VACUUM too low

[IMPALAMAN1]

Hey fellas,
i got my 68 back on the road and the timing (billet msd with vac advance plugged, 6a box, blaster 2 coil) is now set right..... the carb (demon 625? vac secondaries, elec choke not hooked up) with the carb tuned it seems that i have no more vacuum. it is at 5". the motor is a 327 with the 202 heads z28 springs stamped steel roller rockers. intake is the airflow research. cam is the comp cams energy extreme the one just below the 168 with the keith black flat top pistons... 9.5:1. factory crank and connecting rods.

the car has power 4 wheel disc brakes and this is where my problem lies..... 5" of vac doesnt lend it much pedal help.

the car idles at 500 rpm... which seems real low to me.

i do have 2 extra vac tanks.... 1 for the brakes and 1 for the headlights.

https://static1.pt-content.com/images/pt/2008/05/Car042008010-1.jpg
https://static1.pt-content.com/images/pt/2008/05/Car042008011-1.jpg
https://static1.pt-content.com/images/pt/2008/05/Car042008014-1.jpg

[MonzaRacer]

Advance your base timing to about 16-18 degrees before top dead center then hook up vac advance(if you have one,) to manifold vacuum.,

[jb442]

...billet msd with vac advance plugged...
I would definitely utilize the vacuum advance as MonzaRacer mentioned, connected to a manifold signal (vs ported). Is there any reason it was just plugged? Would it not run properly when connected? I would think you should be able to get at least 10" of vac with the advance hooked up... Idle should be smoother and your part-throttle performance should be better as well, as long as there's no detonation issues.

[68Formula]

Exactly what are the specs on the cam? I could not understand your definition plus it's better when you don't require people to do research first. And this factor can play into the solution. My guess is similar to the others in that you have a pretty large cam for such a small displacement, and therefore more ignition timing is needed.

Other than that, check out your vacuum canistors and lines to make sure there is no leak. Your required idle speed (also cam dependent) should be higher than 500rpm which will affect vacuum levels. Usually only low performance engines idle this low. With those low vacuum readings, I'm surprise your Power Valve isn't open, dumping in too much fuel. Or maybe it is which contributes further to a low vacuum reading. Are you able to get any notieable vacuum changes when you adjust the idle screws? Also, are your readjusting the carb at idle after it's fully warmed up? Without a choke, you will have to run richer than normal to get a good start and idle which means more fuel than required once fully warmed, and therefore lower vacuum.

In summary:
1) Tell us the cam specs so we can give you a better idea of ignition timing and idle speed range requirements.
2) Verify all vacuum routings are leak-free.
3) Hook up the choke so you can tune for cold starts and fully warm.
4) Unplug the vacuum canister so it can be run to full vacuum.
5) Adjust your initial timing up. Hook it to full manifold to see if it improves.
6) Check to see if the PV is opening. Although if you make the other adjustments this probably will no longer be a problem.