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Thread: Vacuum Advance

  1. #1

    Default Vacuum Advance

    So I thought I'd share some information I've found out about finally getting the 230 Tornado civvy ignition system with vacuum advance working.

    I have the 230 with 2 barrel Holley 2300 (350 CFM) and 12V distributor with vacuum advance. Problem was I couldn’t figure out where to pull vacuum off of my carburetor to operate the distributor’s vacuum advance.

    First off for those who don’t know, here is how vacuum advance works:

    The distributor has a vacuum diaphragm connected to a linkage that rotates or advances the ignition based on how much vacuum is pulled. According to some books I have and from my own tests, it takes at least 3 inch Hg (inch Hg, or inch Mercury is a units of vacuum) of vacuum to start advancing the ignition, everything after that advances it accordingly.

    You can NOT pull vacuum from your manifold. Manifold vacuum is high when your at idle, and low when your WOT. This would cause you to have full advance at idle and no advance at WOT (or near full open throttle). You must pull vacuum from a port ABOVE your carburetor throttle plate, but below the venturi of the carburetor. This area of the carburetor has little/no vacuum at idle, and high vacuum at WOT.

    If you are using a vacuum gauge to tune your engine, then that’s when you connect it to a port in the intake manifold for your readings.

    My 2 barrel had no open ports that I could find to run a line that would do what I wanted it to. Then I came across and article showing where the vacuum advance port “should” be located on my carburetor. Funny thing was that on my carburetor there was a hole there, but it wasn’t threaded and it didn’t go anywhere. So I tore my carburetor apart and discovered that indeed there was a channel coming from right above the throttle plate up to this hole in the side of the plate, but it was closed off. So I drilled a passage hole through, and then tapped the outside hole for 1/8-pipe (wouldn’t you know the hole size was exactly correct to be tapped for 1/8-pipe. I would conclude that my carb was intended to have this option on it, but for whatever reason due to application it was not use and so they just left the casting untapped and blocked off.

    I did this mod and everything worked GREAT first time I fired it up. My engine has never ran so well. It fires right up, takes 5 seconds of high RPM to warm up and then it will idle all day long. I am beyond impressed with the increase in power and smoothness at which it runs at high way speed. As a side note too, I am now a firm believer in timing engines via a vacuum gauge. I used my timing light and it did not time my engine correctly. This is probably due to stretching of the timing chain, or maladjustment of the external balancer with the numbers on it, I don’t know why but I could never get it to run right with the timing light. I used the vacuum gauge (which when used you need to disconnect your vacuum advance) and now my engine absolutely purrs.

    Here is what the block between the main carb body and fuel bowl looks like with the "hole" in it. Like I said, it had no threads or opening going anywhere, it was just a dead hole, but this is where the vacuum advance fitting "should" go.

    Here is a picture of the passage way, and the location where a hole needs to be drilled:

    Here is what it looks like with the hole:

    Here is the outside hole I was talking about now tapped and a fitting threaded in. Make sure to use Teflon tape to seal this very well:

    While I’m here I might as well talk about Power Valves as well. While I was inside my carb, I changed out my PV. Here is the skinny on this little cool thing. I don’t know if this is something that is Holley specific, or used in most carbs in general. In the Holley carbs the PV is a spring loaded valve that opens up at a specific inch Hg vacuum. When this opens up it allows more fuel into the intake, thus giving you more power. Now the cool thing is that you can buy different PV’s with different ratings.

    To determine what rating you want this is what you do:

    Go drive your rig (it really needs to be tuned properly using a vacuum gauge, because if not then you will get incorrect vacuum readings). A good strong engine should be able to pull 18 to 20 inch Hg at idle. Mine pulls right at 16-17, so I still could stand to fine tune it a bit, but that could also be vacuum loss due to worn valve seals and/or rings. Anyways, connect a vacuum gauge to your manifold and run a line into your cab so you can watch the gauge while you are driving. Go drive at highway speed (55-60) and give it just enough throttle to maintain your speed. Note what your gauge reads. For me I was right at 8-9 inch Hg.

    Power valves come in 1 inch Hg increments, although oddly (I have no idea why) they are in x.5 values, 4.5, 5.5, 6.5, etc. The PV will be stamped with a number like 55 standing for 5.5 inch Hg.

    Depending on what you want, here is what different PV valves can do for you:

    If you want that extra power right as you step on the throttle, get a PV rated close to your cruising vacuum reading, like a 7.5 or 8.5 PV valve. Performance engines, racers, drag racers, all want that power immediately so they will go with a valve rated in this area.

    If you want that extra power a little bit later into the throttle get one rated less, like a 6.5, 5.5, or even 4.5 so you have a lot of throttle range before the extra fuel kicks in. For off road applications where you are low speed crawling around you don’t want the PV valve to open soon because then you are running rich at a very lower RPM and it just causes problems.

    I had a 7.5 PV valve and changed it to a 4.5 inch Hg rated PV so I have a lot of throttle range to play with, but when I step on it and the vacuum plummets below 4.5 then she opens up and you can feel that extra power. It’s a very easy thing to change, when your carb is apart the plate between the main body and the fuel bowl has the PV valve threaded into it. Unscrew it out and screw the new PV in, your done.

    Well I just wanted to share this stuff with you guys, something I didn’t know before and now I know, lol.
    Last edited by rpgdeity; July 10th, 2009 at 05:46 PM.
    68 M-715
    67 M-725

  2. #2


    Great write up Brandon!
    This post is closed-captioned for the hearing impaired.

  3. #3


    That was very good! Way above our usual standard
    It was like a magazine article, with clear pictures even.

  4. #4
    Join Date
    Sep 1998
    North Central Wisconsin



    Thanks for writing the book on this!!!!
    Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.

    Pro-choice, that's a LIE, babies don't choose to die!!

  5. #5


    Well done, excellent information and clearly explained....

  6. #6

    Smile Great Job

    Great Write up "Brute he should get a promotion"

  7. #7

    Default Vacuum advance

    I too have a civilian distributor on a military engine. Except, I have an early engine with the 1-barrel carburetor and I do not have the same base you have. Anyone have any ideas? The fire department mechanic tied it in to a vacuum tap on the intake manifold. And, based on your investigation, that is not the best place to tap off the vacuum.

    Thanks in advance,
    Mike Cougler, MSgt, USAF Retired, '72-93
    '67 M725, VIN 10030, Delivery: 7/67
    Rochester, NY

  8. #8

    Default vacuum advance.

    Actually the vacuum advance is tied to the same "T" tap on the side of the head behind the intake manifold. This is the same tap where the large vacuum line goes to the vacuum section of the fuel pump for the wiper vacuum. And, at idle, I am getting 30 inches of vacuum at the vacuum advance (maximum reading in the coasting range) on my vacuum test meter.

    Thanks again,
    Mike Cougler, MSgt, USAF Retired, '72-93
    '67 M725, VIN 10030, Delivery: 7/67
    Rochester, NY

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