Reason for low compression?

[saumon]

Hi everyone, <br /><br />First of all, please excuse my english (i'm french speaking). <br /><br />I know there's a lot of o/b mech. and experts here and i'm a bit curious. I have a 1993 Merc 60 hp 3 cyl. (s/n 0D074327) with a slightly lower compression reading on the bottom cylinder (from top to bottom #1:125, #2:125, #3:115). I don't worry too much about it cause it's less than 10% and the engine run great, but i want to know if i can't correct what cause this situation, cause i don't want it to get worst.<br /><br />I do a searh on "compression" on this board and, after reading a few hundred posts, i'm just more confused. Some says it can be caused by a lean mixture on this carb, causing detonation then overheating and scoring the cylinder wall. Others says it can be caused by an over-rich mixture, making some build-up and stuck rings.<br /><br />My question is: is there a way to know what can cause this lower compression (to be able to correct it) other than removing the powerhead to visually inspect it?<br /><br />Thanks.<br />Steve

 

[Nos4r2]

Re: Reason for low compression?

The only thing you can do to attempt to improve it is to run a can of seafoam or redex in your next tank of gas to try and clear any carbon from your cylinders.<br /><br />The only real solution is to remove the powerhead and rebuild-but it's not nessesary. With figures within 10% it might simply be that the factory tolerances weren't as close on that cylinder.<br /><br />Don't worry about it unless it changes noticeably.

 

[andy6374]

Re: Reason for low compression?

As you said, the compression reading are fine. Do as Nos4r2 said and run a can of seafoam thru your next tank of gas. However, a can of seafoam is usually a preventative measure. To truely de-carb the engine buy an aersol can of seafoam deep creep or mercury power tune. Spray into carbs until the engine stalls. Take out the spark plugs and spray the stuff into the cylinders, let it sit (the longer the bettter). Then fire her up. You will probably see tons of black crap out your exhaust if you never done this before. It is worthwhile to do this on all of your cylinders and will probably raise all of your compression readings

 

[Kenny Bush]

Re: Reason for low compression?

you got some great answers. I agree with the SeaFoam in the gas tank. You may even want to use the SeaFoam "Deepcreep" foam application directly into the cylinders. You are still within specifications, but a good decarb is always first on my list.

 

[crem1]

Re: Reason for low compression?

If you read my posting on cylinder compressions for a MERC500 then you would be in shock seeing my engine's output.<br /><br />Piston 1 --- 110 PSI <br />Piston 2 --- 114 PSI <br />Piston 3 --- 129 PSI <br />Piston 4 --- 95 PSI<br />Yes, I did use seafoam to loosen the rings but the compressions haven't changed that much in relation to each other.<br />I am happy with the way my engine runs and the RPM's I'm getting.<br /><br />Alex

 

[Kenny Bush]

Re: Reason for low compression?

Crem1, Your motor is slowly shaking itself to death.....or your guage is wrong. You must be babying that motor to keep it going.... goodluck..

 

[crem1]

Re: Reason for low compression?

Everytime I took the readings it always gave the same scenario.<br />So, I don't think the gauge is wrong. I was trying to get an explanation as to why two are similar and 3 and 4 are just the opposite. Any explanations?<br />Thanks<br /><br />Alex

 

[Nos4r2]

Re: Reason for low compression?

Originally posted by crem1:<br /> Everytime I took the readings it always gave the same scenario.<br />So, I don't think the gauge is wrong. I was trying to get an explanation as to why two are similar and 3 and 4 are just the opposite. Any explanations?<br />Thanks<br /><br />Alex

Don't suppose your lower plug (no.4)looks like it's been steam cleaned does it? Maybe the crank seal is on its way.<br />I'd pull it down and have a look anyway with readings like that. It's impossible to diagnose something that can be as complicated and dependant on so many factors without knowing more factors than you could resonably expect to know.

 

[crem1]

Re: Reason for low compression?

Steam cleaned would infer that the plug was clean and wet?<br />If I remember correctly it was lightly covered in fuel like all the others . I should re-adjust the carburator slightly and make it leaner.<br /><br />Alex

 

[andy6374]

Re: Reason for low compression?

Richer is better than leaner.