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Topic-icon 1978 Evinrude 115 (115893C) Compression Check

  • bembrey
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5 years 3 months ago #28057 by bembrey
I just tested the compression on this Evinrude 115hp. First, stone cold and came up with this:

1 - 81
2 - 81
3 - 88
4 - 86

Then got it to run for about 3 minutes with cuffs (having some fuel issues, I believe) and got this:

1 - 80
2 - 80
3 - 90
4 - 88

It seems to be at the limit of acceptable variance (1 & 2 on the starboard side being lower), but I'd like to hear what others have to say before make a decision to put much time and money into it. Thanks!

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5 years 3 months ago #28058 by mvislander
Replied by mvislander on topic 1978 Evinrude 115 (115893C) Compression Check
Compression readings below 90psi are on the low side, but I would not give up hope yet.
If the engine has been sitting for a while or it has been run on cheap two stroke oil it may have stuck piston rings.
I would try adding Sea Foam to the fuel and running it for a while.

If the engine is oil injected don't add any Sea Foam the oil, only the fuel.
Sea Foam also helps clean gummed up carbs, but yours may need to be removed and cleaned.
You want to make sure none of the cylinders are running lean, as that would cause melt down in short order.
You could check the cylinder temperatures for variation with an infrared thermometer.

What is the condition of the spark plugs that where in it?


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5 years 2 months ago #28126 by bembrey
Replied by bembrey on topic 1978 Evinrude 115 (115893C) Compression Check
Chris, for some reason I didn't pick up your reply and I've been swamped with work. The plugs actually looked pretty good, with no carbon deposits or oily residue. I've been thinking about the Seafoam route and seeing how that goes. The carbs could probably stand a cleaning but I've never tackled that before.

Those tests were done with the throttle at idle and I found that it should have been WOT. It registered 90 across the board then.

Since I posted this I took it to the lake and after having a hard time starting I finally got it to idle for a couple of minutes, then reversed off the dock and went into forward and out to the lake proper at just above idle. About two minutes into the run I throttled up to what was near WOT and it responded okay but didn't hit real high RPM, and after about 30 seconds I throttled back slightly and she coughed a couple of times and quit. After several tries at starting she finally got going, and as soon as I put any throttle into it she died again. Same routine - started hard and once I got her in gear a idle I just puttered back to the dock and put her back on the trailer (it was on a 1972 Jolly Roger, pretty light boat). It was new fuel but I think the carbs could be gummed up, etc., so I'll probably try the Seafoam and see if that makes a difference, and take a look at the carbs.

Thanks for the reply!

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5 years 2 months ago #28135 by gwbell

Definitely not an expert, but I'd guess those numbers are still a little low. Usually, you'd be looking for numbers above 100 psi. First thing I do when working on a motor that hasn't run for a while is remove the spark plugs and put some Marvel's Mystery Oil directly into the cylinders, then rotate flywheel by hand. Might do this a couple of times, if I think it's necessary. Seafoam is a good product to get some of the gunk out once running.

Your engine sputtering sounds like it could be the fuel system. Cleaning your carbs isn't that hard, and I'd also inspect your fuel pumps/fuel lines. Another simple mistake I have made is making sure your vent cap is open on your fuel tank. :blush:


P.S. Any new pics on progress on your Cutlas?

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3 years 10 months ago #29689 by woodlund
Replied by woodlund on topic 1978 Evinrude 115 (115893C) Compression Check
Decent motor should be over 100 psi. Tight moor should be over 120 psi.

Lots of ways to go about this and see if it can be improved.

If you can tilt it up and pull the plugs, you can shoot a big slug of Break-Free (carbon dissolving gun solvent) in each cylinder. To be really thorough, you need to have the piston above the ports so it'll soak w/o running into the crankcase ... So use a small dowel through the plug hole and feel the piston come up to the top while turning the flywheel by hand.. Shoot it in and let it sit over night. Do each cylinder on a successive day then put her back down and re-install the plugs. Fire the motor.

You can also do a piston soak with Stihl Chainsaw de-carbonizing solution. Same deal, Big slug in a closed cylinder over night.

The Sea-Foam in the gas will help. But it's not nearly as aggressive or thorough.

When you are all done with either of these treatments, you can shoot a big shot of Tri-Flow into each cylinder and then fire the motor. Tri-Flow has teflon which I have found to help keep freshly un-stuck rings free and moving on the pistons.

All this should get you what you can easily ... If successful (?), you should have over 100psi on a decent used motor.

If it's an upper cylinder that's low, it may have over heated and partially seized. Only cure for that is to hone and do new rings ...

Overheating can be cured by adding a new "piss" tube to the water jacket at the top and letting it go overboard where you can keep an eye on it. Don't need to bleed off much water. 1/8" tube is plenty :)

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