Re: 1958 35hp OMC drops one cylinder at low throttle

[David Stanton]

I’ve had bad spark plugs do this – try swapping them between the cylinders and see if the problem moves to the other cylinder. You have probably eliminated spark plugs as a cause already, but it’s a quick and easy thing to try.

Take off the inspection cover from the top of the flywheel and check points gaps, and visually look for cracking insulation on the coils. I’ve seen original coils that look pretty bad do intermittent things like this. Like Chris said, when you test for spark as you did, with the spark in free air with no compression, you should get a 1/2 inch spark to insure it runs under all conditions.
Dave