Proper Way of Checking Compression

[samm835]

I am purchasing a new motor(to me) soon. I don't have a cylinder compression device, but a local auto part store will let me borrow one for the day. I know main things to do/check, foot oil, cking wire corrosion, but I need to also make sure I am checking the compression the correct way. I have never used one, but I have seen them used. Please correct me or point me in the right direction of use, here is what I know....un-screw one spark plug at a time, and insert the compression ckr in, turn over motor a couple of times(without starting). I should go through all cylinders the same. The compression should read 110 or better with little varience. I think I read that it should be within 10PSI on each cylinder? Thanks for any help!

 

[Bob_VT]

Re: Proper Way of Checking Compression

10% within all cylinder's is correct. A 2 stoke can run with as little as 70-75 lbs per but over 100 is best.

 

[carholme]

Re: Proper Way of Checking Compression

samm;

Compression Test

http://forums.iboats.com/showthread.php?t=391323

Gerry

 

[jerryjerry05]

Re: Proper Way of Checking Compression

You don't say what motor your testing.Always include year model and serial#.
Compression varies from motor to motor.
I had a 225Merc with 125-130# of comp.And a 85Force with 150# and a 120 Force with 165#
The most important is the results.Your guage will read differently than mine.
The results should be within 5% of the others.All of my cylinders read 150# each.Twin Forces.
Remove the plugs remove the kill switch.Install the tester.Turn the motor over till it stops going up.Then do all the others.Write down the results.Check the gear oil too.J

 

[samm835]

Re: Proper Way of Checking Compression

Thanks for all the help. There is one thing that has me completly puzzled. It is the taking out all the spark plugs and doing this test, I was thinking if all the plugs were out you wouldn't get a good read on the cylinder. I will do it the way you guys told me to, but just out of curiosity.....why does all the plugs need to be out? Thanks for your help.....

 

[Silvertip]

Re: Proper Way of Checking Compression

The cylinders are all independent. There is no connection between them. It is no different than takiing two, three, four six or eight one cylinder engines and hooking them all together. Remove all the plugs. And be certain you disable the ignition. Removing plugs is not only a convenience it allows the starter to spin the engine at the proper speed.

 

[Don S]

Re: Proper Way of Checking Compression

Removing plugs is not only a convenience it allows the starter to spin the engine at the proper speed.

PLUS, If you leave the plugs in, and the throttle in the idle position, the engine will pull raw gas into the cylinder. Which is why in the specs the manuals always state to open the throttle to wide open.
If you suck that gas (and oil) mixture into the cylinders before testing, it can mask a problem, seal some leaky rings, and give you false readings.