Whats up with the spark?

I have a 1987 25 HP Mercury, I am haveing spark issues, it started a about three weeks ago. After running just a few minutes of running in the water Iwould have a cylinder go down ..within minutes it would light back up and be fine the rest of the time we were out. yesterday I unloaded the boat and it would not start...no spark at all, when I got it home Ichecked the ground stud and other connections, took out the plugs and turned the key...good spark in both cylinders. later in the day I tried again no spark. this morning I had only one cylinder and now I have only one cyl. but is is a different cylinder and it is intermitten. I pulled the flywheel and cleaned some surface rust on the mag, but it didn't help Anyone have any wisdom to share I am stuck?
Thanks
Tom

Re: Whats up with the spark?

Bad/intermittent connections? If I were you I'd buff all of them to a shiny bright luster and reconnect.

Re: Whats up with the spark?

Dave, I had this problem when I first bought the boat in May, I cleaned every connection I could find, including any connections in the throttle control. I was at wits end when my spark came on and worked fine for awhile. Could it be the ignition switch?
Tom

Re: Whats up with the spark?

I would strongly suspect components in the switchbox breaking down after the get warmed up. Unfortunatey there is no way to test the switchbox.

Re: Whats up with the spark?

Boatom said:

Could it be the ignition switch?

Click to expand...

Nothing in the switch can cause one cyl to drop. All of them, yes.

Re: Whats up with the spark?

Well this morning I did not have any spark when I checked it before going to work, when I came home for lunch I have a good blue spark on the top cyl. only the next time I may not have any ..would the switch box give me these constantly changing conditions?