Is 13:1 air fuel ratio at 60mph cruise rich?

Discussion in 'The Venerable Q-Jet' started by real82it, Apr 11, 2013.

  1. real82it

    real82it Silver Level contributor

    I recently installed and tuned a 800cfm quadrajet on my 455.

    Car runs great now. Idle is 14.5:1. Off idle and cruise is good up to about 50mph. Throttle response is great. WOT is perfect at 12.3- 12.8 from 2000-5500rpm. My problem is part throttle cruise at freeway speeds. Starting about 2000rpm cruise in 3rd gear or about 50mph the air fuel ratio starts getting rich quickly. At about 55mph on, air fuel hovers at 13:1. Sometimes dips into high 12s for periods of time.

    Is this normal at freeway cruise speeds when the engine is under some load? I am worried any long freeway trip might start fouling plugs or something.

    If not, suggestions on how to tune it to be leaner at freeway speed but still maintain the drivability and WOT performance?
     
    Last edited: Apr 12, 2013
  2. Cliff R

    Cliff R Well-Known Member

    What is the carburetor number, and how is it set up?.....Cliff
     
  3. 70 gsconvt

    70 gsconvt Silver Level contributor

    I am by no means an expert in this field, but I have been reading and learning online (they can't put something on the internet that isn't true. They said so on the internet.)

    Butttttt, from what I have read, a number closer to 14:1 for no load cruising would be a better target. Above that and heading to 15:1 go start to lead to detonation. You may need to start trying different rods to get the best throttle response and AF ratio.
     
  4. techg8

    techg8 The BS GS

    Too rich for cruise.

    do you have a ram air setup that might be causing fuel slosh out the vents or pressurizing the carb?
     
  5. DaWildcat

    DaWildcat Platinum Level Contributor

    Power piston may be rising slightly if the spring is too weak. Worth checking.

    Devon
     
  6. real82it

    real82it Silver Level contributor

    Carb number 17057553 0507
    76 main jets
    46k primary rods (confirmed)
    CC secondary rods

    TA 290-94h cam
    1.6 roller rockers
    Edelbrock stage 1 heads
    TA 1 7/8 headers
    Edelbrock performer intake
    Factory ram air cleaner
     
    Last edited: Apr 15, 2013
  7. Cliff R

    Cliff R Well-Known Member

    Jets are too big for that carburetor number. I would drop the jet size back a couple of numbers, and re-adjust the APT screw for best light throttle power, throttle response, and fuel economy. Wouldn't hurt to get away from the "K" series metering rods and go to a full tapered rod to provide more room for adjustment in that area......Cliff
     
  8. real82it

    real82it Silver Level contributor

    Ok. I will try to drop a few jet sizes. FYI I purchased from SMI. Here is a list of mods:

    Idle down tube(idle jet) and idle air bleed resized for smooth idle quality with performance camshafts.
    CRO(channel restriction orifice) resized for crisp off-idle and part throttle response.
    Proper metering rod power piston spring selected to match engine vacuum characteristics
    Secondary metering rods and hanger selected and high speed air bleeds resized for maxium full throttle horsepower
    Select highest quality cores for best performance and reliability
    Install bushings in baseplate to eliminate any potential vacuum leaks at primary throttle shaft.
    Apply epoxy to main body casting plug to eliminate common "leak down" problem associated with Quadrajets.
    Idle, transition, main, and high speed circuits modified to provided optimum fuel curve for YOUR engine
     
  9. Cliff R

    Cliff R Well-Known Member

    If I had $5 for every completely "rebuilt" and custom tuned carburetor I've had sent here to fix, I could retire tomorrow!......FWIW.

    Unless someone knocked out the MAB restrictions to .118", 76 jets are simply all wrong for that carburetor number.....IMHO.

    I also NEVER tune with an A/F meter. They are an excellent tool, but there are variables in that deal, and different engines will require different A/F ratios across the load/speed range to make them happy.

    Very well thought out engines combo's actually need LESS fuel at every point to make best power and best efficiency across the load/speed range.

    The distributor is also a KEY player here, and changing the amount of timing added by the vacuum advance in conjunction with APT adjustments can have a big impact on fuel delivery requirements at light/part throttle operation.

    Here are my recommendations for tuning the carburetor:

    Put a piece of electric tape over your A/F meter so you can't see it.

    Tune the idle system and mixture screws for best idle quality in and out of gear at the leanest settings. Tune heavy part throttle A/F next, for best power down heavy on the primaries only, don't worry about which metering rod is being used, or where the APT is at.

    Once you determine the best jet size, install metering rod that will provided FULL control of the part throttle A/F from rich to lean via the APT.

    Dial in the APT for best part power, throttle response, smoothness right off idle, transition, and best fuel economy.

    Use the secondary metering rods to tune WOT. Dyno or dragstrip will cut to the chase quickly here. "Seat of the pants" tuning at full throttle is just about completely USELESS once you get close. 3 or 4 runs on a good track changing metering rods will tell you where you are at and where you need to go with the metering rods.

    Once you are happy with the carb in ALL areas, take the tape off of your meter and look at your A/F at every point. These will be your "base" settings, and one can make very small adjustments for the APT from this point forward to try to squeak a little more efficiency and fuel economy out of the set-up, and use the meter to help monitor the changes.

    In any and all cases, it's best to er just a tad rich at full throttle and sneak up on the best settings. EGT's spike pretty high when you start to go lean, just about the time you are finding out the leanest settings the engine will tolerate, and right before you knock a ring land or rod bearing out of it!...........Cliff
     
  10. real82it

    real82it Silver Level contributor

    Hahahaha. Roger that. I will give it a try and let you know. Thanks cliff.
     
  11. real82it

    real82it Silver Level contributor

    Cliff...

    Thanks for the advice. I was finally able to tune this carb using your directions. (But never made it to the drag strip. Been a busy summer with car shows, other projects, life, etc).

    I ended up with 50m rods and 74 primary jets. I kept the CC secondary rods.

    All seemed well until I put the a/f to see where I ended up. Cruise A/F is great at mid 14s. I am concerned about WOT.

    I am running at 13.2 to 13.6 from 4250 to 5500 rpm. I would feel much better if I could get it down into the 12.7 range especially at the higher rpms.

    I can't go any smaller on secondary rods.

    What options do I have to richen up the WOT mixture in that rpm range?
     
    Last edited: Sep 16, 2013
  12. 71skylark3504v

    71skylark3504v Goin' Fast In Luxury!

    You don't make more than $5 per Q-Jet you fix?:confused:
     
  13. Cliff R

    Cliff R Well-Known Member

    See my response in the other thread. "M" series primary rods are incorrect for that carb number. They use .036" tips, and will lean up full throttle A/F considerably. After seeing where the carb was built, it may need smaller high speed airbleed tubes, and larger fuel inlet seat. Minimum for a 455 is .135" at any power level. Unless an electric pump is being used, may need to move up to .145-.149"......Cliff
     
  14. gsgtx

    gsgtx Silver Level contributor

    peak torque is most important not run lean at that rpm. that's where it will hurt the motor most.
     

Share This Page