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Technical Multi Carb Jetting

Discussion in 'Traditional Hot Rods' started by customline3859, May 14, 2022.

  1. customline3859
    Joined: Oct 20, 2017
    Posts: 90


    Hello all. I've been working on dialing a 3x2 carb setup in on my 1955 Ford Customline. It's a mild Y-block build. 304ci. Isky cam, gasket-matched ECZ-G heads, red's headers, Weiand intake. ECG5 carbs. I'm tuning with an air fuel ratio gauge and running my center carb only at the moment. I've been able to achieve 14.7:1 with 60s. I'm hoping that I can determine which jets i need for the outer two carbs without having to go through much more trial and error. Should all the jets be the same in all three carbs? Should they be richer on the outer two? Any info or advice would be greatly appreciated.
  2. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,632

    Mike VV
    from SoCal

    Why would you think that !?
    The secondary carburetors will require their own jetting, and thus...your (or a friend)...attention to the AFR gauge.
    Or do a run to your (somewhat ?) local dyno shop to do it more quickly and easily.

  3. jimmy six
    Joined: Mar 21, 2006
    Posts: 11,497

    jimmy six

    If possible run each carb as a single and get themthe same as the first one. If you your running progressive linkage and idling on all 3, buy or borrow a UniSyn and get the ball as high as you can on all three.
    If not progressive by running all three as a single everything should be fine. Its how I did the dual quads on my Y Block and it’s stock..both have stock jetting. I also run venturi and not manifold vacuum for the distributor.
    If it’s an Offenhauser manifold your on your own as they are different inside than others..
    saltflats likes this.
  4. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 1,846

    427 sleeper

    No recommendations on jetting, but I'd only use the AFR gauge as a reference point and not the gospel truth. The spark plugs will tell the whole story. There's a lot of variable's to deal with, so it's all trial and error. If it feels right, it usually is right.

  5. Pete1
    Joined: Aug 23, 2004
    Posts: 2,091

    from Wa.

    I would forget trying to run one alone or progressive. Use straight linkage. Tune with your AFR.
    I ran 3 Stromberg 48's on a 57 312 Tbird with an otherwise stock engine and it improved it to where I could almost keep up with a stock Corvette.
    Elcohaulic likes this.
  6. 57chevymadman
    Joined: Sep 15, 2021
    Posts: 101


    Another read the plugs and tune accordingly vote here....
  7. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,680


    I personally like progressive linkage jet the end cabs to achieve 12.1 Center carb for cruse end carbs for power. Most 4 barrels are richer on the secondary side. Straight linkage can get you to much carb to soon.
    Johnny Gee likes this.
  8. saltflats
    Joined: Aug 14, 2007
    Posts: 11,367

    from Missouri

    Idle carburetors for dumpers will lead to stale gas when you want it and create a bad bog. ;)
    427 sleeper likes this.
  9. customline3859
    Joined: Oct 20, 2017
    Posts: 90


    Why would I think that? Why would I ask a question on a site that has a wealth of information? Why would I look to an older generation of people that have been there and done that. I do not understand the need to criticize a person for asking a question. I got my answer though. It's trial and error.
    ebfabman likes this.
  10. carbking
    Joined: Dec 20, 2008
    Posts: 3,345


    As a rule of thumb, anytime you are placing a carburetor(s) on an application for which it (they) did not originally appear, USE THE MANUFACTURER'S STOCK JETTING FOR THAT CARBURETOR AS AN INITIAL CALIBRATION!

    Now you have a repeatable "baseline".

    Tune from there.

    As you have an A/F meter you are ahead of many.

    As several have posted, straight linkage works better (easier) than progressive.

    As Jimmy 6 stated, a Uni-Syn is a required tool. Not expensive, not difficult to use; and once you acquire proficiency, you can impress your friends with your tuning prowess.

    And while I am not wishing to start an argument with those suggesting the reading of plugs; IT IS AN ACQUIRED SKILL! Few can do it well, and I have much respect for those that can!

    Another tuning aid that is currently available, and not overly expensive; is an infrared thermometer that may be aimed at the exhaust ports to compare port temperatures. Much easier (at least for me) to read the dial (or digital display) than the plugs.

    Last edited: May 15, 2022
  11. Shain
    Joined: Jun 2, 2016
    Posts: 52

    from Omaha

    Like said already.....I (in the old days) did adjustments by plug readings, (learned from racing cars).....and in concert with air flow reading gauges.

    No electronic stuff to do your thinking for you back them.

    But yes, it was a trial and error thing in the beginning. But you could get the "feel" of the engine when it was tuned best, and ran the cleanest, smoothest.

    Now days EFI does it for you.
    Last edited: May 15, 2022
    2OLD2FAST and 427 sleeper like this.
  12. Kevin Ardinger
    Joined: Aug 31, 2019
    Posts: 447

    Kevin Ardinger

    For what it’s worth, my center carb has 56 jets the outer “dump” carbs ( no idle circuits) have 58s in them. I think mine is a little fat though. 350 Chevy and Rochesters. I cant get gas to sit in the carbs long enough to get stale. Evaporates in a couple days if it sits.
  13. oj
    Joined: Jul 27, 2008
    Posts: 6,319


    Do you see little wisps of white vapor curling up from the carbs when you shut the engine off and the engine up to temperature?
  14. Budget36
    Joined: Nov 29, 2014
    Posts: 9,046


    He gave you good advice, as did others.

    Ease up on being defensive;)
  15. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,076

    from illinois

    To repeat what CARBKING said , the carb manufacturer spent a lot of money to properly set the fuel/ air mix for the carbs you are using whether there's one or more , on a moderate performance engine , they're usually very close .
    carbking likes this.
  16. jimmy six
    Joined: Mar 21, 2006
    Posts: 11,497

    jimmy six

    I run 2 multi carb engines now. Both progressive. Both set with a Unisyn and a vacuum gauge. One engine has internal stock parts and one with a cam and compression. One with early WCFB’s and one with 3 Holleys from 1960 Merc 292’s. All have stock jetting. Both perform perfect even tho the 3-2’s are on an ancient Howard log intake.
    One key is very minor to no leaking throttle shafts. Trying to tune idle with a vacuum leak is impossible to repeat. Holleys are easily fixed today with bushings and Teflon collars.
    The other is to kick it in the butt every time your out so you needle and seats get exorcise. On the outer carbs I’ve done with 3-2s I always set the float levels 1/16-1/8” low so if a more fuel from vibration gets past the needles they get tighter on the seat..
    Your experience may vary…
    Last edited: May 16, 2022
  17. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,680


    Stale gas is not an issue when I drive.
    oj likes this.

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