Performing routine maintenance the other day and decided to check a few things. Did a compression and leak down test…everything was good. Compression on all cylinders was within 5% of each other. Leak down between 6%-11%.
Car runs and idles great, no misses or surging.
Pulled the distributer vac advance hose from the small intake port just in front of the carb and idle dropped a little (not much). Hooked up the vacuum gauge and it showed erratic reading between 6 hg- 11hg. Needle moved back and forth very fast. Reconnected the distributer hose and tried vac reading at the back of the carb (fitting for power brakes and transmission) and got a rock solid reading of 5 hg.

Specs on engine:
1970 400 .040 over
Cam is a roller 284/288
Lobe separation 112 degrees
Duration at .050 Intake 230 exhaust 234
Lift – intake .3533” exhaust .3533”
Heads are #13’s with a high end street/strip racing port job.
1.77 exhaust and 2.11 intake SS valves
HEI
1968 q-jet
Stock 70 intake
LBM’s with 2 ½” dual x over

Question is, why would my reading be so much different at the intake port in front of carb vs. the fitting on back of carb since they both open into the intake?
Which location should I be using to get accurate readings of vacuum?
I may just have to live with the low vacuum due to large cam but would love to up it if possible as it would help the power brakes.
Thanks!

Here's a good link to give you some info on vacuum gage testing.

http://www.secondchancegarage.com/public/186.cfm

Sounds as if you may have a vacuum leak in the brake line or booster.

I had a solid 6" with a 245/517 cam and a vac gauge plugged in the rear carb port. Found that my brake booster and fitting was causing stumbles and misses on acceleration but didn't have an issue (other than no brakes) running with rear port capped. I can't say for sure if this is your issue...but.

High lift cams will always have low steady readings if rest of engine is within parameters.

Good point... I'll take a reading with the booster diconnected

Vacuum at a single port will have the pulsating affect. Attached at the plenum will provide a more consistent "signal".

The better choice for a source for the vacuum advance is a "ported" vacuum port. That is, one that has no vacuum at "idle", but as soon as the throttle is "cracked", vacuum will be there. Look on the carb for a port above the throttle plates.

FWIW

Jim

Thanks for the reply Jim.
I was wondering if that pulsating vacuum disturbed the signal to the distributer. I've read a little on ported vs. manifold for vac advance. Seems to me you wouldnt want vac advance at idle. I might end up advancing intial timing after moving to a ported source for the vac advance. Maybe I'll squeeze a little more vacuum out of this thing yet.