1987 Mercury 70 hp low compression?

dantheACman

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I bought a 1987 70 hp Mercury 2 stroke 3 cylinder. I mounted it on a 1973 16' Starcraft holiday anyway I took it to the lake for a short test run 1 hour. Runs purity good alittle bit of a rough idle never stalled starts easy not has bad as I have had from other 2 stroke 3 cylinders. Before I take on a long family lake event I thought I should go threw the engine anyway I started with compression test found compression extremely low I think 82 82 78 I can't believe it runs so I got service manual all it says is 20 psi difference no specs on how high it should be but my thoughts are never below 100 psi and everything have researched the opion varies so much so I thought I would ask the experts here on Iboat. I never seen a 2 stroke with all compression low like that. What I have always seen is one cylinder go low. I checked it warm cold wot always the same I checked with 2 gauges the same. Thanks ahead of time for any responses
 

dantheACman

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Is there anything I can do to get more compression do I need more compression or do I need to rebuild power head can I take exhaust cover off and inspect stuff like that is my question if I need to rebuild best place to get rebuild kit do I need whole rebuild kit lol
 

roscoe

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Welcome to iboats.

Seems strange that it runs reasonably well with that low of compression.

Have you inspected the sparkplugs?
Do they look normal, a little damp, but not coated with tons of oil?

Also strange that the comp is low across the board.

Would indicate a loss of crankcase compression, not a cylinder or head gasket.

2 options:
Run it as is until it gives you issues.
Or pull the head and inspect for damage.
Then put a new head gasket on and see how she does.

Rebuild "kits" are probably not the way to go.
Replace what needs replacing, be it rings, or crankcase seals, etc.

A full rebuild will run you about $800 in parts.
 

dantheACman

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Yes I looked at spark plugs when I did compression test i couldn't believe the compression numbers myself I went and bought new compression gauge immediately lol same thing maybe a little difference 2 psi between new gauge and old but spark plugs where some wet but not covered I looked threw spark plug holes I can see tops of piston they ar a little covered in carbon build up but not crazy bottom cylinder defently worse its the one with lowest compression 78 new gauge 75 or so but my that's is it has to almost be a gasket
 

racerone

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??-----I do not believe there is a cylinder head gasket on this motor.-----And crankcase compression has nothing to do with compression in the cylinders either.
 

dantheACman

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right I have service manual and yes there s no cylinder head gasket so it doesn't matter it's that low it does shake some at idle but doesn't stall just shakes that's what got me worried so I'm got all worked up over nothing and seems like some miss a little here and there just doesn't purr if you not what I mean but not back firing seems to pull ok bit if I'm can change gaskett I just don't want to blow it the boat is restored and purity cool and motor has power title and trim and looks really good no hot spots but I'm a journeyman refrigeration tech so I get all paranoid lol so do you think I'm alright to take way out on lake I just don't want to strand the family lol I'm all about running engines purring and everything perfect I'm kinda weird paranoid guy
 
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dantheACman

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This is exploded view of power head I'm not sure I know difference in compression
 

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dantheACman

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I really appreciate the help if you guys every have a hvac or electrician question I sure will help
 

dantheACman

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I think I'll buy a gaskett kit and replace all the gaskets and see what happens but we all agree that my compression is extremely low right but manual says it only matters is the difference not the how much one place says no greater then 15 psi difference and one place says no more than 20 difference Im no where's close to 20 or 15 psi difference
 

racerone

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Sorry, but there is not a single gasket on that motor that will cause low compression.-----Do this.---Remove exhaust cover and inspect pistons and rings.
 

dantheACman

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I will i think engine was overheated due to lack of water from weak pump that makes sense to me I will find it on the rings
 

dantheACman

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I'm go a sell t or take to repair shop or sell it and gut the Starcraft and turn into a Jon boat
 

dantheACman

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I took power head off and took exhaust cover off I don't have flywheel remover yet but scratches look deep in cylinder wall of cylinder 1 and 3 not bad on two is this repairable by honing or repairable at all I can't believe this motor ran
 

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flyingscott

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Yes that can be repaired probably needs to be bored and new pistons. I do not think that is going to be a hone pistons are available up to .040
 

dantheACman

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I don't think it's past .040 I would guess .030 I'm going to crate it off and send it away I don't know of any where in Dayton Ohio that does that kind of work
 

racerone

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Finding shops that will do this type of work is getting harder.---I do my own work , both the boring and honing.
 

dantheACman

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I talked to local shops they would try it's called a blind bore but I can't find anyplace locally or anywhere that would do the work cheaper than Boatpartstore has anyone used them for power head rebuild when I total rebuild kit machine work gaskett water pump they claim to give you all this sounds to go to be true
 
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