how to use vacuum guage?

[don duick]

440 with carter AVS carb. mild cam 224@50. I connected the vacuum guage to hose fitting on the front of the carb and got a reading of 6. I then connected the guage to fitting at the rear of the carb and got a reading of 12. I then connected it to my vacuum hose for the brake booster and got a reading of 12. When I revved the motor the reading went up to 15.  I thought it was supposed to drop. Have I done something wrong which reading do I use so I can put the correct power valve on the holley I want to use. when I put the gear lever in drive the reading went down by 3.

[firefighter3931]

Don, you want to get a manifold vacuum reading for proper PV selection. I like to use the idle vacuum in gear (auto trans) as the reference point.  In your case ; the higher reading is manifold vacuum 

Personally, I would install the Holley carb you plan to use and get a reading off that carb. Adjust idle mixture for max vacuum with your timing curve optimized. Once you have that number go down 2-3 and pick your PV. With Holley carbs you have to use a vacuum fitting on the baseplate. Vac fittings above the base are timed/ported vacuum for use with vacuum advance distributors.


Ron

[don duick]

thanks ron.