1979 Johnson 70 dies after long runs **UPDATE: intermittent spark**

[Dabbler_E]

This thread is partly to solicit input, and partly to record what I find out for others.

**UPDATE:
**HERE'S A SYNOPSIS OF RESULTS SO FAR:

**Intermittent spark on one cylinder
**Which cylinder has a problem varies over time (not always the same cyl)
**Appears to show up most when timing is advanced
**Sensor coil and ignition coils both show resistance within specs
**Problem comes and goes

**The fact that it only applies to one cylinder, and which that it moves from **cylinder to cylinder seems to narrow it down to the sensor coils (timer **base), but the resistance test came out OK. I don't have the special tool **to do a sensor coil output. It seems to me that the only other possibility **is the power pack. Could that cause these symptoms? I would expect a **power pack problem to affect all cylinders.
**END UPDATE *****

I have a 1979 Johnson 70 hp that cold-starts great, idles fine, and runs up to WOT no problem. However, on a couple recent occasions it has died (slowed down then stalled) after high speed runs (4500-5000 rpm) lasting for 15 minutes or so. Last night it happened while we were getting out of the way of a popup thunderstorm, and fortunately it decided to die in sight of the dock.

In both cases, it was running fine, then started slowing and stalled within about 10 sec, once the boat came off plane. Could be restarted with difficulty, but it only kept running if spark was advanced. In the first instance, I was able to get it back running in gear (very slowly) by quickly dropping the starting lever and pushing the throttle to advance spark. Last, night, I was not able to even get that.

At first, I thought the fuel gauge was faulty and we were out of gas, but no, there was plenty of gas and the primer bulb was hard. All 3 plugs wet but not fouled. After we got home (motor cold), I was able to start and run it on muffs with no problem.

My thinking at this point is a flaky charge coil, but I aim to take the boat out today and see if I can replicate the condition and do a spark test.

 

[mrcrabs]

Re: 1979 Johnson 70 dies after long runs

Re: 1979 Johnson 70 dies after long runs

I had a like similar experience w/ my 70hp when dirt daubers built a nest in my fuel vent, Looking back I was able to make a wide open run for about 5 min before I started losing power. after sitting a few minutes I was able to make the run back.

I see your from C,dale...Rural Pope Co. here, I guess you run on Kinkaid and Crab Orchard?

 

[Dabbler_E]

Re: 1979 Johnson 70 dies after long runs

Re: 1979 Johnson 70 dies after long runs

Mrcrabs -- yep, mainly Crab Orchard. Kinkaid is a little too much of a zoo for me.

 

[Dabbler_E]

Re: 1979 Johnson 70 dies after long runs

Re: 1979 Johnson 70 dies after long runs

The motor started & idled fine on muffs this morning, so I set off to the lake with a spark gap tester, spare spark plugs, multimeter, inductive timing light, compression gauge, a full tank of gas, and a trolling motor (for return if stranded). My aim was to run it until it misbehaved and then figure out what the deal was. Fortunately, as soon as it was in the water I knew it was still an unhappy engine. Hard to start and would only run with the warmup lever up (spark advanced). Pulled off to a courtesy dock and decided to run it for a while at high idle to check how the plugs turned out (one wet plug would imply a dead or flooded cylinder).

While it was running, I used the inductive timing light on each plug wire in turn. #1 was very sporadic and #2 and #3 were fine.

Great, I thought, now I'll swap coils and see if the problem follows the coil. Before that, though, I thought, "Maybe I just didn't get a good reading on that wire" so I repeated the procedure. Now there was intermittent spark on #3, and #1-2 were fine. Over several minutes, I determined that there was always one flaky cylinder, but which one was flaky changed over time.

So, now I have a strong hunch that it's the timer base. Waiting for the block to cool to check ohms.

 

[Dabbler_E]

Re: 1979 Johnson 70 dies after long runs

Re: 1979 Johnson 70 dies after long runs

Resistance checked out for both sensor coils (20 ohm) and ignition coils (545 ohm). Then I tried to replicate the problem while running on muffs. Using the inductive timing light, all cylinders were firing normally at idle, but when I would lift the warm-up lever (advancing timing) until it ran at about 3000 rpm, one cylinder would get flaky (#3 this time).

Had dinner, then went back and fired up again. This time it showed fine spark on all 3 cylinders even when running up to 3000 rpm.

So:

Intermittent spark on one cylinder
Which cylinder has a problem varies over time
Appears to show up most when timing is advanced
Sensor coil and ignition coils both show resistance within specs
Problem comes and goes

The fact that it only applies to one cylinder, and which that it moves from cylinder to cylinder seems to narrow it down to the sensor coils (timer base), but the resistance test came out OK. I don't have the special tool to do a sensor coil output. It seems to me that the only other possibility is the power pack. Could that cause these symptoms? I would expect a power pack problem to affect all cylinders.