Low compression

[wake]

Hello I have a Evinrude 85hp 1979 V4 Model 85995R. I am just after doing a compression test on it, one of the cylinders is well down in compression down to 75 psi and the others are at between 110 120 psi. The cylinder with the low compression is still firing. At the moment i can not afford a rebuild, Would i cause more damage to the engine if continued to run it this way? and would it help if I decarb the engine or not? Thanks for your help Brian from ireland

 

[R.Johnson]

Re: Low compression

Running the engine will worsen the damage already there. I would pull the cylinder head to determine just what is damaged. It could be simply a leaking head gasket.

 

[MCM]

Re: Low compression

You could start with a decarb then recheck the compression. You may or may not get the 35-40 psi to bring it back in range with the others. Most likely not in which case you'll probably need to pull the head and have a look at what's going on in there.<br /><br />Post back the results of what you find.

 

[MCM]

Re: Low compression

What R.Johnson says too

 

[ledgefinder]

Re: Low compression

Try re-torquing the head & the decarb first. If it stays low, I'd pull the head (easy on those V4s) and take a closer look. <br /><br />It it's a broken ring (if the ring is in chunks) or ring land - that it may self-destruct fast. The piston may break up further, or if the ring is broken up into pieces, the pieces may hang in a port. <br /><br />If it's just a scored wall, and the ring, piston land and piston top look intact, it could run that way for quite some time; particularly if you get the rings well freed-up with the decarb. <br /><br />Amongst other motors, I have a 1979 Johnson 100hp V4 that runs 125/125/95/88 for compression. It's been that way for a long time now, & last time I ran it, it would still push a light 16-footer to 44 mph. What happened was the motor got carboned up & the rings stuck. The compression for the for cylinders varied up & down over a few years, but now the situation is permanent. (Former owner was a airfreight pilot & kept a complete logbook on the boat.) The walls are moderately scored. I can't sell the motor that way, so I has stayed around as 'backup'. <br /><br />Obviously, don't go to sea, or otherwise rely for your safely on such a motor. But if you just need something to ski/fish small lakes or harbors, it may be an economical engine for some time to come.

 

[ledgefinder]

Re: Low compression

BTW, figure out why the enginer scored a cylinder in the first place. May be a dirty carb running lean on one cylinder, dead water pump, or? Post back if you need help on doing this.

 

[R.Johnson]

Re: Low compression

Nothing in a can is going to bring back 35-40 lbs. Right now, that engine can probably be fixed, taking it to total failure would be a different story.

 

[Dhadley]

Re: Low compression

The question is not if you can afford a rebuild but rather can you afford to NOT rebuild it if it needs it?<br /><br />Is the low cylinder on the starboard bank?

 

[Vantoo]

Re: Low compression

I imagine it would be cheaper to repair it now than wait for total destruction.......

 

[wake]

Re: Low compression

Thank for the help guys!! Its looks like i will have to lift the head for a closer look. Will i need new gaskets if i am putting the head straight back on? To answer your question Dhadley yes it is on the starboard bank the bottom cylinder.

 

[Dhadley]

Re: Low compression

Most likely then its a broken ring. It happens mainly from lugging the motor. If you continue to run it, it will just tear up that cylinder worse and worse.<br /><br />Dont worry about buying a head gasket. If it is a broken ring you'll be getting a full gasket set with the head gaskets in it. No scese buying two sets of head gaskets. <br /><br />If it is just a bad gasket, you'll have plenty of time to go get a pair. And the deflectors.<br /><br />Right now, just pop the head off.

 

[ledgefinder]

Re: Low compression

Unless you're REALLY on a budget, you would generally fit a new gasket. Head gaskets contain soft metal elements, intended to deform under the bolt load, and necessary to seal the considerable pressures of combustion - so they're different than a cork or paper gasket sealing, for instance, an non-pressurized oil chamber. <br /><br />Torque-ing it down correctly is also important. Post back if you don't know how to do it.<br /><br />My recommendations to you assume that you would not rebuild this powerhead, if it outright failed, but instead find a good used replacement unit. I am trying to help you limp along with this one as long as possible, hopefully until you can afford a replacement. Not sure what DH is getting at, and I wouldn't be so quick to decide it's a broken ring before looking. I doubt a rebuild of a 25 year old powerhead would be cost-effective - particularly if it's seen a lot of saltwater use.

 

[Dhadley]

Re: Low compression

What I'm getting at is to pop the head off and look. I wouldn't spend money on a head gasket yet. If it is a head gasket, then buy one. <br /><br />If its a bigger problem and the block has to come apart & be rebuilt, you'll need a gasket kit which has the head gaskets in it. No sense buying the head gasket and then having to buy the kit on top of that.<br /><br />If its a bigger problem and you end up replacing the powerhead with a rebuilt powerhead or another good running unit, you wont need head gaskets at all.<br /><br />Pull the head and look. That will tell you what you need to buy or not buy.