Timing by Vacuum Reading?

You didn't say if automatic or manual trans or stall.
For automatics;
setting the Idle-timing, on an SBM, by highest idle-vacuum is a terrible idea.
Unless you have a minimum ~3600rpm stall with iron heads, or a minimum ~3200 stall with alloys,
AND,
your PowerTiming is limited simultaneously to whatever it takes to prevent detonation.
Now, why your street engine would have those stalls, is beyond me, especially if a stroker.
But if a manual trans,
setting the Idle-timing, on an SBM, by highest idle-vacuum is beyond a terrible idea.


The exact right way to set the Idle-Timing is;
by adjusting the transfer-port exposure underneath the Primary blades to a lil taller than wide; closing up the secondaries completely, and leaving them alone at those settings;
then
adjusting the timing down to whatever it takes to hit a reasonably slow enough rpm, that the trans does Not bang into gear, AND stays running!, AND the engine takes throttle without hesitation.
If the exhaust burns your eyes, then you will need to add some bypass air, and then reset the IDLE TIMING again, to get the rpm down.

Occasionally, if you can't get rid of an Idle tip-in sag, you might have to increase the transfer slot exposure slightly, then start over again.
I guarantee this will work up to and including the 292/292/108 Mopar cam. Using this method, I tuned this cam in my 367 to idle in Neutral at 700 (manual trans) and idle around the parking lot at 550rpm,
with just 5* of Idle-Advance, and a starter gear of 9.44
and with no tip-in sag on the 750DP carb. So, I know the method works.
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No, my normal Idle-Timing was NOT 5*; I just used that number to avoid bucking and toe-ing the clutch. The normal idle-timing was 12 to 14. To get the 5*, I used an adjustable, dash-mounted, timing-retard module, which had a 15* working range.
My V-can was modified to provide 22* of PT (Part Throttle) advance.
The module was set initially to provide 7* retard/ 8* advance. This allowed up to 20* of idle-Timing which I used after starting the engine cold, and this eliminated the need for a choke.
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Since that time (1999) I have used this same method on many other engines and combos; and for a while in the early 2000s I was the go-to guy in my neighborhood, for doing this kind of work, which I did at home.
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But for best results, there is a long list of "pre-flight" checks to be done.
The toughest one to find was tight ring-gaps. If your ring-gaps and/or skirts are too tight, the engine will be a beotch to make run smoothly.
IMO
the KB street specs for hypers are too tight. I ended up taking my engine apart and loosening it up a lil, then starting over. I now run it at 207*F, all the time. Since that 292/108 cam, this engine has seen two other cams; idle-tuned the same way.