For those interested, I finally have an update on that 454 I was working on.
Several people suggested one or more valves were stuck, and others suggested a spring was broken. Well, we were both right.
After the engine stubbornly refused to heal itself, we finally popped the rocker covers (which we'd been reluctant to do because of rust issues- it's a marine application) and found no less than four broken valvesprings, plus a fifth that was stuck.
The #7 exhaust was stuck and had bent the pushrod, and #8 had both valvesprings broken, which explains the total lack of compression in those two.
The springs were relatively easy- last year I'd made a tool to do the same thing to my Toronado engine in order to replace the bad stem seals. I could have done the same thing here, but it was more expedient to buy an off-the-shelf unit. It was a $20 import, but worked surprisingly well- though in one case due to clearance issues with the big water-cooled exhaust manifolds, I had to pop the handwheel off and weld on a nut so I could use a wrench.
I did whip up a shiny new fitting for the air connection, I'll post a pic in the morning.
We replaced the bent pushrod and managed, with some determination and patience- okay, and a small hammer- to free up the stuck valve. Looked like carbon deposits, but I couldn't tell for sure.
I finished it off with a cold lash, and finally, after several days of on and off work, she fired up and ran like a champ. In fact it ran so well we had to back off the idle speed screw to bring it back down.
Customer was happy- especially that we didn't have to pull the heads. Me, I went and had a big greasy burger and then a nice nap.