Qjet Setup Questions for Stage 2

Discussion in 'The Mixing shop.' started by Mark G, Apr 5, 2021.

  1. Mark G

    Mark G Active Member

    I have a '73 Qjet that I am setting up for a recently built engine. I am looking for recommended jet and rod recommendations. Here's the car setup: '70 GS convt, 462 inch BBB with TA 288-92H cam, 10.47 SCR and 8.19 DCR, B4B manifold, TA Stage 2 SE heads with JW level 2 porting, 3.55 gears, TH400 SP.

    I have a lot of Qjet experience and I like the carb, so changing the carb is not in scope.

    Looking for advice on primary rods, jets, secondary rods and any other Qjet mods that would be worthwhile on this setup.
  2. Max Damage

    Max Damage I'm thinking about it!

    Quoting from Larry in another thread:

    "First, in 1970, the Stage1 carburetor was 7040246. It was a 750 CFM carburetor. It used .068 jets and 42B primary rods. Secondary rods were AU's"

    This seems like a decent starting point for referance?

    Other Thread:
    BadBrad likes this.
  3. rkammer

    rkammer Gold Level Contributor

    The OP says he has a ‘73 QJ so his is an 800 CFM. With his aluminum heads, cam and compression, he should start richer with jets/rods in the 73-74 range & 42rods. Secondary rods/hangers tuned per other threads.
    Max Damage likes this.
  4. LARRY70GS

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

    Do you have Cliff Ruggles' book? Chapter 6 in Cliff's book deals with High Performance Modifications, and has 3 different recipes depending on how radical the engine is. That is what I would start with. I think the second recipe would fit your engine.

  5. rkammer

    rkammer Gold Level Contributor

    Here is the build specs on the '73 QJ Mark Northcott did for my '71 GS. That motor was very close to what you have other than my TA heads were Level 2 ported Stage 1s and yours are Stage 2s. The sheet shows compression at 9.8 but my final compression came out to 10.56 static with 7.9 dynamic. The Ruggles book will guide you based on your build but a carb that worked out well for me is:

    455_Parts (25).JPG
    Max Damage and lemmy-67 like this.
  6. lemmy-67

    lemmy-67 Platinum Level Contributor

    I still have the stock 750CFM on my 430 (7027248), and have 73 jets in there...maybe a size larger primary rods with AU secondary rods. That build sheet looks like a good starting point.

    I'm not running anything other than the stock big-port heads & matching iron manifolds, and the idle & power is nice & smooth.
  7. 436'd Skylark

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

    Somewhere in cliff's book he states that one of the most common mistakes carb tuners make is jetting carbs way up after making big changes like a cam or heads. It's not neccessary. Swap the power piston spring and use the stock jets/rods as your starting point. You likely won't move far from stock.
  8. Mark G

    Mark G Active Member

    Thanks guys. Between Ray's post with his spec sheet and the '71 stg 1 carb specs I copied from the factory service manual, I think I can get close enough. Definitely will heed the advice not to go too rich. My wife seems to have "recycled" my Ruggles book so I guess I need to buy a new copy.

    Attached Files:

    Max Damage likes this.
  9. shiftbyear

    shiftbyear Well-Known Member

    The biggest issue is idle and part throttle air/fuel enrichment. Having the throttles closed at idle on a modded motor makes a big difference. Enlarging the idle tubes, down channel, idle discharge and correct bypass air works wonders and gives you full accelerator pump shot. Ask Cliff for recommendations. Picture is carb upside down.
    techg8 likes this.
  10. 462 Chevelle

    462 Chevelle 462 chevelle

    Mark N does a really good job. I think the only thing I touched was the secondary air flap adjustment when I went from iron intake to the spx.

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