Outboard compression test questions?

[Rhodes]

I am considering purchasing a 1989 48 HP Johnson boat motor but the motor has been setting up a couple of years. I want to test the compression but I've never performed one of these tests before. In my "how to" searching I'm getting more questions than answers so I need help understanding the proper way to perform a compression test. Some of the questions I have are as such: "Do I take all the plugs out at once or just one at a time as I test each cylinder?" "What do I do with the plug wires i.e. does the spark plug have to be in the boot and grounded to the engine as I turn the engine over if I’ve disabled the ignition by pulling the kill switch?" "Anyone know that the compression should be on a 1989 48 horse Johnson?"

[Chalk]

take one spark plug out at a time, you can use the kill switch to keep the engine from firing or remove all the spark plug wires from the spark plugs

compression should be within 5-10% of each cylinder

If the motor has sat up for awhile..it might be good to put a little oil in each cylinder and turn it over a few times with the kill switch off and then run the engine for a minute afterwards.

[FUTCHCAIRO]

WHAT CHALK SAID, JUST SCREW THE COMPRESSION GAUGE INTO THE SPARK PLUG HOLE AND TURN THE ENGINE OVER 10-15 REVS. AND CHECK THE GAUGE. ON AN OLD 40 THE COMPRESSION SHOULD BE IN THE 120 TO 135 POUNDS ON BOTH CYL. NOT OVER 10 POUNDS DIFFERENCE BETWEEN THE TWO CYL. I ALWAYS SPRAYED SOME LUBE. IN BEFORE RUNNING THE ENGINE FOR THE CHECK. I HAVE A 85 HP EVINRUDE , 1985 YR. MDL. AND IT IS CHECKING AT 125-130 ON ALL FOUR CYL., STILL KICKING OUT ABOUT 85-90 HP. THOSE OLD 40 HP ENGINES HAD REALLY GOOD COMPRESSION AND ARE HARD TO PULL OVER WITH A STARTING ROPE. I AM GLAD THE ONES I HAD WERE ELEC. START.
PA
SEMPER FI

[MarkM]

I have a compression test gauge if you want to borrow it?

[Rhodes]

Thanks guys....both cylinders checked about 142 PSI!

[CarolinaJim]

142, did you run the test with the throttle wife open? The only reason I ask is because I saw below the suggestion to oil the cylinders. This is good suggestion but you have to make sure to burn all the oil off before compression testing, the oil will act as a seal and give false readings. With throttle closed 120-130 is "normal" throttle open 140-150 is about right.

[FUTCHCAIRO]

WITH THE READINGS YOU SHOW I WOULD NOT BE AFRAID TO BUY THAT ENGINE, IT IS IN ABOVE AVERAGE CONDITION AND SHOULD RUN A LONG TIME, MAYBE 10-12 YRS. WITHOUT ANY TROUBLE IF YOU USE CLEAN GAS AND OIL, DO NOT RUN ANY ETHANOLE.
pa
SEMPER FI

[Chalk]

It is recommended to run engine to operating temp, shut off, remove spark plugs, open throttle and then run compression.

But I have done one at a time, with all in and found that as long as they are close, you are good