'79 Johnson 15, Stumbling when warm, and strange DVA output

[Deano_G]

Hi Folks.

I'm troubleshooting a new to me '79 Johnson 15 which I run on my 12' Tinny. Runs fine at low speed or part throttle, but after a minute or 2 of WOT, it stumbles and will die back to just above idle, whereafter it will idle around OK. After 5 or so minutes of idling, I can go WOT again for a minute or so before issue repeats.

From what I've read it sounds like it could be a sign of a power pack on the way out, so I've been working my way through the ignition system. Using my home built DVA adaptor, I got within spec readings for Sensor and Charge coil voltages, (which proves those items are good, and that the DVA adaptor works properly), however I can only get a reading of 5 volts for Power pack output instead of the +150v or so it should be. The 5 volts reading is the same across all RPM.

I've also tested the spark jump, and both coils can clear a 1/2 inch gap OK.

Should the engine even be able to run, with a 5v PP output? I'd think surely not? But then I can't explain why it will start and part throttle fine, and go WOT for short periods of time...

I might just bite the bullet and buy a new PP, and if it's not the issue, then I can keep it as a spare. Any other thoughts?

Fuel system appears OK, helping with primer bulb does nothing when it's stumbling. Lots of water out the indicator, and can keep fingers on the block without burning.

 

[oldboat1]

fuel pump? Mixed opinion here, but consider a cheap replacement (for testing if nothing else). CD2 power pack may turn out to be the culprit. WOT ist verboten except in the lake.

[troubleshooting source: https://bpi.ebasicpower.com/downloads/cditech/CDI-Troubleshooting_Guide-JOHNSON-EVINRUDE.pdf ]

 

[Joe Reeves]

Start with the basics..........................

Check the compression. What is the psi reading of the individual cylinders? The actual numbers... not an expression of good, great, etc

With the spark plugs removed, check to see that the spark will jump a 7/16" gap with a strong blue lightning like flame... a real SNAP!... Does it? Note that the 7/16" gap is important!

The lowering of the rpm after you've run awhile at full throttle.... Is that drop of rpm a instantaneous sudden encounter... OR... is it more like a engine running out of gas... a gradual lowering of rpms?

Using a portable tank?... Does the Fuel Primer Bulb have a tendency to flatten out, collapse somewhat? If you pump the fuel primer bulb when this problem is encountered (acting as a manual fuel pump), does that have an effect on this problem?

 

[Deano_G]

Thanks for you suggestions. I'll be doing a partial disassemble of the motor next weekend (carb clean, thermostat replacement and water passage clean) and will report back with any further findings.

 

[Deano_G]

**UPDATE**

Found what I assume to be the culprit. I decided to dive in and strip it down to a bare block. Given I want it to be reliable, I thought this is the best way to make sure its all working as-new.

Proceeded to pull the water cover, which was mildly gunked up but still able to flow water.

Pulled the head, which had water channels the better part of 70% blocked, and only the rusted out remnants of a thermostat left. I cleaned out the gunk, to find that there was significant corrosion in the head, to the point that there was a slight leak path between one of the combustion chambers and adjacent water jacket.

I guess when it was cold, that there must have been just enough gasket sealing effect to let it make power, but once it warmed up and the metallic parts expanded, then it started leaking and not getting proper combustion.

Cylinder walls are OK with no marks, and only minor scoring on 1 piston skirt. And only snapped off 2 bolts when removing the exhaust cover... Exhaust cover side water channels were pretty clean.

I will finish the tear down tonight, so I can measure and check everything else, and finalise my parts list to buy.

Will post some of the gory photos later tonight once I'm back home.

Cheers, Dean.