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1973 20hp strange compression readings

S

SaltyScot

Regular Contributor
I did a comp test on this motor recently and got some weird numbers. I don't know what they mean exactly. Top cylinder is 120. I tested the bottom cylinder 7 times and got 5,5,120,5,5,5, and after putting a little oil in, 10 psi. I can't figure out why i would get good comp once only. If the piston ring is wasted, how could i get 120 even once? I imagine i need a new powerhead or an overhaul. But i figured i better ask before i start anything like that. Motor starts up for few seconds and then sputters out. I did have it idling ok in a bucket for awhile until i gave it some throttle and it cut out. But the fuel line was definitely kinked at that point. I'm hoping to use this motor on a little hydroplane. Thanks.

Stephen
 
On the side of the block you will find a ----" transfer port cover"-----Held on with 3 screws I think.-----You would be wise to remove that cover to inspect pistons / rings.-----If nothing is wrong you might need a new $2.00 gasket.
 
racerone said:
On the side of the block you will find a ----" transfer port cover"-----Held on with 3 screws I think.-----You would be wise to remove that cover to inspect pistons / rings.-----If nothing is wrong you might need a new $2.00 gasket.
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Will do. Thanks for the tip.
 
I got it off and everything looks fine. But, of course, I can't see all sides of the piston. Is it possible, or worth it, to open up the exhaust side for more viewing? Or would that not be fruitful?
 
As I turn the flywheel, I can hear/feel the compression as the top cylinder goes to TDC, but the bottom one does nothing when it's its turn. Someone mentined to me that a crank seal under the powerhead often goes out in these where it takes out the compression on the bottom cylinder. Any thoughts on that?
 
The crankshaft seal has to do with compression in the crankcase.----That seal has nothing to do with compression in the cylinder.
 
Only thing to prevent it from having comprtessuion is damaged piston/cylinder or hole in jug. Do a leak down to see if its going elsewhere. Leaking seals and bad reeds will not cause cylinder not to have compression​
 
When doing compression test, it is good to remember couple of things. Ideally engine should be warm. Throttle should be set wide open. And the rope should be pull enough fast many times. I cant test my 77, 20hp right now, since it is been sitting over year and that is not ideal situation. I am going to put my merc to my boat during coming weeks. How about the feeling in the starter rope while you pull? My Merc feels really really hard from the rope during each stroke, i always feel like, i cant pull it enough fast. Well, it always takes about 3-4 pulls, even after one night stand. And i have all new powerhead seals, and many other things, since i have rebuild my engine.
 
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Why not, Racer? Secondly, our OP, SaltyScot, could have a piece of dirt stuck in the check valve in the compression gauge. Try cleaning it with compressed air.
 
At the bottom of the stroke the exhaust ports are wide open.----Cylinder will fill with air / exhaust / any kind of air to get results for a compression test.----Compression does not start till exhaust ports close !!
 
When i plugged the top cylinder, the good one, it was hard to pull. With the bottom one, it was easy. So i figured either it needs an overhaul or powerhead replacement.

I sold the motor and for a few bucks more, i bought a boat with a good(120,120) 1979 Evinrude 25hp with controls and a nice steering rack and pinion. I appreciate the help. I needed to know what it needed before i could sell it.
 
Good deal, thanks for the report. Thanks also to Racer for your explanation. I have noticed, however, a lower compression reading if the motor has full underwater exhaust and is being tested in the water tank, or on the water. Another thought........what is so critical, when testing a lower unit for leaks......to do a vacuum test????? How can water get in, unless the oil has first leaked out? What comes first.......the chicken, or the egg?
 
Another thought........what is so critical, when testing a lower unit for leaks......to do a vacuum test????? How can water get in, unless the oil has first leaked out? ​
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You always check both..pressure and vacuum. Oil heats from exhaust passing thru lower unit and expands and pushes seals to shaft to seal...when it cools off it will going into a vacuum and pull water into gearcase. I have see plenty pass one test and fail the other. Think like your canning with mason jars..as jar cools it creates vacuum
 
Excellent, thanks Pappy. More forces in action. Well explained. What do you see as proper vacuum testing level? Also how much pressure....12 to 15 psi?
 
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