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1987 70 hp dead cylinder

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Billt1550

New member
Any ideas?
I just picked up a boat with a 70 hp mercury 3 cylinder in which one cylinder has no compresson. I got the boat CHEAP and am hoping to fix the engine as cheaply as possible. The previous owner found out it was only running on two cylinders, after owning it briefly, and rebuilt the carbs.
I took a compression check and got the following: #1: 120 psi, #2: 110 psi, #3: 0, then 15 psi. after squirting oil in the cylinder. I would like to get this thing going without a complete overhaul, but am prepared to do so, if necessary.
My questions are:
1. If #3 has stuck rings, any chance of freeing them? If so, what product?
2. What is the likely cause for such a condition?
3. If the engine needs an overhaul, approximately how much in parts, assuming boring one cylinder, and a new piston. I plan to re-use as much of the old parts as possible.
Any help and direction in this project would be greatly appreciated.
 
If the block is fixable????
It can probably be redone for $400 If you do the work.

Depending on the damage, the aftermarket pistons can be had for as little as $85 or up to $135
The damage to the cylinder will tell.
Take it to a machine shop and see what can be done?
Boring it out the least amount is always best.
You can find pistons .010 to .030 oversized.
Wiseco pistons are a good bet.
The factory sells a piston kit too.
 
..."Remove exhaust covers and inspect the bad cylinder / piston. "

Huh? The "head" on these is integral with the block. You could remove the right side black covers (under the coils) and take a look. Leave the exhaust side alone--the bolts tend to break off.

Unusual that the lower cylinder is bad--it's almost always the top hole. With so little compression the motor has to come apart. Snake oil won't doo it.

Jeff
 
You can't see anymore with the head's water jacket off than you can by simply looking through the plug opening.

Boring all three cylinders of a motor that old is insanity, for the cost would be astronomical.

Jeff (owner of at least 4 of them)
 
If year is correctly posted its a small bore looper not a crossflow and it does not have covers under the coils. remove the exhaust cover as posted and inspect cylinder, these motors are bad about breaking piston skirts caused by lugging...
 
Got the exhaust cover off. #3 looks scored and pretty bad, with a portion of the piston face missing. #2 is scuffed (horizontal scoring) evident. #1 has some scuffing/scoring.
Three questions:
1. is 120 psi too low for that engine (are all three cylinders bad)?
2. is it worth it to go further, or do I need to find a new motor?
3. What would be likely cause of scuffing/scoring? (overheating?, oil injection failure?)
Thanks to all contributors.
 
Theirs very little hope when you have low compression in an outboard, especially an engine that age. You can always pull the head off and take a look at the low cylinder. If theirs any kind of scoring what so ever, forget it.
 
It all depends on the determination / willingness to learn / skill of the owner.--------This motor can very well be rebuilt at minimal cost to run like new !!!!!!
 
If all 3 are scored check the oil tank as another known problem is cracked caps that allows oil to be contaminated with rain water.
 
kimcrwbr1 said:
You need to disassemble a motor before buying any parts for sure but low compression does not condemn a motor. Mercs are a little more difficult as they do not have a head and the reeds are part of the main bearings. You must first try and determine why the bottom cylinder lost compression? Usually because the cylinder was running too lean or lack of proper lubricant.
Click to expand...

Is it possible to repair it? Yes. Is it always cost effective to try and do so and chase down parts for a 30 year old engine with bad compression and who knows what else? No, not really.
 
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There is a huge difference between a shop doing the work for an owner and an owner wanting to do the job himself.---------Lots of good used parts available if you are persistent and patient.
 
The motor doesn't appear to have been abused. I will disassemble and evaluate. I will have to wait a few weeks to finish dis-assembly. If I do rebuild, I need to get to the root cause of the failure.
P.S.: How much cylinder scoring is too much to overbore, assuming a standard bore to begin with?
Thanks for the input.
 
All outboard pistons are tapered.---------------Size to bore is in the catalogue.------------In the old days it was a good idea to have to pistons on hand, in modern factories the pistons are made with computer controlled equipment and are the same every day of production !
 
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