Igniton shorts at two speeds

[nemer]

I have a 1979 175hp v6 mariner. I have replaced Stator, trigger, both switch boxes, ran the engine in the dark spayed wires, replaced all wires in switch boxes. When I run the boat it is fine until 1800 rpms. Then it misses like cylinders are turning off. When I increase the speed it smooths out and runs right up to 3600rpms where it starts to miss again. Then it runs fine up to WOT. I notice some fouling on #1 and #5 plugs. The carbs have been gone through and cleaned with new needle seat and gaskets. The only thing left that I don't know about is the Idle control box. Could this be the culprit? I disconnected it and taped it off. The boat was smooth up to 3000 rpms. I could not get it go higher it started to bog down. Any Idea's as to what the problem is?

 

[Joel K]

Re: Igniton shorts at two speeds

I'm not familiar with your motor. But I had a problem similar to yours with a 40hp 4 cylinder. I found a bad coil causing it to drop out at roughly 3000 rpm. If you have a DVM with a diode scale and have individual coils for each plug, just check one against all the others and see if you find one or more with no reading. You could probably do the same test on ohms if you don't have a diode setting.
One other time I had two coil wires close enough where they connect at the box that they would occationally arc.