Good leakdown/ bad compression. Why?

[MikeB]

Leakdown of less than 3% on all 6. No hissing through the intake or exhaust that I could hear. Compression all good in the 140's except 1 and 2, both at 95.

I don't understand why leakdown could be good, but compression bad. Maybe my intake valves on 1 and 2 are way loose, not allowing a good breath of air for compression??

Comp was done on lukewarm engine, carbs wide open, starter spinning engine normally.

[MikeB]

Just pulled the valve covers and checked 1 and 2 intakes. Spot on. Anyone have any ideas????

[Por_sha911]

What year and motor? I know that it is possible to have bad valve guides which will still have a good leakdown but burn oil.

[john70t]

Rounded cam lobes?

[MikeB]

Car is a 73 with 2.5 short stroke. I don't see how worn valve guides would allow good leakdown, but poor compression. Seems to me that when the valve is closed, it "covers" the guides

Rounded cam lobes? Well, I guess if the intake valve is not open long enough or with enough lift, that would be plausible. But then, it seems as though the whole cam on that side would be a problem. Cylinder 3 is tight.

I wonder how I could measure cam lift/duration without pulling the heads (yet).

This has me stumped...

[cowtown]

Were the tests done at the same time? If not, it's possible you had a couple chunks of carbon interfering with the compression results. It's a longshot, but the only thing I can think of. Any chance of re-doing #1 and #2 compression test?

And there was oil in the car when this was done, right?

[MikeB]

Colin
Tests were done all at the same time. In fact, I did cyl's 1 and 2 about half a dozen times. I was hoping that maybe my gauge or hose was leaking, but the gauge has held the same reading now for about 2 hours (!). So, that rules out the gauge and hose connection. Plenty of oil in the engine. Other cylinders good comp.

Yes, I was thinking/hoping that some carbon may have been in the intake ports, throwing off my results.

Don't know what to do from here.

[johnsjmc]

Yes A single cam lobe can be worn down .It could happen because of a clogged oil spray jet or poor surface hardening and affect compression on one cylinder. Remove the valve covers and visually examine the cam lobes for wear if possible .A dial indicator on each valve spring retainer at a time while rotating the engine by hand should detect a difference in valve lift cyl to cyl . Note exhaust lift may be different than intake but all ex should be same and all int same.Also check for proper valve clearance on exhaust side as well as intakes a slightly tight ex valve might cause same results as an intake. You could also try a wet compression test .You squirt about 1/2 tsp oil into cylinders and repeat your compression test a substantial rise wet points to worn rings a leaking valve will not be temporarily sealed by oil.

[HawgRyder]

I vote for the cam...if the lobes for the bad cylinders were worn...you would not get the compression #'s. (probably just the intakes).
But the leakdown depends on the valves being closed...no cam action.
Bob

[banjomike]

Took the Bruce Anderson / Jerry Woods maintenance class a few weeks ago. They said it is possible that bad rings will hold pressure when the engine isn't moving but will leak when it is moving. I hope it isn't your rings!

MsB

[ChrisBennet]

Originally posted by banjomike
Took the Bruce Anderson / Jerry Woods maintenance class a few weeks ago. They said it is possible that bad rings will hold pressure when the engine isn't moving but will leak when it is moving. I hope it isn't your rings!
MsB

That's the opposite of the results the original poster got - compression test good (moving rings), leakdown bad (static rings). Interesting though.
-Chris

[john70t]

Mabye leaking injectors/carbs that washed the oil off the cylinder walls?