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Compression testing


johnnugent's Avatar
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02-10-10, 04:45 PM   #1  
Compression testing

I'm looking to buy a boat with an out board motor. It is a 1983 140hp Evinrude. I want to do a compression test on the motor. I need to find a site or someone that can tell me what I have to do to perform the test correctly. the motor is in CT and it will be cold. I think it is a 2 stroke.

 
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Rick Johnston's Avatar
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02-11-10, 04:53 AM   #2  
I wouldn't think about buying a marine engine without hearing it run. For proper testing, the engine should be warmed up anyway ...

A compression tester is inserted in place of one of the spark plugs. With the main coil wire disconnected and throttle wide open, the motor is cranked for a few seconds and the compression number noted. Replace the spark plug and do the same with the rest of the cylinders.

The compression test is about variance, not absolute numbers. All of the cyclinders should read close to the same numbers. If not, the engine may have a problem.

 
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02-11-10, 07:32 AM   #3  
compression test

Do I have to take the other plugs out or can I just disconnect them? We will be running the motor before the test to warm it up. How long do you think we should run it? Do you know of any common problems with this type of engine?

 
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02-12-10, 04:25 AM   #4  
Pulling the main coil wire prevents the engine from firing during the test.

The only problem common to all 1983 engines is ... it's 27 years old. The good side is, if it runs smoothly it was probably well maintained.


Last edited by Rick Johnston; 02-15-10 at 04:29 AM. Reason: Removed "plug" answer. See posts 5 & 7 below.
 
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02-13-10, 04:41 PM   #5  
Compression tests are to be done with all plugs removed so the rotational resistance is equal on all cylinders, the same amount of rotations so counting is important . The engine should be warm and watch the first 2 rotations and compare the pressure readings then as well as after perhaps 6 or 7 . Your looking for an abnormal pressure from the rest. Every gauge reads a bit different so it's subjective as to max pressures. Also on a 4 stroke engine the throttle plate should be wide open.
Bill

 
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02-14-10, 09:52 PM   #6  
I run them for 10 min or so, as it's running I shift it into fwd and rev at idle to make sure it shifts with a clunk rather than a grind. Pull the prop off that way you can shift it safely, you can also look down the propshaft from straight behind and as it's turning you can see if it's bent.
Once the water off the tell tale is worm pull the tether and check that it kills the ignition for your spark test like they posted.
They were a good reliable motor I would change the impeller, gear lube and run some decarbonizer through it and then spark plugs, it should run like a top.

 
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02-15-10, 04:27 AM   #7  
Posted By: Tijoe Compression tests are to be done with all plugs removed ...
Several minutes spent searching the methods came up with about a 60/40 mix in favor of your advice, so I defer to you.

 
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02-15-10, 09:47 PM   #8  
Posted By: Rick Johnston Several minutes spent searching the methods came up with about a 60/40 mix in favor of your advice, so I defer to you.
Thanks Rick, I actually ran a chassis dyno at a MB dealership for 11 years. Engine diagnosis is kinda second nature. Now after retirement I offer my version from time to time.
Bill

 
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