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Author Topic:   How to check engine compression

63WHALER posted 09-20-2002 08:57 PM ET (US)
How is the correct way to check engine compression? Is this something a do-it -yourselfer can do, if so where could one find the neccesary tools and gauges. Would like to check compression on a 1984 Johnson 90 hp.v-4. What should be good compresion for this motor?
Thanks in advance for the help & info.
JBCornwell posted 09-20-2002 11:02 PM ET (US)
Howdy '63.
The factory Service Manual will tell you to do this with the engine warm, spark plugs out, plug wires grounded and throttle all the way open.
You screw the fitting on the compression tester into spark plug hole #1 and crank the engine over at least 5 turns. I usually do this by jumping the starter solenoid so that nothing but the starter is "turned on". Record the guage reading. Repeat for each of the other cylinders.
Compression should be over 100psi in each cylinder and the range should not vary by more than 15%. The range is the most important number. If one cylinder is off from the others by more than 15% you have trouble.
The comression tester can be obtained at any auto parts store. Be sure to get the type that screws into the spark plug hole. Typical cost is about $20.
Good luck.
Red sky at night. . .
JB :)
Knockerjoe posted 09-21-2002 03:00 PM ET (US)
also make sure the compression tester you buy comes with a o-ring on the threads that screw into the spark plug hole. the very cheap ones do not. good luck!
Bigshot posted 09-23-2002 09:54 AM ET (US)
Just yank the emergency kill lanyard and that will cut all electronics except starter and then turn the key. Jumping solenoids can be a tad dangerous if a novice, plus turning a key is easier.

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Author	Topic: How to check engine compression
63WHALER	posted 09-20-2002 08:57 PM ET (US) How is the correct way to check engine compression? Is this something a do-it -yourselfer can do, if so where could one find the neccesary tools and gauges. Would like to check compression on a 1984 Johnson 90 hp.v-4. What should be good compresion for this motor? Thanks in advance for the help & info.
JBCornwell	posted 09-20-2002 11:02 PM ET (US) Howdy '63. The factory Service Manual will tell you to do this with the engine warm, spark plugs out, plug wires grounded and throttle all the way open. You screw the fitting on the compression tester into spark plug hole #1 and crank the engine over at least 5 turns. I usually do this by jumping the starter solenoid so that nothing but the starter is "turned on". Record the guage reading. Repeat for each of the other cylinders. Compression should be over 100psi in each cylinder and the range should not vary by more than 15%. The range is the most important number. If one cylinder is off from the others by more than 15% you have trouble. The comression tester can be obtained at any auto parts store. Be sure to get the type that screws into the spark plug hole. Typical cost is about $20. Good luck. Red sky at night. . . JB :)
Knockerjoe	posted 09-21-2002 03:00 PM ET (US) also make sure the compression tester you buy comes with a o-ring on the threads that screw into the spark plug hole. the very cheap ones do not. good luck!
Bigshot	posted 09-23-2002 09:54 AM ET (US) Just yank the emergency kill lanyard and that will cut all electronics except starter and then turn the key. Jumping solenoids can be a tad dangerous if a novice, plus turning a key is easier.