compression test refresher

HFJ · Jun 1, 2023

warm up engine - how warm is warm? should i run it till the thermostat opens?

pull all plugs, and open throttle fully, correct?

turn over 5 or 6 time for each cylinder?

i don't suspect any problems whatsoever, this is about 100 operating hours after an in-frame rebuild. it's just convenient for me to do now.

Jim L WA · Jun 1, 2023

(quoted from post at 10:27:04 06/01/23) warm up engine - how warm is warm? should i run it till the thermostat opens?

pull all plugs, and open throttle fully, correct?

CORRECT

turn over 5 or 6 time for each cylinder?

CORRECT

i don't suspect any problems whatsoever, this is about 100 operating hours after an in-frame rebuild. it's just convenient for me to do now.

Then I think they say to drop some oil into each cylinder and do it again.

Bruce (VA) · Jun 1, 2023

Minimum acceptable pressure is 90 lbs w/ the lowest pressure reading w/in 75% of the highest reading. Run the engine to operating temp, turn it off, remove all 4 plugs, remove
the breather hose & make sure the choke and throttle are both open. Crank it at least 5 compression strokes or until the gauge stops moving. Write down the first compression
reading (that is the valves seating) then write down the reading after 5 strokes or when it stops increasing. You should have two numbers for each cylinder. Then, add a
tablespoon of oil to each cylinder & repeat the process, but you only need the final reading for each cylinder. Write down the results & post back for help figuring out what it
all means.
75 Tips

HFJ · Jun 1, 2023

thanks guys. the numbers are marginal - 90, and 85 on #4. they start about 75.

i didn't do the oil thing. the pistons, rings, and sleeves were all replaced.

i forgot about removing the breather tube. how much would that likely affect the numbers? fwiw, if i flood it, the gas will drip out. those pieces of hose are unclamped and not tight.

my assumption is valves. i tested it because it was easy to do it today, not because i had any concerns about its performance. i only ask it to mow my lawn, and it no longer puts out a smoke screen the infantry is jealous of in the process. plus, i get to just change my oil instead of constantly topping it off

Abbby · Jun 2, 2023

If it starts and runs fine do not worry about compression

HFJ · Jun 2, 2023

(quoted from post at 08:44:24 06/02/23) If it starts and runs fine do not worry about compression

that was my take too - because it does indeed start like a charm and runs well. i had the hood off to change the fan belt, and since it was so easy at that point, i figured why not.

compression test refresher

HFJ

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Jim L WA

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Bruce (VA)

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HFJ

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Abbby

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HFJ

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