'77 Johnson 85hp weak spark

[iwombat]

Model number is 85EL77S

Well, I went and bought the boat I was looking at with the 85hp. Everything checked out fine (exception below) - compression even on all cylinders lower unit oil nice and clear, etc. Took it out on the river and everything ran fairly well (needs a tune up).

When I went to check the spark it was a little weak. Wouldn't jump a 7/16 with a hot spark on three cylinders. It would jump a smaller (probably 3/8 or 1/4 inch) gap very well. I'm hoping a new set of plug wires fixes that.

As for the rest of the ignition system. Power packs are new, and were replaced last year. But, the coils look like the original coils and are beginning to show their age.

I'm heading down to get impellers for both the 85hp and the 15hp kicker later today or tomorrow as well as new plug wires for V4.

I'm on the fence about just replacing the coils out of hand and keeping the old ones around for spares. If the plug wires don't solve my problem I'll be saving myself a trip anyway. Any words of wisdom on this one?

 

[ezeke]

Re: '77 Johnson 85hp weak spark

If you checked the spark with all of the plugs out, got spark on all four, but did not get close to 7/16 sharp blue spark, you could have poor grounding, which is very common, or not enough RPM, also very common.

Before you replace anything, you might want to disconnect each wire on the ignition sytem, clean them, and reconnect. Pay particular attention to the ignition coil grounds. Make sure the cylinder heads are well grounded to the block as well.

There are many discussions here about the need for 300 RPM on this series of motors [1973-1977] to get enough juice to fire properly.

 

[iwombat]

Re: '77 Johnson 85hp weak spark

Good advice. I'll forgo the coil purchase and do a proper cleaning and inspection with new plug wires (they're getting a little stiff and brittle).

My guess is not enough RPM is the real cause. I'm sure it was under 300rpm. Cleaning never hurts though, and winter is a fine time to go through it all.