Low Compression in 1 out of 4 cylinders

[mmarino]

I have a 1979 Mercury Blueband 50 HP. The previous owner hadn't run it for quite a few years. I got it running, hard starting even after changing plugs. Took it out on the lake. Ran well except for a little rough idling. Ran into a water pumping issue, got it solved with a new impellar. Now for the bad news. Did a compression check on it. #s 1,2 and 4 cylinders show 130, #3 shows 40. A local shop did the check and the owner told me it was in that cylinder, rings or piston, and not a valve issue by the way it acted when he cranked it over and had it running. Left with an under $1200 labor estimate plus whatever the parts would be, something I'm not sure I can afford.

Are there any preliminary measures to take before spending that kind of money? Would decarbing be worth a first effort? Any other suggestions?
Thanks a lot for your time responding.
Marty

 

[trendsetter240]

Re: Low Compression in 1 out of 4 cylinders

I have a 1979 Mercury Blueband 50 HP. The previous owner hadn't run it for quite a few years. I got it running, hard starting even after changing plugs. Took it out on the lake. Ran well except for a little rough idling. Ran into a water pumping issue, got it solved with a new impellar. Now for the bad news. Did a compression check on it. #s 1,2 and 4 cylinders show 130, #3 shows 40. A local shop did the check and the owner told me it was in that cylinder, rings or piston, and not a valve issue by the way it acted when he cranked it over and had it running. Left with an under $1200 labor estimate plus whatever the parts would be, something I'm not sure I can afford.

Are there any preliminary measures to take before spending that kind of money? Would decarbing be worth a first effort? Any other suggestions?
Thanks a lot for your time responding.
Marty

You could try to decarb it, that would only help if the problem was stuck rings on that cylinder that had not yet broken free or damaged the cylinder.

As you said, you're looking at $1200 plus parts (several hundred more at least) and that is to rebuild a 30 year old motor. Not worth it in my opinion.

For that money you could find a good running, newer 50hp on craigslist or ebay.

 

[bryanwess2000]

Re: Low Compression in 1 out of 4 cylinders

When did they start putting valves in 2 strokes? If you decide to rebuild I'd go to another shop. I'd decarb it and buy my own compression guage. It
just sounds fishy talking about valves on a 2 stroke. Either the guy doesn't
know what he's talking about ,unlikely, or he's trying to see how much you
don't know and price the repair accordingly, i.e. gouging. If after you
decarb and the compression doesn't come up you can pull the exhaust
cover to see what the bore and piston and rings look like if you feel comfortable doing it. It's a pain removing 30 yr old gaskets.

 

[trendsetter240]

Re: Low Compression in 1 out of 4 cylinders

When did they start putting valves in 2 strokes? If you decide to rebuild I'd go to another shop. I'd decarb it and buy my own compression guage. It
just sounds fishy talking about valves on a 2 stroke. Either the guy doesn't
know what he's talking about ,unlikely, or he's trying to see how much you
don't know and price the repair accordingly, i.e. gouging.

I was thinking the same but held my tongue.

The only valves on a 2-stroke are reed valves and that is not your problem here. That being said, 40psi is very low on a cylinder. If you are going to bother with the decarb then yes, definitely get a compression tester (they're cheap) and test compression yourself before and after decarb.

Probably just wishful thinking if the mechanic was right on that compression reading though.

 

[mmarino]

Re: Low Compression in 1 out of 4 cylinders

I'll try my own compression check. Are the steps to this procedure accurate?
Remove all plugs, start at top cylinder crank motor over 4-5 times, take reading and repeat procedure down through the rest of the cylinders?
Thanks!

 

[bryanwess2000]

Re: Low Compression in 1 out of 4 cylinders

Yea, I wouldn't think the reeds would effect the compression readings
since they just distribute the air/fuel mix but I could be wrong. I'm no
mechanic I just pretend to be one when I work on my toys, and i know
just enough to be dangerous and get myself in trouble.

I agree though on not forking out $1200 plus dollars on a 30yr old
motor. If your at all mechanically inclined you might be able to just hone
the #3 cyl. and put new rings in. Just watch craigslist and your local
sales ads and you can probably find a good priced motor given the season
and the economy. I'd still get that compression guage and check out
your new motor if you decide to buy another one. Good luck either way.

 

[bryanwess2000]

Re: Low Compression in 1 out of 4 cylinders

You'll need to ground the coils/spark plug wires so you don't mess up
your coils or distributor, I'm unsure what you have, probably coils on a 1979.
Or you can run a wire from your battery and touch it on the starter solenoid
with the key in the off position to prevent the ignition from firing. I'm not sure on the details as I've never used the jumper wire method but i'm sure
there's probably a write up somewhere on here.
Some say to have a fully charged battery. It's never made a difference on my
readings as long as the battery's not drained.
Some say to open to throttle all the way. Again this has never made a difference when checking a 2 stroke only on 4 strokes.
The rest of what you have listed looks right.

 

[lowkee]

Re: Low Compression in 1 out of 4 cylinders

I'd be inclined to crack the block and peek at the piston/rings/cyl wall. If it isn't too bad, I'd be tempted to just hone the cyl a bit and throw a new set of rings on the bad cyl's piston. Sure, it won't be perfect, but it'll last a long time and only cost you a few bucks for rings and a hone (harbor freight.. ~$12 for a hone). The reed valve WILL affect compression if it is burned through or rusted. You may get really lucky and only have that be wrong, but it isn't likely. Fear not, as you can likely get it to at least 90% without too much trouble and with very little cost. Save what you would have spent on a rebuild for your next motor 5-8yrs down the line when this one finally gives in.

 

[trendsetter240]

Re: Low Compression in 1 out of 4 cylinders

You'll need to ground the coils/spark plug wires so you don't mess up
your coils or distributor, I'm unsure what you have, probably coils on a 1979.
Or you can run a wire from your battery and touch it on the starter solenoid
with the key in the off position to prevent the ignition from firing. I'm not sure on the details as I've never used the jumper wire method but i'm sure
there's probably a write up somewhere on here.
Some say to have a fully charged battery. It's never made a difference on my
readings as long as the battery's not drained.
Some say to open to throttle all the way. Again this has never made a difference when checking a 2 stroke only on 4 strokes.
The rest of what you have listed looks right.

yes, what bryanwess said. Though you don't need to ground your plug wires if you have a kill switch. Just pull the kill switch lanyard before running the test.

Also if your battery is low the compression test will read lower than if the battery is fully charged so best to make sure it is fully charged for an accurate reading.

For more detailed instructions go to the link below and click the first thread "Compression test how to's".

http://forums.iboats.com/forumdisplay.php?f=24

 

[trendsetter240]

Re: Low Compression in 1 out of 4 cylinders

The reed valve WILL affect compression if it is burned through or rusted.

See, that's what I thought and said in another thread but was shot down that it would only affect crankcase compression and not cylinder compression. Either way it is far more likely to be stuck rings or scoring on that low cylinder.

 

[mmarino]

Re: Low Compression in 1 out of 4 cylinders

Thanks guys for all of the helpful information. All of you have been more than kind with your time. It is greatly appreciated. If I get brave enough and tackle this, I'm sure it would be helpful to have a manual. Having a hard time finding a shop manual online. Usually find the SELOCs/ I think Clymers. Any ideas on where to find the shop manual? Or are the others adequate for the job at hand?

 

[bryanwess2000]

Re: Low Compression in 1 out of 4 cylinders

A factory service manual is best ofcourse but they can be hard to find
especially for older motors like ours. A seloc or clymer is better than nothing.
Some people say you can find factory manuals on ebay but i've never had any
luck.

 

[Inline6]

Re: Low Compression in 1 out of 4 cylinders

Another suggestion. Like the other guys suggested I would try decarb first. Also you need to make sure you find the root cause of the low compression. If the cylinder is scored it is likely the lower carb is operating in a lean condition. You need to rebuild that carb and make sure there is no trash in it - which could cause a lean condition.

You might consider going ahead and looking at that carb to see if it has trash in it (before trying to remove the cover mentioned).

If you go with repairing the engine yourself I would try to get a replacement piston - if you find #3 scuffed at all. If you are going to open it up you might as well.

Good luck!
Inline6

 

[thefairlaneman]

Re: Low Compression in 1 out of 4 cylinders

Im not expert but I wouldnt put any real money into it. Like said, soak the rings to be sure its not one stuck. I always pull the plugs and dump about an ounce of marvel mystery oil into each cylinder and then slowy rotate the crank every day for 3 or 4 days. Then you know if it still wont pump up you will have to take it apart. Paying any shop at this point will quickly have you spending more than its worth. Plan B is to find another engine like it that has a solid power head and keep yours for parts.Plan C is to sell it for parts and find another engine that you are more sure of.

 

[mmarino]

Re: Low Compression in 1 out of 4 cylinders

Thanks, Guys. Again, I appreciate all of the help and advice. I'll follow up on it after I decide what I am capable of doing or what I am not capable of doing mechanically.

 

[Really Old Guy]

Re: Low Compression in 1 out of 4 cylinders

Im not expert but I wouldnt put any real money into it. Like said, soak the rings to be sure its not one stuck. I always pull the plugs and dump about an ounce of marvel mystery oil into each cylinder and then slowy rotate the crank every day for 3 or 4 days. Then you know if it still wont pump up you will have to take it apart. Paying any shop at this point will quickly have you spending more than its worth. Plan B is to find another engine like it that has a solid power head and keep yours for parts.Plan C is to sell it for parts and find another engine that you are more sure of.

+1 for small amount of Marvels Mystery Oil. 2 stokes like it!