1975 Evinrude 85hp lost compression right side

[heckhole]

My 1975 Evinrude 85hp has 45 and 55psi in the right 2 cylinders (looking from behind boat). We were doing about 40 across the lake when the prop snagged some weeds, we slowed down to clean them off and it wouldn't idle, then wouldn't even attempt to start until it cooled down.

The 2 cylinders on the starter side have 150psi exactly.

The motor made no noises and gave no indication it was about to lose compression. Am I looking at rings? Or am I looking at other big problems?

 

[interalian]

Sounds like the port side overheated and popped the head gasket. Any time there's an overheat, retorquing the heads should be done at a minimum as the gaskets can extrude causing loss of clamping pressure in the joint, and/or head warpage.

 

[boobie]

Sounds like cylinders with the low compression need to have the head pulled and gasket checked.

 

[heckhole]

Seriously? I could never, ever be so fortunate. I will tear it down tomorrow.

 

[interalian]

Heads are usually warped even without overheat. Check with a straight edge and lap them flat before re-installing them.

You can lap them using medium grit sandpaper/emery cloth on a true surface like a thick plate of glass or a scrap of marble/granite countertop. Work the head in a figure eight pattern with even pressure on the head and observe the shiny areas. Work it until it's evenly machined on all gasket mating surfaces.

Oh, and check the rubber water deflectors when you have it apart. They need to be seated well and not swolen.

 

[interalian]

Deflector check:

 

[heckhole]

What are the deflectors for? Thanks for those pictures.

Medium grit sandpaper would be what grit? I have have 800 through 3000 w/emery cloths.

 

[Faztbullet]

What are the deflectors for?

To make water circulate around cylinder jugs....the reason the 2nd pic is bad is that its out of position.Medium paper would be in the 220 range,,,,

 

[interalian]

The cooling water comes in below the lower cylinder and flows around the two cylinder bores before exiting through holes in the head gasket, then it goes through the head, then out the top of the head into the cylinder head cover, then down and out the bottom to the thermostat (one per bank). If the deflector is swolen or dislodged per the "Not good" picture, the water won't move properly and you overheat. Rings fatigue and break, pistons swell and bind in the bores. All sorts of fun stuff.

 

[heckhole]

[No message]

 

[heckhole]

Cyl 3

 

[heckhole]

Cyl 1

 

[interalian]

Top of piston shows impacts from broken rings. Note water deflector is swelled and passage is closed. How does the bore itself look?

Edit: can't see enough of the deflector at top of photo to be sure.

 

[heckhole]

Dome for cyl 3

 

[heckhole]

Dome for cyl 1

 

[heckhole]

Cylinders don't have a single scratch, they look brand new. Exhaust port on cyl 3 has a nick in the bottom corner.

Plugs looked good, gaskets look good.

 

[interalian]

BTW, when the pistons are at top of stroke, you can move them around and look down to see what the top ring looks like. Should be able to see broken rings that way.

 

[heckhole]

I will take a look tomorrow. I'd say they are toast for sure. The exhaust port has a small chunk missing. I'd assume it's not worth cleaning up the dome, getting 2 pistons and a set of rings since I'd imagine a lot of the trash is in the crank and bearings?

 

[heckhole]

I threw a straight edge on the head and it's not warped the slightest bit, in any direction.

 

[interalian]

You can smooth out any protrusions in the head (dome) and use it with no problems. See photo below for what mine looked like when I rebuilt. My motor is running great.

You will need new rings, probably new pistons and maybe a rebore on that side depending on how much damage/wear in the bore. If the bore is worn, oversize pistons are needed and a matching rebore. Is the motor otherwise good? Hard to say about economics of repair on something this old.