compression IL6

renny1 · Jan 28, 2008

1982 inline 6 mercury blueband
i have 120 on each cylinder apart from 1 which is ony 30 before parking up for the winter she was 120 all round is this a strip down or is it something else????????? what i mean by something else could the rings be stuck and if so how do i unstick them

renny1 · Jan 28, 2008

Re: compression IL6

cor blimey i been pushed down to the bottom already lol

blouderback · Jan 28, 2008

Re: compression IL6

Low compression on just 1 cylinder usually points to a bad/stuck/bent valve. Low compression on 2 adjacent cylinders points to a bad head gasket. Low compression on multiple cylinders points to bad rings.

Another test is to squirt some oil in the cylinder and retest. If compression comes back up, you have a bad ring, as the oil will temporarily re-seal the rings.

Sig_Mech · Jan 28, 2008

Re: compression IL6

I believe that your motor is a 2 cycle so there will be no valves to worry about. Try what blouderback advised and shoot a couple squirts of oil into the cylinder with the low reading, if the compression comes up then it very well could be stuck rings.
You could try and unstick the rings by squirting some Seafoam into the cylinder then slowly rotating the flywheel a few turns. It will probably take some time to work so plan on doing this a couple of times a day for a week then recheck the compression. I've also done this on small gas engines using Marvel Mystery Oil and it worked well.
If that doesn't help then you may be looking at a tear down and rebuild.
Good Luck.

renny1 · Jan 28, 2008

Re: compression IL6

thats sounds promising but over here in th uk we dont have them items seafoam etc is redex the same as it has nappa or whatever its called in it..... and yes she is a 2stroke

Scaaty · Jan 28, 2008

Re: compression IL6

You try spraying in some oil tilted up, see what happens.
But.

If you have only 30 PSI in one cylinder, you have a serious problem. And it aint stuck rings I bet.
You have to take a good look at all plugs first for starts. Any shiney clean, its getting water, and ya got a crack somewhere.
Any with what appears to have tiny silver balls attatched, ya torched a piston.
Also could have cracked a ring in a port, and gouged out the wall.
Or it just got hot, and piston is a solid mess.
Helps to say which one is low..(betcha its #3)

renny1 · Jan 28, 2008

Re: compression IL6

this sounds serious now....... you got it in one it is number 3 tell me more is this gonna cost big bucks????????

renny1 · Jan 29, 2008

Re: compression IL6

right so far i have introduced oil into the cylinder an the compression has now gone up to 60 after ten minutes of turning engine over slowly is this gonna keep rising or does it indicate naff rings.... i know it is gonna go up a bit due to the oil sealing it a bit but to double the pressure... all i need now is to double it again any feedback would be good......... what is involved in changing rings(i have a manual...clymer)

renny1 · Jan 29, 2008

Re: compression IL6

well ive decided to strip engine i will start a new thread to keep you all posted on how its going and of course for help..... i sheared one bolt already lol

Chris1956 · Jan 29, 2008

Re: compression IL6

Renny, If you pull off the transfer cover you will be able to inspect most of the cylinder wall and the rings. I will guess that motor ran a bit lean, and the rings are stuck to the piston grooves.

renny1 · Jan 29, 2008

Re: compression IL6

is the transfer cover the one which the spark plugs are on??????

Chris1956 · Jan 29, 2008

Re: compression IL6

Renny, The transfer cover is on the starboard (also starter) side of the motor. There are three individual covers, each over two cylinders.

renny1 · Jan 29, 2008

Re: compression IL6

by deduction i found them all the rings (3 each) on each piston are fine but on cylinder 3 they dont move ....... what is my next step

Chris1956 · Jan 29, 2008

Re: compression IL6

You likely need to disassemble the powerhead and see if the #3 cylinder needs to be bored, and a new oversize piston installed. if the #3 bore is not scretched too badly, you may be abe to install new rings on #3 piston, provided the piston is good.

renny1 · Jan 30, 2008

Re: compression IL6

right as in last post i have taken the covers off and the rings are stuck but using lots of oil and stuff they are begginning to loosen my next question is are they just loosening one side or not ..... also there is a lot of brown gunk on the top of the piston is this carbon build up or something more serious.... the sides of the piston i can see are nice and shiny no score marks on them..... or should i just say sod it and tear it to pieces and re-build... i can see this being a long thread and you guys getting bored with it

Chris1956 · Jan 30, 2008

Re: compression IL6

Renny, You may yet get lucky with that cylinder. Use some "Deep Creep" thru the transfer cover and spark plug hole. Let is soak for a while, and the rings may just unstick.

Crank it over and check the compression. if it gets up to 75PSI or so, do a decarbonization treatment on it and you may just restore it.

renny1 · Jan 30, 2008

Re: compression IL6

thanks chris that makes me feel better as i have the classic case of stuck powerhead manual says 6 bolts have taken them of but cant get no movement from head at all(engine still on boat) we dont get deepcreep here in uk so gonna try all different things.....any idea what the brown gunk on piston is??????

Chris1956 · Jan 30, 2008

Re: compression IL6

Renny, I am guessing that the brown stuff is carbon and other combustion crap. Deep Creep is a carbon removal solvent. PowerTune is another brand of the same stuff, from merc. OMC has yet another brand. You can also try some penetrating oil (PB Blaster/WD-40/Liquid Wrench).

Just so you know, to pull the powerhead, you need to drop the lower unit, remove the cowling and cowling support, starter and dissy and then loosen all the powerhead bolts (8 I think) and use a hoist or strong friend to lift it straight up. The cylinder head does not come off.

achris · Jan 31, 2008

Re: compression IL6

The screws you are removing (and probably breaking) from what looks like a cylinder head is in fact a cylinder head cover only. The cylinder head is cast integral with the cylinder block.. You don't need to remove the cover to strip the engine.

If you do decide to strip and rebuild, then make sure you do the exhaust water cover inside the exhaust system too... Most important... and can be a bit of a fiddle to do... GET THE MERC MANUAL, seloc and clymers don't cover it properly.

Chris..............

renny1 · Jan 31, 2008

Re: compression IL6

the cover that i removed was the cover that i think the water flows round to cool the engine i have snapped 3 bolts on it which are sticking out so hopefully not a problem apart from this has been apart before and whoever done it snapped a bolt and drilled a hole a fraction above it for a new bolt so they are both snapped will add a pic today to see if this one can be resolved

compression IL6

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