'77 Johnson 70hp, ignition issue?

[egclassic]

I recently acquired this motor for my pontoon. I rebuilt the carbs, fuel pump, installed new plugs, and lync'd and sync'd.
The motor runs great, for the most part. Today I put the boat in, started it up and went to accelerate. The motor fell on its face, seemed like it lost a cylinder or something. After running like this for about 30-40 seconds, it picked up and ran great the rest of the day. Where do I start to diagnose this? I assumed maybe a coil and thought about just replacing all 3, since they appear to be original. What is the right way to test the coils?

 

[BonairII]

Re: '77 Johnson 70hp, ignition issue?

Don't waste time testing the coils....just replace all 3 of them.

I had the same prob with my 75hp 3-cyl.....coil was going bad. I replaced all 3 for both piece of mind.

 

[egclassic]

Re: '77 Johnson 70hp, ignition issue?

That's my line of thinking as well. I feel they are all the same age, why not replace them all.
But, not sure if thats my problem yet. The way my luck goes, I'll replace all three and still have the same problem. I tend to jump into things feet first, and cost myself money!

 

[BonairII]

Re: '77 Johnson 70hp, ignition issue?

I tend to jump into things feet first, and cost myself money!

A lot of people here(esp newbies) tend to want to throw parts at their motors instead of systematically diagnosing the problem, so you're not alone.

Impatience(to get the motor fixed) leads a lot of us to make bad decisions.

Back to your motor......
Your coils are old and need to be replaced anyway, so you won't be wasting money even if they aren't the problem(but I believe they are the culprits).

On any new(to the owner) motor: I'm a big believer in changing the t-stat, impeller, LU oil, spark plugs, coils....unless the PO can show you proof that they were recently replaced.

Way too many people immediately assume that their powerpack is bad and replace it, only to find out that something else was the problem. I almost did it myself. I had the same symptoms as you are having with your motor. It started with a "hiccup" on the first launch. Then a cylinder stopped firing the 2nd trip out with my new(to me) motor. I changed the coils and also realized that I was using the wrong spark plugs!
The motor runs like a dream now.

 

[egclassic]

Re: '77 Johnson 70hp, ignition issue?

Ok, replaced all 3 coils last week. Took the boat out today and all seems good.
But, there seems to be a slight "stumble" (for lack of better terms) in the upper rpm band. If I back her off just a bit below WOT, it seems to be ok. Today when I got home, I pulled all three plugs did comp. test and open air spark gap test.
Comp. on all three cylinders was 120.
Spark jumped 7/16" with no problem.
Could my WOT stop just not be adjusted properly?