional magneto and the CD2 system that was introduced in 1977, there are still points under the flywheel. You won’t find too many guys that like this system. It can be made to work correctly, but everything has to be right.