78' Evinrude 75 Running Issue

[Knot Waiting]

So, I finally convinced my best friend to buy a boat; I lost a good deck hand and fuel compensator but gained a fellow boater on the lake. Anyway, he chose an older pontoon as his first craft which has a 1975 Evinrude 75hp outboard pushing it. Made sense given his boating requirements; however, I'm a lifelong Mercruiser I/O guy and unfortunately have found myself "literally" like a fish out of water in helping him with this motor. So...... I appeal to your expertise.

The problem started right off the trailer. Ran for a few minutes at idle and then died and would restart for a shorter and shorter period. I chalked it up to vapor lock on the old fuel cans and after a slight vent modification it was running again.

The toon then planed up from idle and ran "ok" for about 15 minutes but stumbled and died again after coming back to idle (no wake). This time it would not re-start even with the cap off the tanks.

I towed him to a shallow where we could look into it and found (from our on site diagnostics) the fuel, spark, and air all ok. With nothing solid to fix we wasted a few sunburn hours and ran back without issue. Until it died again upon idling to the launch/trailer. He has since replaced the plugs and coils, tested the stator, and swapped fuel tanks.

Any ideas? A pic of the build plate is attached.

Thank you all!

 

[Bosunsmate]

It could be a few things. Have you looked at the plugs electrode combustion colour, any look different from the rest?
Have you tried a drop test on each cylinder?
When it idles and starts to die does bumping the choke help? (that indicate carb issue)

 

[F_R]

Check for water intrusion into the cylinders. All too common on those motors. It comes from a warped exhaust cover plate, and will totally destroy the powerhead.

 

[fireman57]

Could be overheat as these engines have no telltale to show the water pump working. Like they said, check the plugs. If one or more is totally clean you have a water in the cylinder problem. When you said it ran "ok" I'm betting that you were running on 2 cylinders due to water.

 

[Knot Waiting]

I'm a little confused by the water intrusion concept. Wouldn't any water in the cylinders hydro-lock the engine and cause catastrophic failure?

 

[fireman57]

If it is a minor leak into a cylinder the damage won't show up for a while but when it does it's expensive. Best to take care of it now.