Checked for pulse. key off- nothing. key on- while unplugged all orange wires hot, black stripe wire nothing. plugged in- all wires hot.
while cranking-unplugged no change from above. Removed Vac. line from intake side of connection for fpr, had fuel vapor coming out of fitting. yes hose smelled of fuel.

 

My mistake, while cranking, with test probe in black striped wires, the other end to ground, I detect a pulse where the light dims with rotation if the engine.no pulse on orange side( common?)

 

This is probably going to be an old post, but just wanted to post for others what my solution to this problem was, figured this would be a well-read topic. Feel free to move it where it belongs, I'd be honored.

My 1994 Buick Roadmaster with the LT1 and Optispark did the identical thing to the original poster, TWICE. Went from running perfect, to an occasional long crank, to DEAD. Crank but no start, an occasional hit for a few seconds after releasing the key. No codes, lots of fuel, and lots of spark at the coil and plugs. Reason for the spark appearing like it's working, but not, at bottom.

BOTH times, it was simply the hi-voltage coil-to-distributer wire open / shorting to ground!

The first time, it was open. I replaced the wire and coil boot, and reused / crimped on the weird proprietary Optispark boot end (I didn't want to buy a whole new spark wire set just for the coil wire yes I'm cheap.... Worked perfect for 3 years until last week, went dead again. After much frustration thinking it can't be the same problem since I fixed the wire, turned out it was, but this time the spark was leaking out around the coil boot, got a shock putting my fingers or a screwdriver at the end of the boot. I simply removed the boot, wiped off the wire and boot inside and perfect, starts on a dime again.

To help diagnose this tricky behavior, I use a spark tester with a pigtail on it I can attach directly to the coil terminal and then to ground. What seems to happen is, I notice the spark is good and strong when going from the coil thru the tester directly to ground, but when I put the (bad) coil wire between the tester and ground, the spark disappears / tries to go elsewhere. My reasoning for why it behaves this way, is the coil wire being too high resistance / bad connection. Sort of like a blockage in a water pipe, the water (spark) then goes somewhere else, path of least resistance. Also explains why spark is good at plugs, easier in the air than under compression.

I even noticed with the lights off, that when I got the engine to run sometimes with the faulty wire, I could see the spark coil GLOWING in the dark with a blue corona all around it. With a good wire there was no glow at all, so the faulty wire was making it work too hard / blockage / excessive 'back-pressure' (water pipe theory).

Here's my '94 Sedan with the Ironhead Corvette LT1, with some DIY A/C compressor tips. I'm also fortunate to have a '96 Roadmaster Wagon, really like these cars.

Hope this helps someone, yes sometimes it is indeed NOT the Optispark, but something simple (and cheap...

Cheers!~ Bellarmine