Vacuum gauge newbie....

Discussion in 'Troubleshooting & Diagnosis' started by Ron D, Oct 7, 2017.

  1. Ron D

    Ron D New Member

    9
    0
    Sep 26, 2016
    Alberta, Canada
    Having never used a vacuum gauge I figured the best place to get the answers is here...

    I have a 1970 LT1 with a Holley 4150 carb which has a vacuum outlet on the front right side. The hose that comes off of the carb goes to a "T" which in turn feeds the choke pull off and goes to just below the distributor cap.

    Once the engine is warmed up can I remove the initial hose coming off the carb to use my gauge or am I best served to insert it in pace of the choke pull off or distributor cap hose (for lack of a better term)?

    I understand that at my location at 3500 feet above sea level my numbers are supposed to be 3.5 off of what is expected so I assume if one were to expect say 20 at sea level in my case if it shows 16.5 then I am OK?

    Many thanks in advance!

    Newbie here so please keep the eye rolling and snickering to a minimum, hehehe
     
  2. tom3

    tom3 Veteran Member

    10,632
    41
    Aug 1, 1999
    ohio
    I'd unhook the hose to the choke unit to get a reading. Hose from carb should be at a lower point on the carb too, often a high connection is ported vacuum and not a constant source.
     
  3. ol' grouch

    ol' grouch Veteran Member

    1,015
    126
    Jul 4, 2013
    Evansville, In.
    Personally, I prefer to pull vacuum at the intake. A lot of ports on carbs. don't have full vacuum, or only at times. Maybe this will help you. You're right in accommodating altitude.

    Vacuum gauge diagnostic chart.JPG
     
  4. Ron D

    Ron D New Member

    9
    0
    Sep 26, 2016
    Alberta, Canada
    Thanks gents for your input. I pulled the hose off the choke and hooked up my gauge to the 3" hose (choke to "T") and the reading was low - adjusting for my altitude = the at sea level number would be 11.5. I then pulled that hose off the "T" and used my gauge supplied hose into the T and it rose to 14.5 and wasn't swinging though it would drop at times to just above 13 and come back up again in 2 seconds or so. Not good numbers as far as what I've read they are supposed to be...

    I have found that there are what appear to be 2 holes in the rear of the intake that are drilled/tapped or a threaded sleeve has been inserted in each of them. I am attaching a corresponding picture. Neither of these holes have bolts in them but suspect they should have? They might not be drilled all the way into the manifold but were done so to hang a bracket from their location?

    I assume to prove out their drilling depth if when the car is running and I were to cover these holes with fingers I would feel the suck of the vacuum?

    Intake manifold rear.jpg

    I won't be able to fire up the car today as its Cdn Thanksgiving this weekend and we have company (garage is too close to the living room) and she who must be considered has already glared over her glasses at me having opened the door and seen me leaning into the engine bay.
     
  5. Ron D

    Ron D New Member

    9
    0
    Sep 26, 2016
    Alberta, Canada
    Well I snuck out there with a long thin metal rod and found bottom on those 2 drillings so there is no vacuum leak from the 2 drilled holes. sigh...
     
  6. 74BIBELOT

    74BIBELOT Veteran Member

    E3143E14-DA73-4589-82A4-6B880D38AA16.png Those are the holes for the ignition coil bracket, yes, no vacuum there.
     
  7. ol' grouch

    ol' grouch Veteran Member

    1,015
    126
    Jul 4, 2013
    Evansville, In.
    It's been a while since I poked around on a 70's intake. There is often a vacuum manifold with several hoses coming off it on the manifold. Some of them will have rubber caps. Otherwise, I have used a short piece of hose and a tee to read off the vacuum pressure to the vacuum advance on the distributor. Hook the hose to the distributor, attach it to the old one with the tee and hook the guage to the third part of the tee.
     
  8. 70-camaro

    70-camaro Veteran Member

    855
    70
    Mar 25, 2010
    marietta ohio
    I just use the one from the carb with full vacuum.
     
  9. rchydzik

    rchydzik Veteran Member Lifetime Gold Member

    731
    7
    Nov 13, 1999
    Colbert, WA USA
    I teed mine into the power brake vacuum line. Vacuum should go down as altitude increases due to lower air pressure. I think your readings are ok for the altitude, and would be closer to 20in at sea level.
     

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