... and though I hate to admit it, I'm stumped for now.

Sorry if it's a bit long, I'm trying to detail what I've tested/done
already. Spent all ******** evening on it. I've got a feeling I'm missing
something blindingly obvious, but I'm out of ideas for now. Couple of
concrete questions at the end.

The basics: '92 940GL, B230FB engine (531 head/VX3 cam/otherwise same as a
B230F), LH2.4, 935 computer

It came in as one of those "multi problem" cars. Water leak (from the head
gasket as it turned out), very poor starting, sort-of-OK'ish running.
Pre-existing additional problem: Often fails to energize the main fuel
pump when turning on ignition. Sometimes the pump starts running during
starting or you can hear it run for a few secs after turning off the car
(or after it died)

First things first, I put on a new head gasket, exchange head
(resurfaced/checked/new guides and exhaust valves) as it was a dual fuel
conversion that was near the end of its exhaust seats anyway, new timing
belt, tensioner, spark plugs, rotor/cap, water pump... Closed it up
yesterday evening. Tonight I started it up first time to work on the
running problems.

The rest of the story involves only the gasoline part of the dual fuel
system, so ignore the propane for now.

After the head job it sprang into life, briefly. Idled around 2000rpm for
half a minute or so, running sort'a rough, then started adjusting the idle
down (as it should). Sounds like it's running on 3 cylinders or missing.

it's still very hard to start, and when it's running it runs extremely
rough and backfires (although with my hand on the ducting I think it's
also forward-firing into the airbox). It also tends to die after between 5
and about 60 seconds.

Now remember I've got plenty of spareparts, so swapping is essentially
free to me.

Easiest possibilities first, gambling on ignition, thinking that if it's
backfiring there should be fuel but a lack of spark leading to fuel in the
intake or exhaust system.

I swapped the crank position sensor just in case, same for the ignition
wires. No change.

Moving towards the fuel system, I swapped the computer, EFI relay and AMM
just in case they were bad. No change. Then I took out the plugs. Here it
gets interesting: The rear plug (4) looks normal for a car that's been
idling (badly): somewhat black. plug 3 is less black, plug 2 has only the
slightest hint of black, and plug 1 looks brand new, it's never seen an
ignition even in its life.

At this point the wires, computer, AMM, CPS and EFIrelay are swapped.
Since component swapping didn't seem to make any difference (does it
ever? One can hope...) all orginal components were put back in at this
stage.

Here is where it sort of becomes interesting:

Take out the fuel rails, put a bit of kitchen paper under it, and crank it
a bit. injector 4 has about a half playing card gas stain, 2 and 3 about a
quarter (coin) sized stain, 1 less than a dime-sized stain. Sounds like a
fuel system issue, right? Change the fuel pressure regulator. No change.
Jumper the pump from a starterpack, now the kitchen paper get a nice even
soaking of fuel. Fine, problem solved. Too bad: Put the fuel rails back
in, try to fire it up with the pump definitely running (jumpered). Ran
poorly for about 20 seconds and died. This is what it did previously, too,
so no change, again. Took plug 1 out again, still looks like it's never
seen a drop of fuel or a single combustion even in its life.

For good measure I rechecked the timingbelt, that seems to be OK and
correctly set...

So now we're basically left with two issues:

1- looking at the plugs it seems to be running on cylinder 4, cyl 2 and 3
are in for half and cylinder 1 isn't getting any fuel. No wonder it runs
like $#!^ and dies when the computer regulates the fuel down as it senses
the engine is heating up slightly from cold. But why that strange fuel
distribution?

2- what could cause the main fuel pump not to run? Hope it isn't the
propane system overriding it. Oh well, I can always run a 'hot when
ignition on' jumper wire if I have to solve that problem in a hurry.