1979 115hp Merc problem(s)

[Moehme]

Oh boy, where to begin!!!
Kind of a long post - hope someone can help me.

I have a compression problem on the number 3 cylinder.

Bought the motor (with boat and trailer last year for $500). Seller told me engine ran but needed a tune up. Listened to it run before I bought it (seemed to run okay) and agreed a tune up was probably in order. Being the backyard mechanic that I am, I figured I could probably do most of the work myself.

Thought it would be a good idea to check the compression before I get to far into it. Cylinders 1, 2, 4, 5 and 6 all had 120 lbs of compression, Cylinder 3 - maybe 30 lbs (this was last year). Assumed this could be from a stuck ring. Since a lot of folks thought that Sea Foam may do the trick, I gave it a try. Ran a couple of cans through the tank along with Sea Foam "Deep Creep" through the carbs. No luck - blew out A LOT of smoke but still had the same compression readings.

Just checked it a few days ago, now I have the same 120 lbs of compression in the same 5 cylinders but now have maybe 10 lbs in no. 3 - what gives?

A local marine shop told me to try pouring some Marvel Mystery Oil into the cylinder and let it set for a couple of days. Figured what do I have to loose. Here's where it gets interesting.

I raised/tilted the engine, poured in a couple of ounces into the cylinder just to watch it slowly drip out through the carburator. I thought maybe it was running out back through the intake port and somehow making its way to the carb. Then I thought, this can't be possible since the reed valves should stop this from happening.

Now to turn a long problem into a few short questions:
1) Any other possibilties for low/no compression in Cyl. no 3?
2) Why is the Marvel Mystery Oil draining back through the carburators?
3) Does this motor really have reed valves? (Seloc manual says it has a reed block located at the top of the crankshaft but yet nobody sells reed valves/sets)

Any help/suggestions is truly appreciated.

Thanks

 

[16vjohn]

Re: 1979 115hp Merc problem(s)

I'm not a marine engine mechanic or expert by any stretch of the imagination, but I am an automobile mechanic. If I am wrong or sound like an idiot it's probably because I am... I am just trying to help.

If it's a 4 stroke engine, then it's possible you have a burnt valve... that would make sense, cause it's still partially pressurizing the cyl.

The reed valve only controls the intake side of things. If the reed is stuck closed, the 10psi compression makes sense, cause that cylinder is not getting any air to compress. But... that begs the question, why is the oil leaking out if it's stuck closed? I think the best thing to do at this point would be to remove the reed block and have a look see.

 

[Derangedmadman]

Re: 1979 115hp Merc problem(s)

It is either bad or stuck rings or a bad cylinder wall. The oil is coming out back through the carbs because the piston is down low in the cylinder letting the oil feed back through the intake port. Rotate the the flywheel by hand half a stroke and it will let the cylinder fill up with oil (unless the rings are completely broken up).

Being the compression is that low you probably have some major damage.

 

[Chris1956]

Re: 1979 115hp Merc problem(s)

Remove the center transfer cover (starboard side of motor) and you can see the piston and rings, as well as the cylinder. You will likey see a scratched up piston, cylinder and rings. The rings will be welded into their grooves, from overheat or lean mixture.

post back with results

 

[Moehme]

Re: 1979 115hp Merc problem(s)

Remove the center transfer cover (starboard side of motor) and you can see the piston and rings, as well as the cylinder. You will likey see a scratched up piston, cylinder and rings. The rings will be welded into their grooves, from overheat or lean mixture.

post back with results

Thanks (to all) - will try this tonight if I get off work early enough.

Also, I'm assuming there will be a gasket that I will have to replace, correct?

 

[Moehme]

Re: 1979 115hp Merc problem(s)

Remove the center transfer cover (starboard side of motor) and you can see the piston and rings, as well as the cylinder. You will likey see a scratched up piston, cylinder and rings. The rings will be welded into their grooves, from overheat or lean mixture.

post back with results

Okay, removed center transfer cover - rings look good, not welded or stuck in their grooves. (spring back after pushing on them). BUT, piston isn't as nice and clean as the number 4 cylinder (below #3). Could this be the result of overheating and/or lean mixture and if so, would this affect the compression? If not, why the low compression?
(I can try to take a picture of it if this would help).
Also, I checked the piston the best I could by shining a flashlight through the spark plug hole and looking in the intake port. Top of piston looks good, no apparent holes or breaks/cracks.

Now for a couple more questions;
1) Other low compression possibilities?
2) Ordered a new gasket for the transfer cover - should I use a little Permatex (or other brand) liquid gasket in addition to the OEM gasket (as a safety measure)?

Thanks again in advance.

 

[Chris1956]

Re: 1979 115hp Merc problem(s)

See if you can see the cylinder walls. Look for scraches. The only reason for low compression are bad rings (yours are good), scratched up cylinder or cracked block.
Transfer cover gasket can go on dry.

 

[Moehme]

Re: 1979 115hp Merc problem(s)

See if you can see the cylinder walls. Look for scraches. The only reason for low compression are bad rings (yours are good), scratched up cylinder or cracked block.
Transfer cover gasket can go on dry.

Kind of hard to see, but the cylinder wall doesn't look too bad, might have some minor scratches, nothing major though. How bad would they have to be scratched in order to loose 110 lbs of compression and what would cause them to get scratched? Also, obviously the best way to fix a problem like this would be a complete overhaul of the powerhead, but is there some sort of cylinder wall rejuvinator on the mark (you know - quick easy fix)?

As for a cracked block, everything looks good and solid, no visible signs of it being cracked. But brings up another question/possibility - how about a bad head gasket?

Thanks

 

[cannonford57]

Re: 1979 115hp Merc problem(s)

one other thought may be the engine has ingested a reed or part of on #3and scarred the piston and cyl.it would explain both the low compression and the seafoam making its way through the carbs....just a thought

 

[Chris1956]

Re: 1979 115hp Merc problem(s)

That motor has a one piece block and head. There is no head gasket to leak.

I am not sure what to advise. Sounds like it is tear down, bore and replace piston with OS time.

 

[Moody Blue]

Re: 1979 115hp Merc problem(s)

With only 10psi in #3 cyl you either have a hole in the top of the piston or the rings are damaged or some other significant damage.

Regardless, you can't run the motor with that situation so unfortunately a teardown is in order to a)identify the problem and b)fix the problem

Once you identify the problem (and it will be obvious) make sure you figure out HOW it happened and fix the underlying problem so it doesn't happen again.

Good luck.

 

[Moehme]

Re: 1979 115hp Merc problem(s)

That motor has a one piece block and head. There is no head gasket to leak.

I am not sure what to advise. Sounds like it is tear down, bore and replace piston with OS time.

Not what I wanted to hear but if it needs to be done.

I am enough of a mechanic that tearing into the engine doesn't scare me too much, but it does bring up a couple of questions:
1) Is this more of a major undertaking that really should be done by a Mercury Service Center (that will probably end up charging me more than what the engines worth)?
2) I have all the usual tools that most any backyard mechanic owns, but the Seloc manual shows that I might need quite a few "special tools", is this true?
3) What is OS time?
4) If I do end up working on it myself (cuz the Mercury guy wants more than just my first born), how many cuss words will I be yelling and should I buy 1 case of Band Aids or 2?

Thanks

 

[Moehme]

Re: 1979 115hp Merc problem(s)

Kind of hard to see, but the cylinder wall doesn't look too bad, might have some minor scratches, nothing major though. How bad would they have to be scratched in order to loose 110 lbs of compression and what would cause them to get scratched? Also, obviously the best way to fix a problem like this would be a complete overhaul of the powerhead, but is there some sort of cylinder wall rejuvinator on the mark (you know - quick easy fix)?

As for a cracked block, everything looks good and solid, no visible signs of it being cracked. But brings up another question/possibility - how about a bad head gasket?

Thanks

Before I start tearing this thing apart, what's everyone's opinion of "Engine Restore"? The can says not for use in 2 cycle engines but their UK website gives instructions on how to do it (yeah, I'm confused too).

"Two stroke quick repair. This is the same way we cure worn two stroke cylinder bores quickly. Pour approximately 100ml down each empty spark plug hole and allow to drain down and settle the CSL particles down around the piston rings. This is just the same as doing a dry/wet compression test - any mechanic knows how to do this. Then use the starter to turn the engine over a few times before replacing the spark plugs and starting the engine. It will smoke for a while then hopefully it will cure the compression leakages and improve emissions and power."

RESTORE is a SAE-30 high performance motor oil containing billions of micro-particles of the proprietary formula CSL (Copper 60%, Lead 40%, Silver trace) in suspension. RESTORE is suitable for any engine, whether petrol, diesel, dual-fuelled

Worth a try or do ya think I'd be wasting my time?

 

[Lion hunter]

Re: 1979 115hp Merc problem(s)

Engine rebuild in a can, novel idea but a waste of money. Might as well drop lead sinkers down the hole and let them melt and fill up the grooves. Just kidding, Don't buy it it's a waste of money. I would find a clean spot in your garage and tear the engine down yourself. Once you figure out whats wrong then go from there. As for putting it back together it's just opposite.

 

[Chris1956]

Re: 1979 115hp Merc problem(s)

Overhaul of the powerhead on a 2 cycle motor is simpler than a 4 cycle motor. OS is oversize, as in oversize pistons. 2 cycle motors have a requirement to have a certain piston skirt to cylinder wall clearance. if you rebore a cylinder, you need to install an oversize piston and ring set.

There are no special tools required, just patience. The bearings on this motor are uncaged needle or roller bearings. I always end up pulling the crank out of the block, boring or honing the cylinders and replacing the pistons one by one, and then installing the crankshaft and reconnecting the rods one by one. This means that you get cups numbered 1-6 and put all the bearings and cages from each connecting rod into its numbered cup. You then stick the cage on with grease and insert the roller bearings one at a time (26 per connecting rod I think). The new piston will not come with bearings, so you need to reuse the needle bearings. Stick them on with grease. They have no cages, and have spacers to keep them aligned.

Oh yeah. if you have bearing cap screws, you must replace any that you remove with new ones. if you have bolts and nuts, they are reuseable. I have my engine throw a rod after I reused a connecting rod bearing cap screw.

 

[Moehme]

Re: 1979 115hp Merc problem(s)

Overhaul of the powerhead on a 2 cycle motor is simpler than a 4 cycle motor. OS is oversize, as in oversize pistons. 2 cycle motors have a requirement to have a certain piston skirt to cylinder wall clearance. if you rebore a cylinder, you need to install an oversize piston and ring set.

There are no special tools required, just patience. The bearings on this motor are uncaged needle or roller bearings. I always end up pulling the crank out of the block, boring or honing the cylinders and replacing the pistons one by one, and then installing the crankshaft and reconnecting the rods one by one. This means that you get cups numbered 1-6 and put all the bearings and cages from each connecting rod into its numbered cup. You then stick the cage on with grease and insert the roller bearings one at a time (26 per connecting rod I think). The new piston will not come with bearings, so you need to reuse the needle bearings. Stick them on with grease. They have no cages, and have spacers to keep them aligned.

Oh yeah. if you have bearing cap screws, you must replace any that you remove with new ones. if you have bolts and nuts, they are reuseable. I have my engine throw a rod after I reused a connecting rod bearing cap screw.

Thanks, you've eased a lot of concerns. I think I have just one more question before I start the cussing - how hard is it pulling off the powerhead?

Thanks

Okay, maybe one more question - is there anything else I should redo/replace while I've got it apart?