Should be 21" Hg   that's if its a real good engine ; )    but really as Ted said.. 



"Came too close to dying to stop living now!"

Couple things - first what is your altitude - 18" to 20" is considered normal for a stockish engine in good tune at Sea level. As elevation increases this figure should be corrected. If you live in Denver 15" might be considered a good number. Leadville would mean an even lower acceptable number. I don't know what's normal for Quito but it won't be much.

If you have a performance camshaft that will tend to lower the idle vacuum significantly. Steady vacuum is an important diagnostic for a stock engine, but an otherwise low vacuum usually means late or retarded ignition timing and/or a misadjusted carburetor.

Also check that the valve lash or rocker arm pushrod clearance is correct. If the valves are adjusted too tight that will lower the idle vacuum as well, valves start hanging open slightly.

The Mixture Screws are working. Cleaned the air Bleeds with Carb Cleaner which seemed to improve the Idle. Still not great. I have sent One Carb in for proper rebuild so for now going to work with this One. Off Idle it runs great. No hesitation etc.Used Carbs that sit are not as good as freshly rebuilt One for sure.I haven't found any Vacuum Leaks but I would have thought Vacuum should be a little higher around 18.I should check mine and see what it is. Same Setup with Stock Engine.. 

  

Mark - 15 inches Hg is OK to get started tuning it up.
Watching the gauge -
Is the engine responding to the mixture screws now that you've got it idling?

 
 Steve Metzger       Tucson, Arizona

should be close to 20

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Idle with vacuum disconnected. At the Moment the vacuum Can is not adjustable because I think the Spring is Broken. I need to replace it. 10 BTDC. The Gauge reading is 15 with some irregular movement swing down by about 2- 3

  

Mark - is this motor running a std distributor curve - or something like you and I run (or did)? The amount of idle advance will change the vacuum a bit.

 
 Steve Metzger       Tucson, Arizona

I am still working on my Buddies 390 Holley. Vacuum reading at idle in Park is 15. Is that within the Range where it should be? Motor has less than 3000 Miles on it. Motor is stock. Thanks