1978 6 hp Evinrude low compression

[Bullie]

I bought a 6 hp Evinrude 6804B recently to see if I could get it running. It has good spark that will jump 1/2 inch on my testers. It had 110 lbs compression on the top cylinder and 60 on the bottom cylinder. I figured it was the head gasket and replaced it, smoothed the head on sandpaper over glass, put everything back together, torqued it down to specs, and checked compression again. This time I got 90 and 30. The cylinders looked really good. I saw no damage at all. But, there was a little sludgy looking stuff on the level of the gasket.

Will I have to take the power head off to view the pistons/rings? I assume that is what I should do next, check the rings?

 

[Fleetwin]

The 110 psi reading seems a bit high to me. Especially for a 6 HP.

It sounds to me as if one of the pistons rings just may be stuck.

Since you already have the head off. Before removing the powerhead, I would try this:

1. Tilt the engine up so that the pistons are pointing straight up.
2. Rotate the crankshaft until the low compression cylinder piston is up with the intake and exhaust ports closed.
3. Use some Sea Foam and pour it on the top of the piston so that it pools. It will probably leak down slowly so keep pouring.

That may un stick a stuck ring or two.

Also, the engine will probably run with those numbers. While running keep dosing the carb. intake with Sea Foam in a spray bottle. It;s going to smoke a lot.

Keep dosing it until you can stall it with the Sea Foam. Let it sit at least overnight.

 

[Bullie]

I had never had a 6 hp with 110 lbs before. I was surprised. Like I mentioned above, it now measures 90 and 30lbs. I had reinstalled the head before checking the compression again. Giving your suggestion a try. I squirted the sea foam in the plug hole.

 

[Bullie]

No improvement on the compression. Will I need to remove the power head to view the pistons?

 

[raczekp1]

i usually go to the mashine shop to flatten the head surface. every time they remove 0,30 mm of head surface.
my advice is to to go to profesinal mashine shop

 

[racerone]

No , a piece of glass and some emery paper cleans up the head every time.------Removes the minimum amount of material.

 

[Bullie]

No , a piece of glass and some emery paper cleans up the head every time.------Removes the minimum amount of material.

This is what I did. As I had been advised in the past. This is the first time that I was not able to restore compression by cleaning up the head and installing a new head gasket. Racerone, what would you do next? I saw no sign of any scuffing or scraping in the cylinders.

 

[Bullie]

Well, I bought a manual and disassembled the little 6. I don't think the fellow that sold it to me was completely honest. LOL I assume the motor is scrap? Anything worth salvaging there?

 

[Bullie]

What is my best choice? Find another powerhead? Part it out?

 

[bspeth]

depends if you have more time or more money I guess.

 

[boobie]

Tear it down. You may be surprised at what's worth saving.

 

[HighTrim]

You have no idea how many times I find this from dis honest sellers. Another reason I open up every motor that is stuck, even If I free it up.

The crank will likely be junk, but maybe you can save it with a rust dip. Try it, see if the journals are worn or scratched. If the journals are within spec, and the rust cleans up you may be in business. The pistons and rods are not all that hard to find for that motor. If you have it opened up, and plan on keeping her, maybe just buy new with new rings. Hone the cylinders, after ensuring the bores are to spec and not out of round. Then you'll have a motor ready for another 50 years of service.

 

[Bullie]

Thanks guys. I think I am going to break it on down and check everything out. I see that one of the rod caps is broken too.