short lean AFR spot after 0-> WOT

Discussion in 'Troubleshooting & Diagnosis' started by pomartin, Nov 16, 2019.

  1. pomartin

    pomartin New Member

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    Sep 16, 2019

    This is my first time dealing with car carburetor so please bear with me :)

    Edit: it is a 1980 Z28

    The problem is there since I have seen the car for first time, abou 6 months ago. Now I came to carb tunning and I'd need a bit of help.
    I have a problem with lean condition which lasts for a second when I perform no throttle to WOT quickly- AFR switches from 14:1 to 17:1 and in 1 second drops back down to rich values (12-13:1).
    If I open throttle to WOT slowly this does not happen.
    If I open throttle from 1/2 throttle to WOT quickly AFR rises a bit, to around 15:1 and then drops to 13:1.

    Please take a look at video of AFR on youtube (vac. secondaries blocked):
    best seen lean spot:
    slow acceleration:
    another video:

    AFR at idle is around 12:1, a bit rich but I can not manage to make it idle in gear otherwise.
    Primary jets are 77 and secondary 83.
    Squirter nozzle size is 31 (mounting screw is not hollow) and carb has pink color (3 holes) pump cam installed, bolt in middle hole.
    There is no clereance between pump and lever, also pump does not bottom out.
    I have tried with power valves (carb is boost referenced) from 2.5 to 6.5 but they do not affect lean condition at WOT.
    I have blocked vacuum secondaries and lean spot at WOT is still present, therefor I think there is a problem with primaries.
    I sized up jets to primary jets 86 and secondary 86 and blocked power valve- result is overall richer mixture but change in AFR is still there in conditions descriebed above, but the leanest point is in this case a bit richer.
    Squirter in primaries shoot fuel out instantly when throttle lever is moved.

    Is this a squirter and pump problem? Would bigger squirter make any difference?
    Current squirter size is 31.

    Motor is SBC 383 stroker.
    B&M144 supercharger, 2PSI boost (due to compression ratio too high)
    Road demon 725 carb with vacuum secondaries, boost referenced, primary jets 77 and secondary 83., plates are A12401X and A12400X.
    Canfield heads 23-500-65-0, compression ratio about 9.7:1
    I have no idea about cam, but it pulls good till 6500 rpm.
    About 6" of vacuum in gear at idle
    Hydraulic rollers and full roller rockers
    18°initial and 32°total timing- only centrifugal advance.
    vacuum advance is used and provides about 8° of advance at idle.
    Rear ratio is 3:7.3, 205/75 R15 tires.

    Any input is appreciated.
  2. bourbon_scotch

    bourbon_scotch Veteran Member Lifetime Gold Member

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    Jan 10, 2003
    oh... your carb is too small
  3. BC_Homegrown

    BC_Homegrown New Member

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    Sep 9, 2018
    Powell River BC
    Sounds like a bog off idle as well in 1 of your vids. That is alot of motor for a 720, could be a little small.

    Have you checked the fuel supply at WOT?
  4. Joekool

    Joekool Veteran Member

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    Feb 4, 2002
    Hollister CA USA
    You should be running a double pumper, a single accelerator pump isn't gonna be enough to supply the amount of fuel your motor needs until the power valve starts flowing. Your jetting is fine or else you'd be lean across the board. There is an extra capacity pump diaphragm and housing available if you don't have one already, it takes the accel pump capacity from 30cc to 50cc.

    At 2psi boost, why even have a blower?
  5. jeff swisher

    jeff swisher Veteran Member

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    Apr 26, 2018
    Yukon Oklahoma
    Forget looking at that AFR gauge it will drive you crazy with a carb.
    The carb can't keep up with abrupt throttle opening it will always go a bit lean then catch up.
    Best thing to do is tune it so it runs great and get the part throttle cruise in a good AFR territory and idle in a good territory and full throttle in a good spot.. the transition is a muther.
    At least when looking at the AFR gauge..
    Get it so it does not bog during transition.

    You say 6" idle vacuum in gear.
    Is the in gear RPM really low.
    850 RPM as a minimum in gear and have my engines set at 1100 in park.

    Look at cruise vacuum readings and see if this issue happens at 10.5" or so.
    You may be able to run a 9.5 power valve depending on cruise Vacuum readings.

    You will not add any additional fuel from the 9.5 power valve at idle from a Holley.
    You must be into the boosters before fuel can flow from the power valve area.

    If it was my car i would see if I could eliminate the vacuum advance and run more initial timing.
    I have no idea if your engine can tolerate more initial timing. but i would try for 24 initial and keep total the same.
    When you open the throttle quickly and drop vacuum reading you also loose that vacuum advance.

    But you may be all in on timing by 2400 rpm I do not know.. it could be all in at 4500.
    I like it in quick if the engine combo likes it.
    2 PSI should give you another 50HP anyway.
    If ya have the stuff laying around might as well use it .
    80sz and G72Zed like this.
  6. G72Zed

    G72Zed Veteran Member

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    Sep 8, 2015
    X2 ^^ A great reply and wright up by Jeff Swisher.
  7. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Nov 4, 2004
    Ringgold, GA
    I can agree with this, been there, done that, got frustrated.

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