Compression Variance 1978 Mercury 1150 / 6 cylinder

[frankenmerc]

I have recently purchased 1978 Mercury 1150 that I am bringing back to life,changed all the wires, stator,trigger and switch box and went through the carbs and rebuilt the fuel pump and got her running to discover this on the compression readings.
#1=110
#2=110
#3=110
#4=85
#5=110
#6=110
I have tried several things to rule out stuck rings by spraying PB blaster in that cylinder and letting it set and and spinning it over without starting hoping if it was it would free up with no luck.I am planning on pulling the powerhead to get the bottom cowl off to repaint and thinking I should pull off the port covers to take a look at the rings to see if they are ok or if i have broken one or whatever else might be going on in there.
If I discover that the rings are ok should leave as is or plan a complete rebuild or just change the one piston/rings and hone if needed.
All in all she runs pretty good just concerned about that #4 cylinder,Any thoughts / input would be appreciated.

 

[JB]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Try a decarb treatment before you start tearing it apart. Could simply be carbon buildup.

 

[Maxz695]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

I agree with carbon build up as possibly being a factor but why just the one cylinder? If you have to take it down you can replace the one cylinder rings or piston wrist pin rod etc. As needed at that point. Inspect the cylinder well for gouges or deep groves. lack of fuel oil to this cylinder may have caused this condition. Check the reeds if disassembled for wear and seating. The carbs should be checked over well too.

 

[Texasmark]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

I agree that only one cylinder being that far off with the others being exactly the same doesn't deserve a tear down. I just checked my engine recently, a lot newer than yours and it was at 118. So your 110 numbers are great....I certainly wouldn't tear that down if I could avoid it.

Why not look up "decarb" on this site and do it. It's a heavy dose of Sea Foam in a little gas and works wonders on engines. I too will go with a stuck ring(s). The fact that it's #4 and not #1 or 6 adds to the "don't fix it if it ain't broke" mentality. As I have gotten older and lazier, I have subscribed to that axiom and have been pleasantly surprised by the results. Additionally, did you test it more than once and in the process ensure that your gauge was firmly mounted in the plug hole?

Course if you are just itching for spending money and a shot at the unknown, have at it......the tear down. Grin.

Mark

 

[Willyclay]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

I certainly wouldn't tear that down if I could avoid it.

Amen, brother! I would beg, borrow or rent a borescope/inspection camera to look inside that suspect cylinder to gather as much info as possible before making any repairs. Good luck!

 

[Faztbullet]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Just pull the transfer port cover and look inside, what you see will likely not be solved with a decarbon....

 

[frankenmerc]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

#1=110
#2=110
#3=110
#4=110
#5=85
#6=110
View attachment 175356View attachment 175357

Thanks for the input some very good advice!
There is one mistake made on my original post it was not #4 the cylinder in question is #5 sorry for confusion I transposed my numbers the # above are the ones i started with.
After taking JB's advise on the decarb i went to Wal Mart and picked up a can of sea foam and mixed it with about 4 gallons of gas and 2 cycle oil and ran it for about 45 min on the stand and this was the results on the compression,and a lot of corbon build up driped out by the next day a good sixe puddle.
#1=115
#2=115
#3=115
#4=115
#5=90
#6=115
and I have Tex's mentality if it aint broke dont fix it is why i mentioned only paying attention to the one in question #5 if i would have to take it down cause i would think a cyl with 115 psi is good.
and I aint in a rush to spend a bunch of money....Grin, dont have but about $400 in it now with everything i have done including the purchase price of the motor.
Faztbullet,I think I will pull the powerhead soon and take port covers off and peek inside to check out the rings and see if i have a broken one,all i will be in it for at that point is the price of a couple gaskets the powerhead and port cover gasket.
I will go through the carbs again and double check my float/ (levers) settings according to specs.
Thanks again and will let you all know the outcome when i get it off.

 

[Texasmark]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Thanks for the input some very good advice!
There is one mistake made on my original post it was not #4 the cylinder in question is #5 sorry for confusion I transposed my numbers the # above are the ones i started with.
After taking JB's advise on the decarb i went to Wal Mart and picked up a can of sea foam and mixed it with about 4 gallons of gas and 2 cycle oil and ran it for about 45 min on the stand and this was the results on the compression,and a lot of corbon build up driped out by the next day a good sixe puddle.
#1=115
#2=115
#3=115
#4=115
#5=90
#6=115
and I have Tex's mentality if it aint broke dont fix it is why i mentioned only paying attention to the one in question #5 if i would have to take it down cause i would think a cyl with 115 psi is good.
and I aint in a rush to spend a bunch of money....Grin, dont have but about $400 in it now with everything i have done including the purchase price of the motor.
Faztbullet,I think I will pull the powerhead soon and take port covers off and peek inside to check out the rings and see if i have a broken one,all i will be in it for at that point is the price of a couple gaskets the powerhead and port cover gasket.
I will go through the carbs again and double check my float/ (levers) settings according to specs.
Thanks again and will let you all know the outcome when i get it off.

Do some more decarbing and give it time to soak between runups. You may have a lot of it. Since you gained 5# initially, that is very good news and tends to drive one to continue till no changes are observed.

You can pull off exhaust water jacket covers without removing the powerhead with the right sized hands and tools.

Don't worry about your carbs. You have 2 cyls per carb and only one cylinder with a problem. The carb wouldn't give you a low compression reading anyway.

Mark

 

[smokeonthewater]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

further, parts for that engine are NLA and honestly it's cheaper to buy another engine than to try and gather enough parts to rebuild that one.

 

[frankenmerc]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

After much anticipation on this situation I decided to pull the power head and look inside the intake and exhaust ports and this is what I found on the # 5 cylinder.
on the intake side all was good when I pressed on the rings they sprang up and down and then when I pulled the water jacket cover the rings were not springing back as I was checking the rings and they appeared to be stuck on the exhaust port side.
no scoring was observed on the piston as though it had not been in that condition long it was sitting on the ground on it side when I bought it for some time.
My question now is which way to go from here any thoughts or opinions on this situation would be greatly appreciated.
Thanks

 

[JB]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Running Seafoam through the fuel is not a thorough decarb, merc. Follow the directions on the container. Get that stuff soaking in the cylinders.

 

[frankenmerc]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Sounds good, going to run down to wally world and get a can and get working on it.
Thanks

 

[Texasmark]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

You just answered your own question. No spring back of the ring is a stuck ring and carbon sticks rings (unless the ring is broken, but multiple rings broken????). Stuck on the exhaust side is where the engine exhaust heat can bake on the carbon. Go to the archives and get the "decarb" process and do it. I learned about that process and Sea Foam on here 9 years ago. Haven't been without it in my gas since.

Mark

 

[frankenmerc]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Well after a little more probing,I got screwdriver and pushed on the piston and to my suprise that piston has about a 1/8 inch of play .i can push down on it and it springs back (wow) was not expecting that.
None of the others do that and it was not knocking before ran fine just the low compression on #5 and a little moisture from the exaust plate gasket.
What are your thoughts on this .
Thanks

 

[Texasmark]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

My first thought would be why would one cylinder go bonkers when the other 5 are identical? Unless it does some secondary work that the others don't , like supply pulses to the fuel pump or whatever it would do differently that might make it fail sooner. I guess it's a defect in materials and workmanship that took 35 years to show up. H!@# I don't know.

How long you going to keep the engine? How many parts will you need to tear into it and where are you going to get them and what are you going to spend on the job. Like smoke said, may be cheaper to chunk it.

Soooooo, since you have nothing to loose, why not put the covers back on, put the power head back on the LU, get some Sea Foam and a small tank of gas and do the decarb. Then check your compression again. If it's within around 10-15% of the others run it. You only have about 10 more psi to go.

Mark

 

[woodsyfeller]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Not to burst your bubble or sway you from trying to fix that engine but you can find another 115hp that has good compression on all cylinders for around $1000.00-$1500.00 that runs well. That engine is not DIY friendly if your messing with the pistons, cant change just one since there is no head for that engine. sure you can disconnect all the pistons and change just one, but why go through all the effort and not do them all .You will need special ring compresser to put the pistons back in and if you pay to have a mechanic do it . It would be cheaper to buy another motor.

Im guessing the reason why the #5 piston has low compression is that it was topped out in cylinder head and sat for how ever long and ring got stuck. The cylinder walls taper in at the top.

 

[frankenmerc]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Thanks for the input,i am working on decarb with sea foam and fogging oil and it seems to be getting in there some but not sure how this is going to work out never the less i am giving a shot soaking and spraying i have both sides open so can access fairly easy.
Couple things parts availibility is not an issue have several for parts and for me this is a passion for the in line 6 more than necessity and by no means an expert at this just looking for advise on repairs, if it dont work out I may pull the crank and change the piston and rings if necessary as a last resort.
Thanks again for the input.

 

[Texasmark]

Re: Compression Variance 1978 Mercury 1150 / 6 cylinder

Thanks for the input,i am working on decarb with sea foam and fogging oil and it seems to be getting in there some but not sure how this is going to work out never the less i am giving a shot soaking and spraying i have both sides open so can access fairly easy.
Couple things parts availibility is not an issue have several for parts and for me this is a passion for the in line 6 more than necessity and by no means an expert at this just looking for advise on repairs, if it dont work out I may pull the crank and change the piston and rings if necessary as a last resort.
Thanks again for the input.

Just an opinion worth what it cost you. If you aren't a perfectionist, or tinkerer, and you get your problem reasonably well solved with minimal effort are you going to go all out anyway? Since you are bringing up the compression on your whole engine, #5 is already within 10% of the others before you started. Surely it will come up some more. If you test it before you tear into it you may not choose to.

My 2c,
Mark