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Bad compression Mercury 50 1980 1983

C

calle

Member
Hi, I tested the compression on my merc 50 hp today, cant say I like the results... The engine was cold and I dont have a startermotor so I cranked the flywheel with a wrench a few turns on each cylinder. Motor was cold and all the plugs were out. This is the results (psi)

1 58
2 51
3 0
4 51

From what I understand theese compression sucks so I double checked the gauge on my yamaha 6 hp. On that I have a rope starter so I turn much faster then with the wrench on the 50 hp. On the Yamaha I got over 100 psi with both plugs out-

So Im wondering what might cause this? Did I did something when measuring on my mercury? Could it be the cylinder head gasket? If not what could it be? Any advice forward would be appreciated.

Has anyone changed the head gasket on a motor like this? The bottom bolts are hard to reach, do I remove the engine mount (marked with red arrows) in order to access them or do I have lift the entire powerhead out? Also what is the bolt next to cyl 3 indicated by the green arrow?
 

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There is no cylinder head gasket on that motor.----There is a water jacket cover gasket.-----The " bolt " is a pipe plug for a place the get pressure to a gauge.-----You can pull it over with a rope.
 
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racerone said:
There is no cylinder head gasket on that motor.----There is a water jacket cover gasket.-----The " bolt " is a pipe plug for a place the get pressure to a gauge.-----You can pull it over with a rope.
Click to expand...

But from what I can read out it should be the gasket highlighted in yellow. If no cylinder head gasket - how is it supposed to get compression?
 

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I hear what you say when you say I must accept that fact why dont you comment on the questions I posted? Would make it easier for me to understand the design of tje motor. The blown diagram I got from internet using the motor data for my motor. Are you saying that the gasket listed as no 10 does not seal the compression head chambers from the cylinders? If so I still wonder how the motor can achive compression can you please explain?

If I remove the intake I will be able to see part of the piston I suppose, and can check compression on the piston rings. I poured oil into number 3 but it didnt make any difference
 
Ok so whats the most likely cause for this poor compression? Bad o-rings could maybe explain it on number 3 but the others are so even in comp. Is there a gasket between crankcase and cylinder block?
 
There are no gaskets / seal involved that can cause bad compression.------You have major damage to a piston / cylinder.------Remove intake bypass / transfer port covers to inspect for this damage.----Item #16 will reveal it I think.
 
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Ok thanks I will check that later today but bad news I guess - most likely is crap as I understand it. If a piston is bad I guess chances are that that cylinder is scarred to. Even changing a piston on an motor this old is questionable if its worth the effort as I see it. A new piston shipped to Sweden seems to be about 100 USD
I got the motor, small sportboat and trailer about a month ago. The previous owner said the starter motor was faulty and that ”ignition had be set” - guess I was tricked. Got the whole set for approx 450 euro so wasnt that much but sucks to discover this I was hoping to be able to restore the motor during the winter.
 
From what I see the crankshaft seems to be split in half with no gasket between. Im curious how this design can be tight since the crankcase is under some slight compression?
Also since the top cover seals around the spark plugs (plug threads are in the top cover from what I can see) is there not a change that the foult can be here? But the gasket is very cheap so I guess its not meant to seal compression as you say. I just cant get the design straightend out. Anyway Ill check the pistons
 
Just throwing this out there but double check #3 cylinder again. I can't see how you could possibly get any sort of decent compression readings turning the engine over by hand...
 
In all likelyhood it needs new OS pistons and bored .020 over. Research parts price and availability before making any decisions find out if the cylinders can be bored or if the engine is shot dont spend any money.
 
Jay1234 said:
Just throwing this out there but double check #3 cylinder again. I can't see how you could possibly get any sort of decent compression readings turning the engine over by hand...
Click to expand...

I tested it twice and when I turn it by hand I feel the lack of compression clearly. On the cylinders I feel when the specific piston that I test is compressing (the other spark plug holes are open) but on no 3 its no difference relative to the ones that have the spark plug hole open :(. Also I noticed that the spark plug was clean compared to the other three cylinders.

But sure the test is not to standard so to speak. Perhaps that accounts for the low readings on cylinder 1, 2 and 4 which are low but even, I dont know...
 
kimcrwbr1 said:
In all likelyhood it needs new OS pistons and bored .020 over. Research parts price and availability before making any decisions find out if the cylinders can be bored or if the engine is shot dont spend any money.
Click to expand...

Yeah I guess thats the case its probably next stop scrapyard for this one - too bad. Boring up cylinders and getting over sized pistons i get the feeling dollars would be going fast. Im guessing if im lucky its just the piston rings but I dont know how often they break leaving the cylinder unharmed
 
racerone said:
I believe I already mentioned using a rope on the flywheel to pull it over for the compression test.
Click to expand...

Right, I will do that - thanks. But no 3 is obviously out of order. Sorry for all the newbie questions - when removing the exhaust cover discussed earlier to inspekt the pistons/cylinders - how to reach those bottom screws. The bottom one is covered by shift mechanism. Do I need to release whole powerhead from bottom cowl or do take away gear linkage (which I dont see how to do, its very tight down there at the bottom of the cowl)
 
I also mentioned removing cover #16 as it is much easier !----With 0 compression the damage will be evident.-----And before you ask , the reed valves have nothing to do with cylinder compression either.----For me this engine would be an easy fix.-----Have all the used parts to do so.------I believe you should take this engine apart.-----The things you will learn will serve you well in the future.
 
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racerone said:
I also mentioned removing cover #16 as it is much easier !----With 0 compression the damage will be evident.-----And before you ask , the reed valves have nothing to do with cylinder compression either.----For me this engine would be an easy fix.-----Have all the used parts to do so.------I believe you should take this engine apart.-----The things you will learn will serve you well in the future.
Click to expand...

Right you said to remove item 16 thats opposite to the exhaust my mistake. Im trying to get my head around this, the carbs are located opposite to the spark plugs but the blown diagram I posted earlier indicates that item 16 is opposite to the exhaust cover. That would be the same side as the ignition coils. So fuel-air mix is transferred thrugh channels in the crank house to that side I assume? I took of a junction box (?) and the bracket för that but it was not beneath there. So I removed the 5 screws that seem to hold the coils to see if I could get to the intake ports, but the coils are stuck. I dont get much wiser from looking at the blown diagrams I have access to online. Help would be gremerc 50 coils small.jpgatly appreciated.

Its a good point to get to the bottom of it as much as I can. I dont have any parts for this enginge and besides from this there is more to fix but I might aswell do what I can
 
Ok so I got the coil pack off, had to pry a little, think it was glued to the intake cover. With the intake cover cone I could see the side of the pistons and on no 3 there was damage. In the intake channel I found 3 pieces of wire approx 2 mm in diameter (dont know where those came from??), another of those pieces were jammed into the top of the piston as can be seen from the image taken from the inlet port. Its not easy boroscoping the cylinder from the spark plug hole with a iPhone 6s in one hand taking pictures and a flashlight in the other shining through the inlet port :) but I got an image - I dont know how clear it is - of the piston viewed from the top where the wire piece is visible (red arrow) and unfortunally theres a scrath mark in the cylinder wall that seems to go all the way down to the port :( So i guess that explains the lack of compression. I dont se any other solution to this than boring out the cylinder and installing a oversize piston which for me is beyond reasonable repairs for this motor. So i guess thats thats but I want to thank everyone especially you racerone who contributed to my efforts in getting to the bottom of this.

bore.jpgIMG_8977.jpgIMG_8976.jpg
 
If the engine has the stock bore the scratch in the chylinder walls can be no more than .015” deep. Mercs are a complete different animal you need to properly ID your motor to get the correct powerhead that one will likely go to the scrapper. I cant imagine how roller bearings got jammed in the cylinder?
 
My guess is that those " wires " are the wristpin needles.-------Take this motor apart.----You will learn a lot about simple , precision machinery.
 
kimcrwbr1 said:
If the engine has the stock bore the scratch in the chylinder walls can be no more than .015” deep. Mercs are a complete different animal you need to properly ID your motor to get the correct powerhead that one will likely go to the scrapper. I cant imagine how roller bearings got jammed in the cylinder?
Click to expand...

How come it cant be more than 0,015" deep if its standard bore?
 
racerone said:
My guess is that those " wires " are the wristpin needles.-------Take this motor apart.----You will learn a lot about simple , precision machinery.
Click to expand...

The bearing between piston/crankshaft? Yes probably. Actually its gone now went to a new home, some guys bought it for parts hopefully it will come to good use. Sure I could have learned from it but that would have been it I wouldnt have got it running again
 
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