Compression test by JB

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tashasdaddy

tashasdaddy

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JB
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1. Fully charge battery.
2. Start engine and warm her up.
3. Remove all spark plugs and disconnect fuel line.
4. Rig a jumper to engage starter solenoid without turning ignition on. Pull the emergency kill switch on rope start engines.
5. Screw guage into #1.
6. Turn engine over until reading on guage stops increasing or 5 times, whichever is more.
7. Repeat #5&#6 for all cylinders.

All psi readings should be within a 10-15% span.

* It has no effect to open the throttle.
* Don't get psi hysteria. Remember this is a 2 stroke engine and you are an amatuer at compression testing.
* Rope start engines will usually give lower psi readings than your know-it-all neighbor claims are required. Same for big JohnnyRude V6s
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RED SKY at night. . .
JB
 
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Jeff2012

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Re: Compression test by JB

Thanks for the reply,
What 2 places do you exactly hook up the jumper?
 
JB

JB

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Re: Compression test by JB

The starting jumper (step #4) goes from the +12V cable to the terminal that the starter switch normally puts +12V on to start.
 
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xmos

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Re: Compression test by JB

What is the normal compression range for a 1982 90hp Johnson Seahorse after the engine has warmed up? Thanks.
 
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CaptainHook

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Re: Compression test by JB

xmos said:
What is the normal compression range for a 1982 90hp Johnson Seahorse after the engine has warmed up? Thanks.
Click to expand...

I have one that runs great and is around 135-140 psi in each cylinder.
 
captseacraft

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Re: Compression test by JB

I tested my 85' V4 90hp yesterday cold,closed throttle and the left two cylinders were exactly the same at 115psi the right two at exactly 120psi.
Today I de-carbed by warming up the motor at lunch and pulled the irridium br8hix plugs.shot quicksilver engine tune into the cylinder numoruse times turning the motor over each time.Then I filled the cyliders one last time and went back to work.After work I turned it over a few rotations to clean out the de-carbing spray,reinstalled the plugs,ran the motor till it cleaned up and proceeded to check the compression once again.this time I did it with wide open throttle and warm engine.I got 120lbs on the top left cylider and 118 on the lower and both rigt side cylinders were at 125 psi each.
My engine is a strong runner and I am glad that the compression was so close and feel monitoring the compression is a safe practice every few months, Best
CaptSeaCraft
 
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JB

JB

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Re: Compression test by JB

There is no specified minimum compression.

If all cylinders are within 10% the engine is healthy.
 
tashasdaddy

tashasdaddy

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Re: Compression test by JB

no need to bump. this is in the TOP SECRET FILE, 1st thread in this forum.
 
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xmos

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Re: Compression test by JB

Capthook....ty. My compression was 110, 110, 110, 120 starting from cold. Engine runs good.
 
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jiml100

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Re: Compression test by JB

for the compression check on my tracker boat with a 60 hp johnson 3 cylinder i can just pull the kill lanyard instead of jumping the plug wires correct ? thanks
 
JB

JB

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Re: Compression test by JB

There is no mention of jumping plug wires, Jim. Whatever will disable the ignition during the test will suffice.
 
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boobie

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Re: Compression test by JB

Just make sure your ignition is disabled with the plugs out and cranking over. I've seen some mtrs go "boom" if this is not done.
 
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Robbiecd

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Re: Compression test by JB

Regarding step 6: Do you use the key to turn the engine over, or do you connect the jumper? What is the purpose of the jumper? I want to make sure nothing goes boom. I'm assuming the plugs should NOT be firing when you turn over the engine?
 
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wilde1j

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Re: Compression test by JB

You can use a remote starter switch with the ignition "off" and the lanyard pulled to be double safe. You don't want the plugs firing during this test or you can maybe get the BOOM.
 
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stumpnocker

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Re: Compression test by JB

Remember, remove ALL plugs before checking any of the cylinders.

Thought it worth mentioning this is an excellent time to check your spark while you're at it, since the plugs all have to be removed anyway. No need to jumper anything out, just make sure each plug is grounded against the block when you turn the motor over. Kill 2 birds with one stone this way...
 
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wilde1j

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Re: Compression test by JB

Why are you replying to a to a TWO YEAR OLD THREAD??? And no, you don't want to check the spark when checking compression. Read above if you do know why.
 
JB

JB

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Re: Compression test by JB

stumpnocker said:
Remember, remove ALL plugs before checking any of the cylinders.

Thought it worth mentioning this is an excellent time to check your spark while you're at it, since the plugs all have to be removed anyway. No need to jumper anything out, just make sure each plug is grounded against the block when you turn the motor over. Kill 2 birds with one stone this way...
Click to expand...

Sure, and kill yourself in the bargain. Even with the fuel line disconnected and the throttle closed fuel and air are being pumped out of the plug holes during this test. Allowing a spark in the vicinity is several steps beyond. . . .um. . . just not smart.

I am going to close this before we get any more dumb ideas.
 
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