Good news! A black GM Vacuum Advance Regulator came up for sale on Ebay at a decent price ($40) and I bought it.
This is a “GM Spark Delay Valve”, part 10115852, which is for 1982-85 Chevy S10 and GMC S15.
I can confirm, it holds at idle to 7.5 inHG to the distributor vacuum advance. This is the closest so far to our OEM brown one that holds idle to 10 inHg to distributor vacuum advance. So it may do for a replacement of our non-existent brown ones.
So what this means, if you’re running the OEM brown one, it’s regulating about 16 BTDC at idle. With this black one, it’s regulating about 11 BTDC at idle. Still a decent advance. For me, since I advanced my normal timing to 22BTDC@3000RPM (instead of the standard 18BTDC@3000RPM), I was getting 20BTDC at idle with the brown one, a bit too advanced for my liking at idle. Now I’m getting 15 BTDC at idle with my advancement.
The hookups are located a bit differently, and the one for vacuum (delay) is the large one instead of the one for throttle tap (carb). But I verified if you hook up to Delay, Carb, and Dist just like before, it works fine.
I’ll run this for a few days and report back. But first run, idles fine. I had to open my Idle screw 3 flats (1/2 turn) to get idle RPM back up to where I wanted it. I also noticed, when giving it some pedal, my pickup response is a bit quicker. I tested my brown valve, and the Carb hookup (throttle tap) seems a bit sluggish, or sometimes needs a lot of pressure to open. These brown Jag valves are 30+ years used. The black ones are NOS, that seem to still be out there.
FWIW, I ran the green one for a few days, but I didn’t like it. Just idled a bit funny, especially when cold. Timing was just not advanced enough, I guess.
