How bad is it?

[ziguan]

Hi,

Found low compression on the top cylinder of my 83 mercury 70hp.
mercury 70 low compression
(what caused them to came loose?)
Look like there are .015 and 0.030 O.S pistons available.
Can it be salvaged? I need some advise before I take the head off please.


Thanks in advance,

 

[Chris1956]

What is the compression of all 3 cylinders?

No cylinder head on that motor, just a water jacket cover over the spark plug holes into the cylinder. All pistons + crankshaft must be removed from the block for boring and honing.

Pistons are $100+ each. Boring is likely about $75 each.

 

[dwco5051]

They were known for burning the top piston from running the cylinder lean do to fuel delivery problems. Very low or no compression probably means a hole burnt clear through in the piston.

 

[racerone]

The 3 cylinder 65 / 70 HP is not high on my " good motor " list.------Remove bypass cover and look at piston / rings.

 

[ziguan]

Thanks for all the replies.
The compressions are 70/115/120(top->bottom), engine still runs, difficult starting.
Looks like there are couple deep scoring caused by the bearing needles.
Could someone please tell me what would cause them to loose(to prevent it from happening again)?
Is it ok to just bore one cylinder? or bore all three?

 

[racerone]

Going to be expensive.-----Best look for another motor.----Your location ?

 

[ziguan]

Going to be expensive.-----Best look for another motor.----Your location ?

Boston

 

[racerone]

There are better more reliable motors out there.----Takes time to find a good one.------Yes it is acceptable to bore one cylinder.

 

[ziguan]

There are better more reliable motors out there.----Takes time to find a good one.------Yes it is acceptable to bore one cylinder.

Any recommendations?
Are Tohatsu good outboards? seems much easier to find parts for Mercury outboards.

Thanks,

 

[The Force power]

They were known for burning the top piston from running the cylinder lean do to fuel delivery problems. Very low or no compression probably means a hole burnt clear through in the piston.

I belief that the coolant water played a role also as the water reached the top, it has passed the other cylinders already
(but that's in most cases??)