1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

[dicarlo]

Hey guys,

New member here! After a bit of run arounds with my 1980 Evinrude 100HP not running as it should (not accelerating or getting up to speed as it should) and after rebuilding the carbs and new plugs, I finally checked the compression. Yes I know something I should prolly have done to begin with. I am fairly knowledgable with motors but just started to dabble in the world of marine motors.

I have low compression on two cylinders, both on the same side (not sure how you know which number cylinder is which) its the tow on the right side if you are looking at the motor from the back of the boat. Top is 60 and bottom is 75. The other two are 115 and 117.

What can cause this? And any suggestions? I am going to try sea foam and spray some into the spark plug holes.

Motor runs good as far as starting goes and idles well. Just missing the power though.

Any help is appreciated.

 

[Joe Reeves]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Standing in back of the engine, facing the spark plugs.... the right side is "starboard", the left side is "port".

If you look at the way the spark plugs are staggered, you'll notice that the top starboard plug (cyl 1) is higher than the top port plug (cyl 2), followed by the bottom starboard plug (cyl 3) which is higher than the bottom port plug (cyl 4), so the cylinders are numbered as follows:

2......1

4......3

Hopefully you simply have a bad head gasket. Remove the cylinder head and inspect the sealing surfaces of the head and block (crankcase) and the head gasket itself. If simply a bad gasket, carefully clean the surfaces of the head and block, install a new gasket and torque the head bolts to 18 to 20 foot pounds in the following sequence.

9...10
5....6
1....2
4....3
8....7

Let us know what you find.

And yes, the propershooting method is to check the compression and spark before all else.

 

[James R]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Joe Reeves advice is very good. However if you have no service record on this motor the basic should be addressed after determining that you can get compressions up . Carbs should be stripped and cleaned and rebuilt with new kits and the lower unit should be pulled and the impeller and oil replaced. I get too many blown motors after these things have been ignored by new owners of used motors.

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Standing in back of the engine, facing the spark plugs.... the right side is "starboard", the left side is "port".

If you look at the way the spark plugs are staggered, you'll notice that the top starboard plug (cyl 1) is higher than the top port plug (cyl 2), followed by the bottom starboard plug (cyl 3) which is higher than the bottom port plug (cyl 4), so the cylinders are numbered as follows:

2......1

4......3

Hopefully you simply have a bad head gasket. Remove the cylinder head and inspect the sealing surfaces of the head and block (crankcase) and the head gasket itself. If simply a bad gasket, carefully clean the surfaces of the head and block, install a new gasket and torque the head bolts to 18 to 20 foot pounds in the following sequence.

9...10
5....6
1....2
4....3
8....7

Let us know what you find.

And yes, the propershooting method is to check the compression and spark before all else.

Joe,

Thanks for the response. Is it a good idea to seafoam before checking the head gasket? Is it possible to have carbon build up that is causing the low compression?

 

[rolmops]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

With seafoam you can decarb the engine and it may bring the compression up a bit,but the compression would have been lower on the other side as well.
You can dump a bit of engine oil in the low cylinders (teaspoon,not more) and check if that brings the compression up. if it does,you may have stuck rings.

 

[boobie]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Don't waste the money on seafoam yet. Pull the head and see what you find first.

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Don't waste the money on seafoam yet. Pull the head and see what you find first.

What exactly should I be looking for when I pull the head? Besides the obvious, damaged gasket, pitting, ect...


Thanks for the help guys.

 

[Joe Reeves]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Remove the cylinder head.... if what you see is not obvious to you (good/bad), take pictures and post them here for us to see.

 

[emdsapmgr]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Agree, pull the head. Typically, a carb problem might affect the two opposing cyls that the carb feeds fuel to, such as the two top or two bottom cyls. Would be somewhat unusual for a fuel issue to affect two cyls in the same head. A cooling problem might affect one head. If one side of the engine overheated, it could have damaged the head gasket, or cause other internal damage to both cyls. Do you know if the overheat horn works on your engine? Are the cooling passages clear around both cyls?

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Well, I found the problem. Not what I was hoping. Both pistons are toast. All gouged up with some damage to the head as well. Here are the photos I took of both pistons and inside the head. Screw driver in cylinder #3 to give you an idea of size of the gouge on the head. Any idea what would have caused this sort of dammage? There were no visible loose pieces of metal or any sort of material. I assume whatever it was was melted / burned or just not visible.


What are the next steps? Obviously rebuilding but can I rebuild only one side? By the looks of this I would assume the head would have to be machined out? Do I need to bore out both sides if so? This would be my first rebuild so it is somewhat new to me but I am pretty mechanically knowledgable and learn easily.

Oh and I dont want to be dumping tons of money into this thing since its an 1980 motor. But I have all winter.


Thanks guys!

 

[emdsapmgr]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

The engine was severely overheated on that side-causing the top rings to break and get chewed up in the cylinders. The rubber water diverter on pic #1 is deformed, completely cutting off water flow around the cyls. The bottom diverter looks to be out of place also. See attached pic. That's a normal install on the rubber diverter. When your situation occurs water is still flowing-trying to get to that side of the block. It is possible that you may not always get a overheat warning horn....

 

[Joe Reeves]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Yep... what emdsapmgr said. That center water deflector on your engine is swollen so as to completely block the water flow. The choice is to completely tear down the engine for a major overhaul... or replace it.

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

On average what do you think the cost would be? I am assuming new pistons and rings, getting the head bored out (am I fine with boring just those 2 cylinders?) and new head gasket. Prolly new cylinder cover too.

Is there a good place to look as far as pulling the motor apart? I have never pulled a marine motor apart, just cars. For the head to come out I am assuming I need to drop the lowers?

 

[emdsapmgr]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

You'll have to pull the powerhead and completely disassemble it. Crank, pistons, all come out. One good thing I see is that both bad cyls seem to be well oiled. I don't see any cyl liner scuffing. It is possible you might be able to just re-hone the cast iron cyls on that side. You will need two replacement Bombardier pistons with ringsets. Standard size. That would be a best outcome. Might as well buy a powerhead gasket set too. If any of the cyls two are too out-of-round (beyond factory tolerance) you will have to have those two cyls bored .030 over and then purchase oversize pistons. In any event, you will want to hone the bores of the two good pistons and re-ring those two good pistons for your overhaul. I'd also go through the thermostat housing and put two new thermostats in. A good place to start is with a genuine factory service manual-check out this website: outboardbooks.com

 

[daselbee]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

I have done a few just like this. I am in Mims.

PM me with your number, I can help.

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

This is all great info and super helpful! Just to be clear, if I do need to bore out the two cylinders with the bad pistons, is it okay to go .030 over in just those two and not the other two cylinders (the good ones) I should get new rings for all four cylinders either way?


Also from your experiences what do you think part cost is? under $500?


and Daselbee I will PM you. Thanks!

 

[emdsapmgr]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

It is ok to over bore just one (or more) in the same block. Just be sure to get the matching factory oversize pistons. You can't mix pistons from two manufacturers in one powerhead. Best to install 4 new ringsets-that way you don't have to go back into the block in a few years to update the original rings as they start to wear out. Good sealing of the piston rings is what makes a 2 stroke run. Don't skimp in that area.

 

[dicarlo]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Pulled most of the motor apart tonight. One cylinder needs to be bored for sure. Looks like the bottom cylinder (#3) is the one that had a broke ring.

Everything came apart easy with no broken bolts. Heres a few photos for you guys. If anyone has a recommendation for machine shops in Orlando area please let me know. Otherwise I am gonna check out a place in Titusville another helpful member recommended. I have taken tons of photos of the tear down and will do the same on the rebuild and put together a write up when I am done. Hopefully that can help another guy down the road!

ALSO anyone know what tool I need for removing the rods in picture 5. Special socket of sorts?

 

[Sureshot]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

12-pt 5/16 socket I believe.

 

[Joe Reeves]

Re: 1980 Evinrude 100HP - Low compression on 2 cylinders. Need help

Sureshot.... Yep, that's it.