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Thread: Quick and dirty SU Tuning

  1. #1
    Nova Scotia,Canada,Earth Blue's Avatar
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    Default Quick and dirty SU Tuning

    For balancing the carbs here are a few quick and dirty methods I use when I don't have a flow meter or WB A/F meter:


    Setting Idle
    0 Assume good plugs, valve adjustment, no vacuum leaks, good fuel pressure and float levels set correctly.
    1. Remove air box
    2. Warm up car and remove all linkage connections so that the carb's throttle plates are resting on their set screws.
    3. Adjust each set screw so that the suction felt when you put your knuckles against the throat of each carb and feel the air flowing between your fingers is the same on both carbs.
    4. Stick you head down between both carbs and listen to the intensity of air whistling sound from each carb. It should be the same amplitude
    5. Repeat 3&4 until happy and your rpms are 600-800rpm (exact rpm should be suited to your taste and performance when moving from stop or what the FSM states... your call baby)

    Setting Off Idle
    6. Reconnect linkage
    7. Make sure the screw from the fast idle servo is not pushing on the throttle rods (that screw is the one next to the front carb).
    8. Turn the fast idle screw at the center of the linkage (way up high) so that the car is idling ~ 3000 rpm
    9. Adjust the balance screw on the linkage (next to the back carb) so that the air flow is the same in both carbs (as you did in 3&4 above)
    You can also observe the point at which both throttles open when you push the main linkage. The balance screw should be set so that both throttle plates open at the same time.


    Setting Mixture
    0. Warm up car
    1. Screw both jets all the way up to the top
    2. Screw the jets down 2.5 turns (default starting point)
    3. Take the car for a run and listen closely for popping in the exhaust (rich), popping in the intake (lean) and note power
    4. Screw both jets up or down 1/2 turn and take another run to see impact.
    5. Repeat 3&4 and do 1/4 adjustments until you are happy
    6. Take the car for a spirited 30min drive with plug wrench then while driving fast on a long stretch, hit clutch, kill motor then pull over to side of road, look at all plugs and compare colour. If there is a difference between the front 3 and rear 3 then the fuel jets need tweaking. Adjust the darker 3's carb by turning up the jet to lean the mixture a 1/4 turn
    If all plugs are too dark then turn both jets up 1/2 a turn.
    7. Repeat 6 until you are happy. All plugs should be the same colour and very light brown. Listen for popping out the front to tell if you go too lean. Don't worry, you can take your time and do this leisurely over a couple of days to fine tune and colour match your plugs.


    Power and Timing
    1. Fill up with premium gasoline
    2. Advance timing to 10 BTDC
    3. Grab a 10mm wrench
    4. Take car for a run and note power
    5. Pull over and use 10mm wrench to advance timing a bit (turn distributor clock wise) Don't worry about a timing light
    6. Take it for a run and note power increase and listen for detonation under load (try acceleration on a hill to force a load)
    7. Repeat 5&6 until you are happy with balance of power vs. detonation
    8. Check plugs again. You may need to add more fuel to get the last bit of power
    9. Make note of timing at idle using a timing light.

    Follow Up
    1. Turn each jet up and count turns. Record these numbers for future reference then turn the jets back down.
    2. Measure timing with a timing light at idle with no vacuum advance and at 3000 rpm Record these numbers
    3. Measure engine vacuum and record number
    4. Measure air flow (with proper device) and record numbers
    5. Measure O2 with WB at idle and at various RPM's underload and record numbers
    6. Re-balance carbs at idle and 3000 rpm's with airflow meter
    7. Adjust jet height using WB O2 sensor
    8. Close mouth of each carb (one at a time) set rpm to 2000 and run rough on one carb and measure WB. Compare both carbs and strike balance.
    9. Do the "lift each piston 1/16" and compare how engine behaves as well as "Norm's lift each piston fully to disable" trick to also compare each side to the other's behaviour.
    9. Measure 02 with WB at various rpms' under load and select or augment needle to taste. Repeat 9 until happy with WB and butt dyno performance.
    10. If you have the luxury of a dyno then use it.
    11. Once jets and needles are set Blueprint (measure everthing) including depth of jet from bridge of carb with vernier caliper's depth gauge.
    Last edited by Blue; 04-13-2011 at 03:35 PM.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


  2. #2
    Registered User Powderkeg's Avatar
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    Default

    Philip,

    Thanks for the tip.

    I look forward to your posts and glad to see here!

    Dave Ruiz

  3. #3
    Registered User Dobber's Avatar
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    Default

    Thanks for taking the time to post all the great tips.

  4. #4
    Fired Up! JLPurcell's Avatar
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    Default

    Philip
    Thanks, I will keep this for future reference.
    JLP
    Jerry Purcell
    1970 HLS30 00029 Owner since 1976 Now owned by Les Canaday (Classic Datsun Motorsports)
    1972 HLS30 93606 IZCC Registion #105 Original owner back home after full restoration by Classic Datsun Motorsports

  5. #5
    Nova Scotia,Canada,Earth Blue's Avatar
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    Default SU Tuning Pictures

    Some drawings.
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    Last edited by Blue; 05-07-2011 at 06:23 AM.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


  6. #6
    Nova Scotia,Canada,Earth Blue's Avatar
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    Default more pictures and diagrams

    more pictures and diagrams
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    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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