Rich at cruise

Discussion in 'Fuel and Air Systems' started by WCMachine, Sep 15, 2018.

  1. WCMachine

    WCMachine Well-Known Member

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    I have been trying to lean out my cruise mixture. Specs are AED 950HO, super victor, performer heads, cam= 234 in 238 ex .571” in .576 ex, 410 magnum. 18 degrees timing at 1000 rpm idle.
    I am running around 10.5 AFR at cruise rpm 2500-2800 @ 40-45 mph level ground. AED suggested increasing the idle air bleeds from .07 to .08. The only change I noticed was the mixture screws went from 3/4 to 1 1/4 turns. Cruise AFR stayed about the same.
    So I went down From .08 primary jet to a .075. This brought the cruise AFR to 14.5 but then engine would not idle until I turned the screws out 2 1/2 turns. This brought the cruise AFR back to 10.5
    What am I missing?
     
  2. hangn0ut

    hangn0ut Well-Known Member

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    Your idle mix screws should not affect your cruise afr. Perhaps the idle speed is open to far where it's idling off the transition slot. Are the secondaries open the same as the primaries at idle? Does this carb have an idle air bypass? Are you running a pvc system? Seems to me that you need more air at idle. But using the idle screw is not the way to do it.
     
  3. hangn0ut

    hangn0ut Well-Known Member

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    Increasing the main circuit air bleed will lean the cruise mix. As the carb opens into the transition slot the idle circuit shuts down. The idle circuit should not affect the main circuit and vice-versa.
     
  4. WCMachine

    WCMachine Well-Known Member

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    Also, If i lean out the idle mixture to get max vacume. It will not idle reliably in gear. It wants a richer idle mixture in gear.
     
  5. crackedback

    crackedback FABO Gold Member FABO Gold Member

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    Smaller idle feed restrictors...

    I had a spreadsheet to help figure some of this stuff out. Check sizing of IFR in your metering block. If they are .035 or larger, find a piece of .018-.020 wire, small bend and put it in the restrictor. See if the cruise leans out.

    I've "fixed" a ton of 950 hp style carbs doing this, especially the ones with fixed IFR's.
     
  6. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Yup!
    An engine can run lean when the only thing to overcome is friction.
    If you want to check the vacuum in gear, that's different.