Low Compression on Cylinder #2

[FWDan]

History: My 1995 70HP Force has been dying at idle lately. It is difficult to get into gear, before it dies. Many times I have to use fast idle to start, even after it is warmed up. For cold start, fuel enrichment seems to be working properly. I increased the idle speed, which helped, but it still does not start and idle like it usually does. Using Frank A’s Carb and Timing tutorial, I planned to do the link and sync, then check/set idle and mixture. To start the process, I took the spark plugs out and decided it would a good time to check compression.

Compression Results: The readings were #1-135, #2-75, #3-145 psi. Adding oil in the #2 cylinder increased the pressure a little, up to 85 psi. Although, I probably did not get a good distribution of oil in the cylinder. The 85 psi on #2 concerns me, so I put an endoscope in #2 and found it looks different than #1 and #3. I will try to attach the video of #2. I’d like your thoughts on what the video is telling me. Does this look bad? Is this causing the low compression? I suspect it could be rings, cylinder wall scoring, piston, or head gasket issues. What is my next troubleshooting step? I think my main repair choices are: 1) Keep running it as is until the motor completely dies, 2) Pull the head and piston #2 only without a complete teardown, measure and repair as required, 3) Take the motor completely apart and evaluate and repair all bad or suspect components. Obviously, time and cost increases dramatically for option 1 through option 3. I would be doing all the work myself, so that helps tremendously on my out of pocket cost.

One more thing: The motor died on me this summer at high throttle. Long story short, it ended up being water in the fuel, which was fixed. Could the condition of cylinder #2 in the video be the result of water in fuel? With all your knowledge and experience, I’m looking for troubleshooting and repair advice for the low compression. Thanks.

 

[pnwboat]

Looks like top edge of the piston has a divot or chunk missing. Kind of hard to tell from the video. If there is a small divot, that is usually a broken ring.

 

[Nordin]

As pnwboat says and your choice in my opinion are first #2 then #3.
From the video only a change of the piston will not help you enough.
Score in the cylinder wall and you have to bore. Only hone it will not rise the compression enough to make it run decent.
With too much different in the compression numbers there is impossible to tune the engine well.

 

[FWDan]

In the attached photo, is the red arrow what looks like a divot? I thought that was carbon deposits. However, I can look closer tomorrow and maybe get better pictures.

 

[las]

At this point I would consider getting another engine that runs with good compression.
There is quite some scoring in that cylinder and my concern is that a rebore/ re-piston on 3 cylinders would cost more than it's worth. And you always end up replacing far more stuff than you intended.
If I was to try and make this engine run on a budget I would get another piston or maybe even just the rings, to fit that cylinder, chamfer the edges around the ports and give it a hone with a flexhone or similar tool.
Then maybe run 3% oil. Ideally jet the engine a hair richer and beware of fouled plugs in the future.
But I agree, it wont run like it should with a cylinder on lower compression, but sometimes we have to make do with what we got!
It all depends on where your at financial, timewise, skills and lots of other stuff

 

[jerryjerry05]

The motor you have was one of the worst Mercury/Force put out.
The single carb feeding 3 pistons was a problem from the beginning.
I'd try to sell the parts, you'll get more for the pieces than selling outright.
Fixing? around 1200$ just a wait and see when/if it breaks again.

 

[FWDan]

Well, I guess it doesn't cost anything to take the head off and take a closer look. Maybe I can see what's going on with the #2 piston. Taking the piston out would be the best way to see the cylinder wall, piston and rings. Then I can see how bad it really is. Can I remove the piston without splitting the crank case?

 

[las]

I have no idea, but maybe you could access number 2 trough the reed plate opening?
Go for it and let us know what you find.

 

[jerryjerry05]

You can remove the piston without splitting the cases,
Remove the head, then from the front, remove carbs and reed assy.
Just be careful and watch for dropped bearings, some come in a cage some don't.
Also the under the pack and electronics are the port covers(3) you can remove the center one and inspect through the holes.
Also on the other side the exhaust chest cover can be removed and you can see through the exhaust holes. IF??? you remove the exhaust? the replacement can be tricky and usually leaks. Have the surfaces of the block and cover milled and the leaks are minimum.
FrankA (RIP)made a video on how to do the removal without taking it apart

 

[FWDan]

I sure would like to see Frank's video. He posted a lot of material. How would I go about finding it?

 

[jerryjerry05]

I looked for the video and all I could find was 2 but neither covered the one you want to see.
Possible he didn't make one for that??? Sorry
He posted about it in this forum, maybe use the search function.
One thing he stressed was make sure you count the bearings as you take them out and replace them. 16 is the count unless they come caged?
Magnet: use it sparingly, the bearings get magnetized when left on the magnet for a while. If magnetized? they can pickup small metal particles.

 

[FWDan]

Searching the archives I found some good articles about removing the rod cap through the intake and pushing the pistons out the head end. So I went ahead and removed the head, and the piston from my trouble cylinder #2. The piston is damaged and both rings are broken. Sections are missing from each ring. The cylinder wall is damaged, but not quite enough to catch a finger nail on. I can feel the rough but it's not deep enough to actually catch my nail in the worse area. How can I determine the the depth of the damage? It seems to me the top ring caught the edge of the exhaust port damaging the piston and the cylinder wall. Or did the ring fail first? What do you think?

 

[jerryjerry05]

What came first the chicken or the broken ring??
Who knows??
The damage is bad enough it needs a bore job, might get away with .010 maybe as much as .020?? The people at the machine shop will be able to say.
But I'd really think about a different motor, these 70hp with the single carb are famous for failure.
The parts of your motor are worth way more $$ if sold individually.

 

[FWDan]

Yeah, a quick look at Craigslist in my area this morning shows a 1984 Mercury 75 for $1500. 11 years older than my motor and probably wore out too. There's a 1996 Johnson 125 with 10 hours on a rebuild. Asking price? $5000.
Where's a good place to find used motors? If I cannot find a different motor, fixing the one I have might be the only way to go.

 

[las]

It's a tough one.
Many times the junk you buy of others hands are just that, junk. Poorly maintained, kept running when it should have been repaired or serviced. Seized nuts and bolts and with low compression or any other symptoms under the sun.......
Some times it's better to buy a complete boat and sell off the boat!

As I see it you could scrap the engine.
Repair it the cheapest way and run it till it dies on you, could be sooner or later!
Repair it the right way and keep it well serviced and maintained looking forward.
It's a tough one, pick your poison.

 

[FWDan]

I'm leaning toward repairing it the right way. I found a powerhead rebuild kit that includes: 3 pistons, rings, wrist pins, wrist pin bearings, wrist pin washers, circlips, rod bearings, upper crank bearing, rod bolts, gaskets, seals, O-rings, gasket sealing compound, Loctite, crankcase sealer, crocus cloth, and assembly grease for $425. How much would I expect to pay a machine shop to bore all three cylinders?

 

[roscoe]

Bore the bad cylinder and replace the one piston.
Replace rings on all 3 pistons.

 

[jerryjerry05]

I pay 40$ for 1 hole, 80$ for 4. Have to pay for setup the extra holes are easy after initial setup.
Not sure where you are and the local costs???
First you need to figure out why the #2 went bad?
OR??? you'll be redoing the motor again.
Could have been sucking air somewhere? Lots of times one small backfire can blow the port cover gasket and start sucking air and problems start.
Like I said I don't know where your at but Facebook has, Market Place and ebay's everywhere.

 

[las]

I'm leaning toward repairing it the right way. I found a powerhead rebuild kit that includes: 3 pistons, rings, wrist pins, wrist pin bearings, wrist pin washers, circlips, rod bearings, upper crank bearing, rod bolts, gaskets, seals, O-rings, gasket sealing compound, Loctite, crankcase sealer, crocus cloth, and assembly grease for $425. How much would I expect to pay a machine shop to bore all three cylinders?

That sound like a good deal! Is it oversize pistons?
Maybe find a machine shop and have them look at it to determine what oversize you should be ordering.
Maybe have them chamfer the ports at the same time, I know I would do it when in there!
And like Jerry said, try and figure out why it clipped the rings. Chances are when you reassemble it you also fix any leaking gasket it had beforehand.
It could also be due to carbon build up and you just got unlucky.

 

[The Force power]

What came first the chicken or the broken ring??

The Rooster