engine and drivetrain cold compression numbers- what is ok?

[jetboy]

So I am no expert at compression so please bear with me.

I have an old engine on a stand - I removed the manifolds, changed the oil and filters, put some Marval M oil in the oil and a bit down each spark plug hole, hooked up a battery.

then did each cylinder down the line: got approximately:

25, 165, 125, 125

But when I went back to the first 25, I got 75 the next time - and it seemd to get better the longer i turned the engine over.

I have some general questions: how many revolutions should you need to get a firm reading? is it wierd that the more revolutions I did the better number i got? the one that got 165 seemed to jump perfectly there and stay, the others took a bit more work.

in any case, it looks like I will have to tear it down. but wanted to see what people have to say about compression tests done this way- if they are reliable or not etc.

[TJs692002]

A cold compression is not really 100% accurate. Hot compression test is the way to go. You could have some carbon build up under a valve in the low cylinder that you have. DO you have a leak down tester? A leak down will tell you if you are loosing air through the rings or through the valves.

[jetboy]

i dont have a leak down tester, I have heard about such a thing, but not sure how one does it or how much its going to set me back.

actually the intake had some white carbon/corrosion build up on that cylinder - and not on any of the others. I cleaned it up as best I could- but maybe it just needs to break in again (I hope)

[TJs692002]

Get the motor running. Once it runs, it will blow out the loose carbon in that funky cylinder and your compression numbers will go up. Try to do a hot compression test and you will see what I am talking about.