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Thread: Carb Tuning 101

  1. #1
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    Wayne Presley's Avatar
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    Carb Tuning 101

    I thought i'd jot down a little primer on carb tuning a Quick Fuel or Holley Ultra double pumper since both have removeable air bleeds, idle feed restrictors, power valve restrictors. The principles apply to the standard Holleys but since the only jets that are adjustable are the main jets and idle mixture screws, you have to really modify the main body and metering blocks to accomplish the fine tuning. So lets get started with the basic concepts of what makes a carb tick.

    A carb delivers the main fuel to the motor not by vacuum developed by the motor but by a pressure drop caused by the air flowing through the venturis sucking the fuel from the float bowls, the fuel in the float bowls has atmospheric pressure pushing the fuel into the motor. At idle the vacuum developed by the motor does pull the fuel in. The fuel is delivered in stages starting with the idle circuit, then the transfer slot, power valve and then the main jets. And the flow doesn't stop in one circuit when the next one starts so you have to work from the bottom up to the top.

    Mechanical Set Up
    First you need to set the carb up with the very end of the transfer slot (T slot) just visible so that it looks like a square (or even slightly smaller) when viewed from the bottom.



    Not this


    Now do not open the idle screw to adjust the idle as you'll open the Tslot and it will mess with the idle mixture. You can open the secondary screw under the bottom plate using the allen head screw if needed to increase the idle speed. Set all 4 of the idle mixture screws (IMS) to 1 turn open from seated. Go ahead and bolt it on, hook up the fuel lines, run the pump and check for leaks.

    Idle Setting
    Start the motor and get the motor up to 150 F and set the base timing to 14 BTDC (good starting place for SBF). With the motor at temperature, open one IMS 1/4 turn and see if the idle speed goes up. That tells you that the mix is lean and you need to open all the IMS 1/8 turn more and retest. If the idle speed goes down then try turning them 1/8 turn closed since it's a bit rich. Note the vacuum in the motor at idle (around 850-900 rpm) and select and install a power valve that is half of that reading (ie: vacuum 13", install a 6 5 PV, vacuum 9", install a 4 5 PV). If you are using a wideband O2 sensor like an Innovate LM2, try to get the mix close to 13.5-14.0 AFR. If you don't have one, you are trying to get the highest idle speed. Now you are ready to tune the light cruise portion of the carb. For the rest of the tune you will need a wideband O2 sensor.

    Light Cruise
    In this section you will adjust the idle feed restrictors (IFR) and idle air bleeds (IAB). Go out and run the car in 3rd gear from 1500 rpm on a flat to slight downhill and just enough throttle to lightly accelerate to 3000 rpm while datalogging the wideband O2. In the example it took 12 seconds to get from 1400 to 2100rpms to give you an idea on how little the throttle bldes were open. Look at the datalog and you want the AFR to be in the 13.5-14 range at 2500-3000 rpm. If it's richer than that, go smaller on the IFR's. Lean, go bigger. Now look at the start of the run and see if the log goes from lean to rich or rich to lean. If it goes lean to rich, put in a smaller IAB to bring the fuel in sooner. If it goes rich to lean then you need to put in a bigger IAB to delay the circuit flow. The example below goes richer from low to higher rpm so to flatten out the curve I need to go smaller on the IAB's. In this case I went .002" smaller. Since the end of the run was at the desired AFR I did not need to change the IFR size.

    https://imgur.com/n1vtESJ

    Tuning Primary Main
    start out in 3rd gear about 1500 rpm and press the throttle only open far enough for only the primary blades open and the secondary opening slot ending in the linkage (you don't want the secondaries to open) and let it rev to 4000 RPM while logging the AFR. Look at the log and see if the AFR toward the end of the run is above or below your target of 12.5-13. If you look at the example below, the brown line is the AFR on the initial run. Its too rich early and too lean late so the power valve circuit restrictor (PVCR), main jet (PM) and HSAB need to be changed. The PM will add fuel late and the PVCR and HSAB will remove fuel early. I went up 5 sizes on the PM, .002"smaller on the PVCR and .005 larger on the HSAB to acheive the yellow AFR line. Above 3000rpm the motor is pulling more air than the partially open primary blades can supply so the vacuum is going to increase and it is enough most of the time to close the PV. You can see the PV starting to close at 3300 ish RPM and is fully closed at 3600 ish RPM when AFR goes leaner.

    https://imgur.com/n1vtESJ

    Tuning Secondary Main
    It is done exactly like the primary main with out having the PV to adjust the initial and overall fuel since most secondary blocks do not have a PV. In the full throttle base run below the yellow line was the baseline full throttle run before the above jetting changes and you can see that the final fuel mixture is close to the desired AFR of 12.4-12.8. But remember that all the jets are flowing at full throttle so any adding or subtracting of fuel done earlier will affect the AFR plot now. So I had not changed the IFR's, went down .002 on the PVCR and up 5 sizes on the primary mains (PM) I'll need to go down about the same on the secondary mains (SM). The second graph shows this change on the brown line and it is also rich early and desired AFR at the top of the RPM's. So I changed the SHSAB's from .035" to .045" to get the yellow AFR line in the second example.

    Full throttle baseline run

    https://imgur.com/1dpJvKZ

    Full throttle after SM jet change (brown) and after SM jet and HSAB change(yellow line). Look how much more torque (blue line) between 2375 and 4000 rpm with the mixture cleaned up.
    (note: The brighter red and blue lines also reflect 2 more ignition timing over the other run. The slightly leaner high rpm AFR is due to the HSAB reducing the fuel slightly and the extra timing burn the mix more completely)



    Accelerator Pump
    Now that all the fuel curves are set you can fine tune the accelerator pump enrichment (AE). Looking at the below example you can see that 1/3 throttle stab(from 15-19 seconds on the graph) and full throttle stab (from 23.4 to 25.8 seconds) both result in the AFR going lean. The leanness on the 1/3 throttle is enough that the motor stumbles, notice the blue torque line drop(between 16.5 and 18.5 seconds) then pick back up. The FT stab isn't nearly as lean so going to address the primary AE first. It has a pink pump cam and .031" squirter on the carb so I went up to .036" and that was enough to solve the low RPM stumble with out being too much AE above 2500 RPM.

    https://imgur.com/qQu2RrG

    You can create data logs like these with the Innovate LM2 and the software with that comes with it.

    https://imgur.com/A6pjAD3
    Last edited by Wayne Presley; 06-11-2011 at 01:18 PM.
    Wayne Presley www.verycoolparts.com
    Xterminator 705 RWHP supercharged 4.6 DOHC with twin turbos

  2. #2
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    Wow...thank you so much for the education.

  3. #3
    Senior Member riptide motorsport's Avatar
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    very useful.............thanks
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  4. #4
    FFR6671 Fifty-Two's Avatar
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    Great info Wayne! Very much appreciate you taking the time to write that up and post it for us. It definitely takes some of the mystery out of carb tuning.

    - John
    Mk 3.1 - FFR6671 - "Trying" to recreate a Semi-Comp 427...
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