Compression low on Mercury KE4 Rocket

[JordanSewruk]

I tested my outboards compression and the results were 40psi, I know It should be around 110-130psi. Does this mean I now need to dismantle the powerhead to check the pistons or is there some other test to do or any common mistakes I may have made while testing the compression? Thanks in advance.

[DAN UMBARGER]

Before you go taking the powerhead apart make sure your tester is accurate. I have 4 compression testers and all 4 are different. I took one gauge and hooked it up to my air hose with a regulator in between and set my regulator at 100 psi. I took the glass and the back off the gauge and bent the air bladder inside the gauge until it matched the air pressure on the regulator. Not sure if that’s wise but it worked for me. There probably is a more correct way to do it so someone else may chime in. But at any rate don’t disassemble it until your sure it really is low.

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JordanSewruk

[Tubs]

If it only has 40 lbs. compression there would
be almost no resistance when you pull it over.
If someone ran that motor till it was so worn
out it wouldn’t start or run anymore it would
still have more than 40 lbs. compression. I
think your going to find that someone has been
in to it ahead of you.
Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

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opposedtwin, JordanSewruk

[Pondrocket]

Jordan,

I see you are a Junior Member. Would like to welcome you to AOMCI first off… As Dan stated make sure you are accurately testing the power head.. Where are you located once we know we can get you in contact with a Chapter by you and you can possibly go to some meets and get advise on the wonderful world of Antique/Classic outboards then go from there.

Thanks

 

Travis
AOMCI VP Communications
AOMCI Webmaster
webmaster@aomci.org

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JordanSewruk

[green-thumbs]

Think 2 cycle…the crankcase has to be tightly sealed, the reeds have to be free of rust for the charge to be crammed into the cylinder. The rings have to be free in their grooves and rings, piston and cylinder need to be in reasonably good shape.

40# dry is low. I have torn down a few KE4/Mark 7  First one was a learning experience. Unless need can be shown, not a good idea to go looking for trouble that is not there.  Best to diagnose the patient before the operation. Do the tissue test in front of carburetor opening to see if reeds are sealing,  dirt and oil under magneto plate may indicate the upper crankcase seal needs replacement,. Oil added thru spark plug openings may raise compression indicating worn rings or cylinder or pistons.

If rings are stuck in grooves, I would pour in some Marvel Mystery Oil and see if rings will unstick. Running motor may also cure problem.

If worse comes to worse and you need to tear down powerhead, it will be a learning experience. You will need to learn the Mercury Way of doing things, techniques and make a few simple tools. The tools are well worth time and effort to make.

My 0.0002 cents worth

Louis

 

Photos show ready to reassemble powerhead and new upper crankshaft seal (7415) installed. Crankcase with bad crankshaft was picked up at swap meet. Needed to source cylinders, but, rest was on shelf…left overs from parts motors and what all. Sort of like the Cadillac that Johnny Cash sang about.

 

 

• This reply was modified 3 years, 9 months ago by green-thumbs.

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opposedtwin, JordanSewruk

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