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'75 Evinrude 70HP Lacking Power

F

fluketilyoupuke

New member
I recently bought this outboard, and at first it gave me no trouble. It was a little sluggish getting out of the hole, but I think that was related to the motor being slightly small for the boat. I had a 88spl on the boat before and that motor really made the boat fly.

What happened to me the other day while out fishing was that the motor would at first get up to speed just fine, but then slowly bog down until eventually it would drop out of plane. I would then arrive at my fishing spot and kill the motor. When I went to move again it would do the same thing. At the end of the day when I had to make the last long haul back to the harbor the motor completely bogged down and stalled out and I could only get it to run while at idle.(I had to be towed in by a passerby)

Here is what I have done and I know about the motor.

It has good compression. 125 all cylinders I tested it myself.

All cylinders have spark. I put an in line tester on and all lines were sparking nice and even from what I could tell but Im no expert.

All the electrical wires seem to be in good shape with no corroding connections or burnt parts.

The waterpump is working. The engine is almost cold to the touch while running. Maybe this is a problem? I am going to remove and inspect the thermostat.

I cleaned the carbs and fuel pump but did not replace the seals. All seemed to be in relatively good shape. After I did this the motor started up but would quit when put in gear and it seemed to get worse the more I tried. Also hitting the choke does not help at all.

I thought that maybe it is the gearcase seizing up or something, but when I put the motor in gear when off, I am able to turn the motor by spinning the prop without too much resistance.

My limited understanding of outboard mechanics it leading me to believe my problem is some electrical part like the stator or timer base.

If someone could tell me how I could easily test these parts without just replacing them and seeing what happens, that would be great. Any other advise or questions are also welcome.

Thanks all!!
 
What gap is the spark jumping? It should be a bright blue lightning like spark that makes a snap. It should easily jump a .5" (12mm) gap. Weak spark may manifest itself in a multitude of ways, not always easily attributed to the source.

My 77 70hp had issues somewhat like you're describing, and it was intermittent spark on the middle cylinder. Ran great on the hose, but horrible in the water.

Also check all of your grounds, mine had a lot of gunk between them and the block causing bad grounding issues which will cause intermittent ignition issues as well.
 
if you want to know how to test the electrical components look up what you want to test coil, stator, timer base assembly.....ect on youtube they usually have those test videos that are step by step you will just need a digital multi meter
 
I am not sure what gap the spark is jumping I have one of those cheap testers you can get at harbor freight for $5. It only has a bulb that lights up when there is spark. Is there another tool which would be more helpful?

Was it poor grounding which was causing your problems?
 
fluketilyoupuke said:
I am not sure what gap the spark is jumping I have one of those cheap testers you can get at harbor freight for $5. It only has a bulb that lights up when there is spark. Is there another tool which would be more helpful?

Was it poor grounding which was causing your problems?
Click to expand...

Look up 'air gap spark tester'. You will want one that's adjustable, so you can set it to .5"

Just because a plug will fire at idle, or even outside the cylinder, doesn't mean it will fire when under load. If the ignition system can jump a half inch gap, that means it's strong enough to fire under load, inside a pressurized combustion chamber.

I did some other stuff while doing the grounds, so I'm not sure if they were my problem, but bad grounds are never a good thing.
 
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