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procedure for compression chek....

Old 06-03-2006, 06:15 PM
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Default procedure for compression chek....

on a v-6 outboard....do you pull 1 plug at a time and check it or do you have to pull all of them ?? how long do you have to turn it over to get a correct reading ?? thanks alot....
Old 06-03-2006, 07:26 PM
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Default Re: procedure for compression chek....

Make sure the battery if fully charged, pull the safety lanyard, pull all plugs, put throttle all the way open, turn the key on each cylinder atleast 3 revolutions. Write down the cylinder # and all cylinders should be within 10% of each other.
Old 06-03-2006, 07:28 PM
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Default Re: procedure for compression chek....

Reel Boobs - 6/3/2006 6:26 PM

Make sure the battery if fully charged, pull the safety lanyard, pull all plugs, put throttle all the way open, turn the key on each cylinder atleast 3 revolutions. Write down the cylinder # and all cylinders should be within 10% of each other.
......thanks a ton reel....
Old 06-03-2006, 08:32 PM
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Default RE: procedure for compression chek....

Old 06-03-2006, 09:46 PM
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Default RE: procedure for compression chek....

Mumblerone - 6/3/2006 7:32 PM

.......thanks mum.....will compression read lower if the motor is cold as opposed to op temp ?? reason being mine wont start now as it sits in the driveway.....thanks again man...
Old 06-03-2006, 10:01 PM
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Default Re: procedure for compression chek....

Make sure you ground the spark plug wires to the block if you have a CD ignition system. I bought a set of alligator clips with jumper wires from Radio Shack for this purpose.
Old 06-03-2006, 10:24 PM
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Default Re: procedure for compression chek....

capecodguy - 6/3/2006 9:01 PM

Make sure you ground the spark plug wires to the block if you have a CD ignition system. I bought a set of alligator clips with jumper wires from Radio Shack for this purpose.
What's the point in doing this?
Old 06-03-2006, 10:58 PM
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Default Re: procedure for compression chek....

jethro1 - 6/3/2006 9:24 PM

capecodguy - 6/3/2006 9:01 PM

Make sure you ground the spark plug wires to the block if you have a CD ignition system. I bought a set of alligator clips with jumper wires from Radio Shack for this purpose.
What's the point in doing this?
.....ditto.... ;?
Old 06-03-2006, 11:34 PM
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Default RE: procedure for compression chek....

Cold will give you completely different values. Probably not helpful and misleading numbers if not at operationg temp. I've never done it cold. ;?
Old 06-03-2006, 11:59 PM
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Default Re: procedure for compression chek....

Grounding is unnecessary. Running motor prior to measuring isn't too important. And I wouldn't get too hung up over WOT either (I've measured the same motor at WOT and not advanced at all and gotten similar results). You're concerned about deviation between cylinders more than the absolute values. I've always used max of 10% deviation high to low as a good rule of thumb. You wan't find any specific numbers in shop manuals and some motors like the J/E big block V6's tend to have pretty low norms (i.e. ~ 80-90 psig), Merc 2.5L V6's a little higher while the J/E V4's are typically much higher (say 130 to 150 psig)
Old 06-04-2006, 12:43 AM
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Default Re: procedure for compression chek....

WildeOne - 6/3/2006 10:59 PM

Grounding is unnecessary. Running motor prior to measuring isn't too important. And I wouldn't get too hung up over WOT either (I've measured the same motor at WOT and not advanced at all and gotten similar results). You're concerned about deviation between cylinders more than the absolute values. I've always used max of 10% deviation high to low as a good rule of thumb. You wan't find any specific numbers in shop manuals and some motors like the J/E big block V6's tend to have pretty low norms (i.e. ~ 80-90 psig), Merc 2.5L V6's a little higher while the J/E V4's are typically much higher (say 130 to 150 psig)
....thanks wild......
Old 06-04-2006, 09:46 AM
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Default Re: procedure for compression chek....

capecodguy - 6/3/2006 8:01 PM

Make sure you ground the spark plug wires to the block if you have a CD ignition system. I bought a set of alligator clips with jumper wires from Radio Shack for this purpose.
You don't need to do this if you have a way to crank the engine with the ignition off. Pulling the kill switch is an easy way to do this. My mechanic used a jumper wire with a pushbutton switch on the starter solenoid. Either will work fine.
Old 06-05-2006, 12:16 AM
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Default Re: procedure for compression chek....

I have always done the compressions tests cold - not because of accuracy or laziness, but because bringing an I/O up to operating temp and then trying to pull the plugs will result in burning your arms. I started tsting them cold this weekend - nbr 2 cyl. 163#, nbr 4 cyl. 165#, nbr 6 cyl 0 psi. Well now I know why it's been running like crap!!!
Old 06-05-2006, 12:47 PM
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Default Re: procedure for compression chek....

my view.
1. compression checking on a warmed up engine reads higher than on a cold one, but proportions amongst cyl is the same. If you use your own bycicle valve , they will last a lot less on a hot engine
2.make sure no fuels sits around the area where you are testing and that you disconnect plugs and disconnect them from the wires and put them somewhere ( Sounds silly, but I have seen a fool keep the splug unscrewed, attached to their wire and dangling around. This is #1 way to start a fire
3. my mechanic uses same trick as rwidmn's , cool, saves 1 person helping you

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