Changed headgaskets, now low compression!

[Ben12345]

Car is a 1985 500 sel Euro. M117.963.
Changed both headgaskets, had both heads machined (tops and bottoms).
Car started first time on reassembly. Idles poor and at 1000 RPM.
I pulled off the spark plug wires one by one, and on removing no 6 nothing changed.
I have spark, albeit a bit yellow.
Stuck the compression tester in and I have ~20psi Aghhh! put some oil in with no change. I'm hoping that I only need to adjust the valve clearance. I'm off back out there to pull the rockers to see if it makes any difference.

Any advice to a desperate man?