Register now to get rid of these ads!

Technical AFB carbs on a supercharger

Discussion in 'The Hokey Ass Message Board' started by hemihotrod66, Sep 21, 2023.

  1. hemihotrod66
    Joined: May 5, 2019
    Posts: 968

    hemihotrod66
    Member

    I have a blown 392 hemi in my 34 Ford and it was always running lean at highway speeds....Tried replacing jets and metering rods but never seemed to work right,,, Running two 750 CFM carbs...Removed the metering rod covers while the engine was running and the metering rods where always in the lean position due to the vacuum made by the blower....Found an old Hotrod Quarterly magazine and it has a good article on building and tuning a blown hemi....I removed the springs from the metering rods and blocked them to the rich position....Worked great for me...Just thought I would pass this along...
     
  2. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 3,171

    427 sleeper
    ALLIANCE MEMBER

    A blower make's boost, not vacuum, and boost doesn't happen until W.O.T. The fact that you had to block the needle's means that the spring's were wrong or the needle and jet combo were wrong, or a combination of the two. Are these REAL AFB's or Edelbrock's?
     
    Desoto291Hemi and Tim like this.
  3. Bob Lowry
    Joined: Jan 19, 2020
    Posts: 1,573

    Bob Lowry

    I used 2 Holly 750's on my 671, and made sure that they were boost referenced. Lots of info. on
    how to do it yourself so that everything works like a regular intake. Just a thought....
     
  4. hemihotrod66
    Joined: May 5, 2019
    Posts: 968

    hemihotrod66
    Member

    Blower makers boost in the manifold not in the carbs....One side sucks and the other side blows....
     
    Last edited: Sep 21, 2023
    ekimneirbo, Ned Ludd, SS327 and 3 others like this.
  5. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 3,171

    427 sleeper
    ALLIANCE MEMBER

    Correct. But the effect of the blower doesn't come into full play until W.O.T, or close to it. At street speed's, the effect of a blower is minimal at cruising speed's. There is no more vacuum with a blower than there is naturally aspirated.
     
  6. tomcat11
    Joined: Mar 31, 2010
    Posts: 980

    tomcat11
    Member

    That's an interesting modification. How did you block the rods? Do you have a link to that article? There are different springs available for the vacuum pistons in the AFB carbs for the express purpose of tuning to the vacuum signal, but you probably already know that. The late great Pat Ganahl published a book called "Street Supercharging" which has a carburetor section in the back and covers various modifications for running the AFB's as well as the pro's and con's of the Carter's, Holley's, and Rochester's.
     
    kadillackid likes this.
  7. hemihotrod66
    Joined: May 5, 2019
    Posts: 968

    hemihotrod66
    Member

    I removed the springs which were the stiffest that the tuning package had...I took a vacuum hose and cut it into about 3/16 long and put it under the pistons that control the metering rods....This kept the vacuum signal going to the pistons but they don't move keeping it in the rich position..I also drilled the main jets to 1/8 and left the secondary's alone....The article is in a old Hotrod Magazine's Street Rod Quarterly type of magazine dated July of 1971.... It has a good article on building a 392 by a guy named Cotton Werksman....Now that I know it works well going to make some spacers out of aluminum tubes...
     
    Last edited: Sep 21, 2023
  8. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,850

    gimpyshotrods
    ALLIANCE MEMBER

    These, and their Edelbrock cousins, can be boost referenced too!
     
    big duece likes this.
  9. hemihotrod66
    Joined: May 5, 2019
    Posts: 968

    hemihotrod66
    Member

    Here is the magazine for that hemi tech article.... Scan.jpg
     
    tomcat11 and Deuces like this.
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 57,322

    squirrel
    ALLIANCE MEMBER

    You might want to put an O2 sensor on one of the headers, and an AFR gauge in the car, and drive it for a while. Might be surprised what you see.....

    I did this when setting up the 750 AFBs on my blown big block Chevy. I started with stock jetting, and the only change I made was to one size larger cruise portion of the metering rod. The carbs have been on the engine for 30k miles now, it's been in the high 9s, gets 11 mpg on the road, and doesn't foul plugs or burn pistons or anything undesirable.

    But you'll also see the mixture go wonky under some conditions, rich when you think it should be lean, go lean for a moment when you think it should be rich, and generally run a bit richer than you'd like much of the time. The carbs are not really made to be "perfect", and it turns out that "good enough" works.

    Boost referencing might be something you want to try, it could let you run it a bit closer to optimum under some conditions. Just don't expect miracles.

    If you haven't tried stock jetting/rods on the carbs, you might want to do that before you get too far out of range. An aftermarket carb of about the right size, with it's designed jetting, usually is driveable under most conditions, although it will tend to be a little richer than optimum.
     
  11. mohr hp
    Joined: Nov 18, 2009
    Posts: 1,129

    mohr hp
    Member
    from Georgia

    I built a similar engine to Squirrel's (Roots BBC w/ Edelbrock 750's) that's been in 3 cars. I found the same results when tuning with an A/F monitor.
    Carburetors are incredibly adaptable, but highly inaccurate at maintaining proper ratio through varying conditions.
    However, close is generally good enough.
     
    SS327 likes this.

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.