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Compression test

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Yeah, cap.
Exhaust pipe is an exhaust valve issue, through the carb, butterfly open, is intakes, oil fill cap, rings (or piston).

Compression tests are dynamic, meaning they are reading what your engine is doing while rotating.
It will show issues leakdowns will not (like, a cylinder that is very wide in the centre of the bore), and a leakdown is static, which means you are looking for issues with the engine at rest.
Sometimes, if you have a completely shot guide, and air pressure from a leakdown happens to seat the valve, you won't see it, but on a cranking compression test, you will.

To simply say one type of test and not the other is ludicrous.

I do a comp test FIORST, then do a leakdown if it warrants.

That said, VW air cooled seem to prefer leakdowns first.
 
TOC,

One more time, please, for my pea-brain. Just so I understand you.

You write: "Exhaust pipe is an exhaust valve issue..." Does this mean that if I apply air -- say to cyliner #1 -- and listen at the rear exhaust pipe for a "hiss," it will indicate if I have an exhaust value issue in cylinder #1?

And if I listen near the carbs, with butterfly open, and I hear a "hiss," then I can tell if I have an intake valve issue in #1?

The oil cap, I understand.

The method you are suggesting sounds really good and usefull, but I just don't know exactly where you are telling me to listen.
 
Interesting comments. I don't bother with compression tests very often, but my experience has been that after 4 or 5 hits, the needle doesn't move much whether there is a sealing problem or not. You're basically looking at how much the cylinder leaks during each compression stroke, which doesn't change much from turn to turn unless perhaps the oil pressure comes up and squirts oil on the cylinder wall.

How many hits it takes also depends to some extent on your compression gauge. The little short one that fits directly into the hole comes up quicker than the one with the 2' hose to the cylinder fitting. Doesn't say much of anything about engine condition, only how much air space there is within the gauge. If there wasn't any air space at all, it would come up all the way on the first hit.
 
Yes, ear (or hose) at carb inlet, ear (or hose) at tailpipe, ear (or hose) and oil fill cap.
On an older engine, you may get some from all three. A LOT is an issue, and at that point, a leakdown will give percentage of loss.

On pushod engines, I take the rocker shaft assembly off to do it. You apply air, you need to make sure you're on compression, and at TDC or lock the engine, or it will spin. Removing the rocker shaft assembly keps the valves from springing open if it rolls.
 
Gentlemen...

OK... I think I have it now. Many thanks! I am going to have a friend who knows how to do these leak-down tests do it for me. I'll watch... and with the information you have given me, I'll have some idea of what I am seeing and know what questions to ask.

My compression hose is about 18" long.
 
Randall...you ain't lived until you've seen a small block with a flat cam take 20 hits to come up to the compression reading of the rest.

Then again, when you've got a broken rocker arm, and run a test on a 302 Ford (new car) and get 245, because it's now acting as a comprssor.....
Saw that once.
 
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