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Low compression on all 4 cyl's of 1985 Mercury 50 2-stroke

R

rwbrown3

New member
I bought this engine last summer to put on a 18 ft aluminum boat. Last week I started getting it ready for the water and noticed writing next to the spark plugs. The previous mechanic had written the compression numbers next to each plug. Each cylinder is labeled #80, #80, #80, and #80. I remember the mechanic telling me that the important thing to note is that the compression numbers are equal (or close) on all cylinders. The motor runs great, just a little light blue smoke coming out of the exhaust, it never hesitates to start, unless I flood it pumping the ball, and seems to run all day. I use it the boat in Cook Inlet to get from the village where I teach, to Homer a trip of about 24 miles. I just installed a tach last week so I could know how hard I was running the engine. Last year I rarely if ever ran the motor over 3/4 throttle because I did not know how much the motor could take, better to go slow 13kts or so and make it home than to try and set a water speed record and be set adrift waiting for a tow. I guess my question is I have a compression tool to test compression myself, is it as simple as taking the plugs out and cranking the engine, I read somewhere that I needed to ground the spark plug wires but not sure if I understand the "why" of that. And should I check compression after letting the engine warm up or do it cold? Also is #80 really bad if it seems to run just fine. Any help is greatly appreciated.
 
Here we (may) go again. These are tiny cylinders compared to an automotive engine, and using an automotive tester can result in low readings. A Merc like that would run terrible with such low compression.

Jeff
 
fastjeff said:
Here we (may) go again. These are tiny cylinders compared to an automotive engine, and using an automotive tester can result in low readings. A Merc like that would run terrible with such low compression.

Jeff
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Hey Jeff, I don't know why a tester used in automotive applications would differ all that much in readings, I would think PSI is PSI either way. The readings were done at a very reputable marine shop in Olympia Washington, Tom's Outboard. I was concerned about the low compression readings myself and have considered them to be wrong, but also considered that no matter what they are, all of the cylinders were the same and regardless of the compression the engine does run fine, or maybe I don't know what a good running outboard is, after all this is my first outboard. I have a compression tester and want to use it and my question about using it is do I need to ground the spark plug wires to the engine? Also can I leave three of the four plugs in the engine with spark plug caps off and test one cylinder at a time by just cranking the engine? Would it hurt to do it when the engine is cold, then let it warm up and do a second reading to compare the difference?
 
Thanks Racerone that's what I was thinking also. I want to check the compression myself and regardless of the numbers I'd like to see if all the cylinders are equal. The motor does seem to run well. I bought the motor and controls for 1,000 bucks thinking that at least if I got a season out of it then I wouldn't take that big a loss. Going into season two and so far the biggest expense has been a new prop, I must've looked like a farmer on a tractor plowing some the places I have taken my little boat, I think I'm coming out with a good deal on a old motor. The serial number on the thing stumps me and maybe the previous owner too, he told me it was a 1986 model, but from I read Mercury 50 2 strokes were 3 cylinders in 1986 and 4 cylinders prior to 86. So far I've lucked out with the parts I ordered like an impeller and such as I use 1985 as the model.
 
Posting a picture often get results on quick identification.---So what is the serial # on it ?----The 4 cylinder 45 / 50 hp models are just excellent motors !
 
Seeing it was done in a competent shop, the readings are probably correct, unfortunately.

Excessive volume in the hose of a tester tends to read low with small displacement motors. A correct tester for these has a minimal volume in its hose.

Jeff
 
If it is running good give it a good decarb and do the compression test again. Get a can of seafoam spray and bring it up to operating temp and at a high idle spray the seafoam in the carbs evenly until it starts smoking real good and quickly shut it down. Wait 1/2 an hour and then start it and run WOT for about 20 minutes. It could be carbon buildup on the rings 80 and even is a good sign. Is it reaching proper WOT rpm? If not get a prop with less pitch what is the WOT rpm?
 
Excessive volume in the hose of a tester tends to read low with small displacement motors. A correct tester for these has a minimal volume in its hose.
Click to expand...

Nahhhh aint going to be off much..... There aint but about 5 psi difference between my Merc screw in tester and my OTC tester and it has a 12"hose.
 
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