Hi - on my 74 1.8 removed passenger side head to remove something that fell in (!), removed some "carbon build-up" before replacing - pressure now is 115 front, 90 rear (while the drivers side is 120 and 120). Unfortunately did not check pressure before removing this head. Does anyone know if/how this will run with this low compression on one cylinder? Thanks for any advice.

(Suspect I should remove the head and have it done properly...)

It'll be fine(for a while). I've seen them continue to run with 50 all way round. Is the engine out of the car? If so, its a good time for a quick hone and ring change, while your getting the heads refreshed.

Probably a silly question, but you did use new head gaskets, right?
Mark D.