qjet 704124 KA 455 carb going on a built 350

Discussion in 'The Venerable Q-Jet' started by Niterider66, Jul 9, 2021.

  1. Niterider66

    Niterider66 Member

    Looking for input on running a 455 carb on my 350 engine. The carb was professionally restored to original. WOT is great once I'm rolling, terrible out of the hole. I'm thinking things need changed to run this carb on the 350 engine. Idle mix screws are all the way out and don't seem to do much. Vacuum runs around 12-13 Hg at 950 rpm idle, (higher than it should need to be). My idle needs to be adjusted so high to keep the car from stalling once placed into gear that I think the throttle blades are cracked open some and that they are pulling fuel from the main metering at idle. Can anyone tell me what i should be changing related to the idle channel restrictors or tubes. Are there other things I should look to changing to run this 455 carb on a well built 10:1/ .030 over with TA 284 -88H cam, TA Dual Plane intake, with ported heads, Tri-Shield 2800 rpm stall and stock manifolds with a 3.73 rear gear.
     
    Dano likes this.
  2. Dano

    Dano Platinum Level Contributor

    You're missing a digit in the carb #. I've wondered the same as you for an upcoming build. Maybe @Rob Ross or @Cliff R will chime in here.
     
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Ignition timing? A cammed engine likes more initial timing, at least 12*, if not more. This allows you to close the throttle blades enough to get the engine operating on the idle system, not the mains (nozzle drip). This may require a distributor re curving to limit mechanical advance so you can run more initial advance. I would try that first before changing idle system calibration. The 455 carburetor already has a higher capacity idle system because a 455 needs more air at idle than a 350, but a 350 with a bigger cam may like the 455 carburetor.
     
    techg8 likes this.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If you want to experiment, temporarily advance the initial timing way up there, and see if you can close the throttle so the idle mixture screws become responsive. You should be able to kill the engine by turning them all the way in. Just don't drive the car that way as you will be over advanced.
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If it is a 455 carburetor, it is either a 7041243, or a 7041242 (Stage1 carburetor)
     
  6. Dano

    Dano Platinum Level Contributor

    7041240?
     
  7. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    You have a lot going against you here.

    If the carb is for a stock 455 then you'll need to do a lot of recalibrating.

    First there probably isn't enough vacuum to close the power piston and seat the rods, couple that with the jets being way to big for the 350 and the carb is dangerously rich. That explains why it only runs ok at WOT. Same for the secondaries.

    Larry is right, the idle circuit may be close but it can't overcome the power piston not closing..
     
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Is there such a number? 7041540, not 7041240. Below is the 1971 Chassis Manual specs for 1971 Quadrajet.

    1971Quadrajet.jpg
     
    techg8 likes this.
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  10. Dano

    Dano Platinum Level Contributor

  11. Bigpig455

    Bigpig455 Fastest of the slow....

    for whatever reason ( and 71 was the only year) the 540 was the standard 455 48 state Q-Jet for 1971. A terrific performing carb too!
     
    Brett Slater, techg8 and Dano like this.
  12. Dano

    Dano Platinum Level Contributor

    Interesting... Never knew.
     
  13. techg8

    techg8 The BS GS

    ^^^^^Yes!^^^^^ timing is first
     
  14. Niterider66

    Niterider66 Member

    you are correct. 7041240 is the correct id#.
     
  15. techg8

    techg8 The BS GS

    First thing,
    455 carbs have a lot of idle air bypass in the baseplate. Too much in fact for a 350.

    Bypass air let's you close the throttle and still get air into the engine. On a 455, this helps prevent nozzle drip at idle. Used On a 350, it's basically a vacuum leak. You'll have a high idle and the mix screws will be 4 turns out or more as a result.

    Unless youve got an extreme 350, I would close off the idle air bypass holes in the baseplate.

    Next thing...
    As larry mentioned, retarded ignition timing at idle can lead to open throttle settings and primary nozzle drip.

    The other cause is lean idle circuit mixture.
    Whether its clogged idle tubes or a vacuum leak causing the leanness, sometimes it's bad enough that we chase an idle right into nozzle drip throttle settings.

    Bottom line
    1 timing 1st...12deg initial
    2 plug the idle air bypass
    3 pull idle tubes and clean/ size .035-.036
    4 if you still have problems, check for vacuum leaks
     
    sean Buick 76 and Bigpig455 like this.
  16. Niterider66

    Niterider66 Member

    I have recurved the distributor it is currently set at 12 initial and 34 total at 2800 rpm. I think I read that the big blocks had an different idle air circuit altogether than the small block carbs? The BB had an air idle bypass circuit through the horn and the small block within the carb downtube type air idle.
     
  17. Niterider66

    Niterider66 Member

    yes thats the one
     
  18. Niterider66

    Niterider66 Member


    Carb # 7041240 KA is what the carb has is stamped on it. I looked it up once on a site and it was listed as 455 automatic trans carb ? I need to know what I should start ordering or if it needs sent out to a professional rebuilder that can set it up to my engine package. It listed 70 jets and 456 rods. I'm historically a Holley guy. Years ago i had a SD-455 Trans -Am and that q-jet was the best carb I ever laid my foot into.
     
  19. techg8

    techg8 The BS GS

    BBB qjet 800cfm had idle bypass air channel from the air horn, down thru the carb body, and thru the baseplate into the intake.

    SBB qjet as no bypass holes in the air horn nor the baseplate. it forsakes bypass altogether in stock form.

    There are differences in the placement and sizes of the upper idle air bleeds between the carbs, but that has nothing to do with bypass air.
     
  20. Niterider66

    Niterider66 Member

    What is the optimum way to check for a vacuum leak? If my base timing is above the current 12 deg initial the car doesn't want to start well cold and really bad turnover when hot. I know my compression is high for pump gas and I installed a big battery to crank it. At 12 initial the car will start ok though it turns over a bit much and seems rich. If the vac advance can is unplugged the car wont idle without even more idle screw turns. I should add that the car is at fast idle in park (due to turns out on the idle screw) and will near stall as soon as it is placed in gear. Seems to worsen after full operating temp. Also once fully up to temp the car doesn't like to restart. As if flooded but will crank fine unless I advance the initial base timing beyond 12 deg.

    I have been studying the idle air circuits and the things guys do to tweak them and why. I need to fully comprehend the circuits and locations, terminology and so on before I start plugging or drilling. My first thought was jets and rods being too large as it is a BB carb originally. The specs. state 70 jets and 456 rods from factory. I presume they are in fact those. I'm just not knowledgeable " yet" about the q-jets.
     

Share This Page