I'm going through my distributor and correcting some sloppy end play. Took a closer look at my vac. advance unit and cleared away some crud to see the number. It has 626 10 stamped on it. This doesn't seem to match anything on Lars vacuum spec document. I connected a vac. pump and it starts to move at 4Hg and is done by 9Hg. Does this seem correct? I have 78 L-82 with distributor #1103291.

Where did ya find the info?

I stumbled across the delco part number 1973626 on the NAPA site.  The numbers stamped on the vac. unit (626) are the last 3 of the delco part#.  On the NAPA site, this is a VC1703.  I found the specs for this somewhere but can't put my finger on it now.  I will continue to hunt.  The delco number has now changed to 19138061.  Regardless, I see ~10* with timing light at the crank when it's fully engaged.  I'm still a bit confused on the crank vs. distributor* and what number to use when calculating total timing. 

Got my distributor back in here's what I ended up with. I only get 18* mechanical advance max so I set my initial at 16*. This gives me 34* with an additional 10* of vacuum advance. The only thing I notice is a 'very slight' stumble when transition from idle while cruising. I haven't had time to investigate it yet. Any ideas?

Probably a carb issue, or possibly EGR.

Adams' Apple  I remembered where I found the vac unit specs.  In the Haynes repair manual.  Here's what it shows.

I wouldn't have a clue why Haynes wouldn't publish the specs... I may be able to come up with it tomorrow. I have a "Vintage" program on the shop computer. I'll see what it has to offer.

IF I read your earlier post correctly, you have 16* initial, and 18* mechanical, which is 34*... If you add another 10* from the vac. advance, that would be a total of 44*...at what RPM is this all coming in?
I have found, as have others in the Corvette repair business, that the engines seem to run best with the timing set where the emissions decal states it. We can sometimes get by bumping it up a deg. or two, but it always has some sort of trade-off...either it will start pinging when hot, or they become hard to start when hot.
The "Haynes" spec of 12* initial sounds a bit high, to me(as does the 16*). Does your car still have the decal on the driver side, by the hood latch? If so, what do it say the timing should be?

Checked the decal,  12* BTDC @ 900RPM.

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