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need tips on jetting a quadra jet for a motor with a cam

Discussion in 'The Garage' started by down4thakrown, Jun 15, 2005.

  1. down4thakrown

    down4thakrown 1/2 ton status Premium Member GMOTM Winner

    Aug 5, 2003
    Likes Received:
    never had a cam put in a motor. how would i go about jettin the carb for the cam? im goin to rebuild the carb this weekend and wanna change the jets while im at it. so anybody know some good sizes that would work in a 350 with a 274 lift duration 4x4 comp cam? i know how to put them in im just not sure what sizes i should use. should i jet it up,down, just leave it???? i dunno what sizes are in the carb now but the secondary hanger says O and needles say D R.
  2. dyeager535

    dyeager535 1 ton status Premium Member

    Dec 13, 2000
    Likes Received:
    Roy WA
    Only time I've ever spent on a Q-jet's primaries was when I put EGR calibrated carbs on two non-EGR equipped engines. One had a lean surge at cruise, I fixed that and the other I prevented this on, by running one jet size larger.

    Short of running an O2 sensor solely to determine if mix is at 14.7:1 or not, not many of us have an exhaust gas analyzer which is pretty much what you need.

    Doug Roe's book on the Quadrajet has some info in it about jetting and the power piston IIRC, but it's more work than I've ever wanted to put into a carburetor. There are enough variables in the quadrajet on the primary side that without some way of measuring your differences (and racing only measures WOT) it's pretty hard to tell if you are making a difference or not.

    My dad played around pretty extensively with his 454's Q-jet (K20, 6000lbs, fairly wild cam, 465, 205, Gear vendors OD, 4.11's, 32 or 33" tires) to try and wring a few more MPG out of it. He's up to about 11 on the freeway. Knowing how he drives, I think he might be able to wring another MPG or two out of it just by modifying when he shifts, but from what most get out of their big blocks here, he seems to be getting better than average MPG out of it. I know he played around with the jets and rods on the primary side, but he also purchased a power piston spring kit that came with 3 springs, and played with those.

    He didn't have anything to determine mix, (O2 sensor, exhaust gas analyzer) so it was just by seeing how the different changes affected driveability/economy.

    If you've rebuilt Q-jets before, you know how "easy" it is to tear the air horn off to change the primary pieces, I'd probably wait and see how it does as is before playing around without establishing a baseline.

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