[tccrab]

Just a thought...
Probably way off base, but...
It's mechanical or it's pressure.
Mechanical could be that somehow the cam cover when tightened onto the head causes the cams to push harder on the valves and not allow them to seat. On just one side.
That just does not make sense though, as there are cam retainers under the cam covers, so...
Lifters, is there any way that yours are damaged? On the wrong cams? The intake is different than the exhaust.

Pressure. Hmmm...
Somehow you're getting so much blow-by from lower in the engine that it's pressurizing the area under the cam covers and that air pressure is holding the valves open? That's just plain crazy. I must be a nut.
Maybe something to do with too much oil pressure?

Gosh, I'm just pulling ideas out of my butt, I can think of dozens of reasons why none of the ideas is even close.

I remember that you've had the heads off a few times, what condition are the head gaskets? Is it possible to put them on the wrong side?
Did you do anything with the lower end at all? Rings bad, maybe?

Here's my thinking,
Cam cover off, engine starts.
Cam cover on, engine won't start.
Problem is something to do with the heads being sealed.
Either oil pressure or air pressure is keeping valves from sealing in head.
Have you tried starting the engine with the oil filler cap off?
!!!
That would tell you if it was air pressure or oil pressure holding the valves open.
Wait a minute, have you measured the tolerences in the Cam Covers where they act as cam bearings? What if you've got a bent cam or one of the holes is slightly out of round, allowing the cam more movement that it should have? It wouldn't need much to keep a few valves open.

My brane (mis-spelling intentional) hurts. Must be overworked.

Just tossing out some stupid ideas...

Good luck!

TomC
'90ZR1 #792