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Old 07-17-2019, 10:16 PM
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ndutton ndutton is offline
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Compression Test Notes

Compression test details from another thread:
Originally Posted by Dave Neptune View Post
. . . . . . . Do a compression check again and then do another with a bit of oil sprayed into the cylinders like Al suggested.

Be sure the choke is open, the throttle is wide open, spark plugs removed and the ignition lead disconnected for a proper compression check.

Dave Neptune
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others
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Old 07-18-2019, 10:56 AM
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I think I also got this from Dave:

Test the engine while its warm, as you'll get different readings hot and cold.
Your readings will be higher on a hot engine, but if it passes cold it will pass hot.

1. Make sure water intake is shut off so you don't fill exhaust system with water which will then enter the cylinders.
(You're not actually going to run the engine during the test so it won't need any cooling)
2. Remove spark plugs. Cover holes with a rag.
3. Remove coil wire, disable fuel pump if electric (remove fuse or disconnect wire)
4. Open throttle to wide open and leave it there.
5. Be sure choke is OPEN completely
6. Screw tester into spark plug hole 1 and crank engine with starter for a few seconds.
You'll see the pressure on the gauge go up in steps and max out. When it tops out, that's the reading.
7. Repeat on other cylinders.

If the readings are low or vary more than about 10% from one cylinder to the next, add a tablespoon of motor oil through the spark plug hole of the low cylinder(s).
If this increases the pressure significantly, it indicates worn rings or valve guides (the oil acts to seal the gap somewhat).
If it doesn't change it's more likely a valve that's sticking or not closing all the way.

'Lone Ranger'

1978 RANGER 30
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The Following 4 Users Say Thank You to roadnsky For This Useful Post:
Dave Neptune (07-18-2019), GregH (07-19-2019), Sam (07-18-2019), thatch (07-18-2019)
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Old 07-18-2019, 01:09 PM
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I can add one more thing to the list:

Make sure the cranking battery is up to charge for all the tests. Even a change in cranking rpm can affect accurate comparisons of each cylinder's pressure readings.
Whiskeyjack a '68 Columbia 36 rebuilt A-4 with 2:1

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