Help: 3 cyl GTV Stumper (Fred, Tom, Wes... listening?)

[Robert Brady <bobb@domain.elided>]

I'm running out of options and need some advice.
Since a track event in November, my '74 GTV has been running on 3
cylinders, sometimes 4.
Here are the symptoms:
- - pulling #3 cyl. plug wire while idling affects no change in idle (all
others do)
- - #3 cyl plug appears to spark fine from visual tests; can hear arc when
I pull the wire while idling.
- - when driving, it runs rough, but every once in a while "clears its
throat" and picks up all 4
- - #3 cyl. plug is black (not fouled or excessivly sooty, just black
instead of tan like the others)

Here's what I've checked/done so far (in approximate order)
0. replaced lever on SPICA pump (old one broke and was field repaired at
the track)
1.  checked compression: 190, 190, 185, 160 (#3 is fine, #4 is a little
low... go figure)
2. checked ignition; coil was not testing to spec, replaced: marelliplex
coil, ignition module, cap and rotor (car ran better, still not right
though)
2.5 replaced plug wires with old spare set; no change
3.  checked timing, okay
4.  replaced #3 injector with a spare.... (#3 injector was broken
internally), car ran MUCH better, thought everything was cured, but
still the symptoms above persisted, even though full-throttle
performance (measured by timed runs) was 90% back to normal.
5.  poured a bottle of "injector cleaner" in figuring replaced injector
may be dirty/clogged
6.  reset fuel injection settings; was basically fine from the start
6.5  checked valve clearances: fine; inspected cam lobes: fine
6.6 swapped #3 and #4 injectors; no change in performance; #3 still not
firing properly
7.  replaced all the injectors with a matched set from a spare car;
figured maybe the #3 injector didn't have the right oriffice to match
the others.... no change in performance; actually, maybe worse.
8.  confirmed fuel going to #3 injector by loosening fuel line while
idling.

Any suggestions before I pull the FI pump and/or head?

I plan to replace the plug wires but since swapping them with old wires
didn't change anything, I'm not very confident this will do anything.
My only other thought is that the spring that fits inside the injection
pump outlet may be broken on the #3 cyl causing fuel to dribble out
instead of pulse as it should (I'm not sure this holds any weight either
since the car can run reasonably well even if the pump is timed 180
degrees out).

One last tidbit of information which may or may not be pertinant.  When
I opened the throttles all the way with the car off last night, #3 cyl
made a hissing noise and some vapor came out of its intake port.  None
of the others did that.

Thanks in advance,

Bob Brady, DVAROC
Yardley, PA
'88 verde
'74 gtv (1500cc)

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