3.0 CIS Compression Problems

[evopanop]

Hey everyone, new member here. I just purchased a '77 911 that had a '78+ 3.0 CIS motor swapped into it. My problem is, one of the cylinders has absolutely no compression and another cylinder has only roughly half compression. The engine runs with no perceivable noise. Any ideas?

Thanks in advance, everyone.

[BART'S 911 SC]

check crefully for cracked valve springs,each valve has two of them if one is cracked engine still will run but valve is not close all the way relising some pressure,guess how i know that...

[tcar]

Don't run the engine until you find out what's wrong. You can do more damage.

[Gunter]

That would have shown up in a PP exam????????
A leak-down test will show where the problem is.

[evopanop]

Thanks for the great replies so far, everyone. I'm picking up the car this afternoon; so, I'm just trying to get as many ideas as to what could possibly be wrong, before I dive into this thing, so that I kind of have an idea of what to look for when I pull the motor apart.

If anyone else has any ideas, please keep them coming.

[ianc]

Like Gunter says, a leakdown test will show exactly where you're losing compression. Make sure the valves are adjusted correctly on those cylinders if you're getting leaks past the valves. Check for broken head studs when you get the valve covers off. If you have to tear the motor down, I'm sure the source of the leak will be very apparent.

ianc