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Dopey39

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Like a lot other posters here, I'm losing power at WOT. I thought it was a electric issue but the switch box and coils have been replaced. The stator and trigger all check ok on the multimeter. So maybe a fuel/air issue? I have replaced all fuel lines from the tank to the carbs and rebuilt the fuel pump and carbs too. I replaced the gasket on the water manifold too.

Could the exhaust gasket or head gasket cause this???

Each cylinder is showing 110 psi compression.

Below is pics of the plugs. left is top. middle is middle and right is bottom cylinder.


IMG_1475.jpgIMG_1476.jpg

Thanks
 
No model / serial # was posted.----I don't think it is a bad head gasket.----Simply because there is no head gasket on a 3 cylinder 2 stroke Mercury outboard.
 
i'm kinda having the same issue with a 1994 75. Its killing me i can't figure it out! Share your info if you get yours worked out and i'll do the same.
 
Will do. I have replaced almost everything on the engine. It is weird, last night my stator quit. I had test fine on the meter maybe that had something to do with it
 
The whole story on the motor.

1991 Mercury 75hp, ELPTO, ser# 0D0956xx. The motor sat for many years. It sat in a yard and hadn’t been started. When I got it I got it started and it ran fine at idle but would lose power at WOT. I have done the following:

Changed the lower unit oil (looked ok)
Rebuilt all carbs
Replaced all fuel lines from the portable gas tank to carbs and lines that go to carbs
Compression check 110 psi all 3
Replaced water manifold gasket
Replaced switchbox
Replaced all coils
Rebuilt fuel pump
Replaced spark plugs
Checked exhaust system for clogs, was clean
Pulled electric panel and checked all bolts and grounds
Timed engine with light
Checked impeller
DVA checks showed in spec

Last trip out I didn’t notice any difference. When I got home I did some checks and found no power at any of the plugs. A stator check shows an open circuit between red and blue wire. I order a flywheel puller that is supposed to be here tomorrow.
 
Dopey39 said:
The whole story on the motor.

1991 Mercury 75hp, ELPTO, ser# 0D0956xx. The motor sat for many years. It sat in a yard and hadn’t been started. When I got it I got it started and it ran fine at idle but would lose power at WOT. I have done the following:

Changed the lower unit oil (looked ok)
Rebuilt all carbs
Replaced all fuel lines from the portable gas tank to carbs and lines that go to carbs
Compression check 110 psi all 3
Replaced water manifold gasket
Replaced switchbox
Replaced all coils
Rebuilt fuel pump
Replaced spark plugs
Checked exhaust system for clogs, was clean
Pulled electric panel and checked all bolts and grounds
Timed engine with light
Checked impeller
DVA checks showed in spec

Last trip out I didn’t notice any difference. When I got home I did some checks and found no power at any of the plugs. A stator check shows an open circuit between red and blue wire. I order a flywheel puller that is supposed to be here tomorrow.
Click to expand...

This sounds eerily similar to mine. lemme know if the new stator fixes the surging at WOT. I have the red stator and it only has one speed windings. Merc customer service said he thought my ignition system sounded happy...i just wonder what a stator does when it "starts to fail" . $400 gamble.
 
Well it was not the stater I pulled my fly wheel off stater and trigger are fine so now I have decided to pull the powerhead and check the lower crank case seal I can’t think of anything else to check at this point
 
Dopey39 said:
Well it was not the stater I pulled my fly wheel off stater and trigger are fine so now I have decided to pull the powerhead and check the lower crank case seal I can’t think of anything else to check at this point
Click to expand...

Whats the bottom plug look like? How about the top of the bottom cylinder... if you are getting water in thru the bottom seal my understanding is all that will look like its being washed and will be clean. But thats just what i've read...who knows. My bottom cylinder is nice and carboned up like the rest of them.

Something the mechanic said today to me was if any item is bleeding voltage to the same ground as the coils and switch box it can cause weird issues like this...i'm planning on checking that.
 
That’s weird. I found my kill switch was bad and I hooked it from the switch box. Maybe I should test run it before I pull the head
 
Dopey39 said:
That’s weird. I found my kill switch was bad and I hooked it from the switch box. Maybe I should test run it before I pull the head
Click to expand...

Yep I have voltage on the plate all the electronics ground to. How do I find the offending component without burning up other components. Think I'm getting somewhere!
 
I would start at the switch box, see where its grounded, unbolt that and see it you still have voltage. Then go one item at a time.
 
Dopey39 said:
I would start at the switch box, see where its grounded, unbolt that and see it you still have voltage. Then go one item at a time.
Click to expand...

Well plot thickens on mine...sorry not trying to hijack your thread..but we are experiencing similar issues with similar motor.

So i'm getting negative voltage if i put the red meter(set to measure DC) lead on the plate the electronics mount too(ground). It increases if i turn the key to the on position. if i start the motor it jumps all around.

I have no idea what negative voltage to the plate means...i would expect to see positive voltage if a component was shorting to ground...i'm not much on electricals tho.
 
I just pulled the seal off and it looks great. I did notice this check valve that is on the lower part of the powerhead with a rubber hose that goes to another check valve at the the top of the case. This valve was blocked was dry oil/gas so i have to get another one. Maybe that will help.

IMG_1479.jpg
 
I didn't know it was a check valve when I first got it out I ended up breaking the screen on the bottom. I have to wait for the new gaskets anyhow. But thank you for the advice.

Any thoughts on how to remove the 30+ year old paper gasket from everything???
 
I will give you two ways I use, first I use a stiff putty knife (some are very flexible you want one that is downright stiff) and sharpen the tip like a wood chisel blade just one angle not a v like a cold chisel till it is as sharp as you an get it an use that to get what you can without gouging the aluminum and whatever's left use a spray gasket remover to soften it so it can be scraped off.
 
I forgot, did you realize that those two check valves are different? They check the flow in opposite directions, one allows air out and the other only allows air in.
 
I didn't pull the top one. It is pressed in and to be honest, I'm too scared to try to remove it. It did let air come into the top chamber. I'll have to see if prevents air from leaving.
 
That the upper/lower crankshaft bearing lubrication. It pulls puddled fuel of bottom bearing and uses it to assist lubing top bearing
 
With that bottom valve being plugged do you think that may have had been part of my problem with my engine losing power at White open throttle
 
With that bottom valve being plugged do you think that may have had been part of my problem with my engine losing power at White open throttle
Click to expand...


Nope...How does motor act when it looses power?..just slows down or like something turned off?
 
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