750 qf vac sec running rich

Fuel and Air Systems

  1. rigger3006

    rigger3006 Well-Known Member

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    carb is running super rich at idle, can turn air/fuel screws all the way in and vacuum does not change at all. carb is untouched out of the box, ive read others have had this problem but never got a solution from those brand x sites. car goes like a hot damn but plugs are fouling and getting headaches from exhaust at idle, any thoughts?
     
  2. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    #1 Primary throttles open too far at idle.
    #2 Fuel levels set too high (Check this first as you standard oeprating procedure any new or worked on carb)

    Generally less throttle is needed at idle when more initial timing advance is used. Say its set at 10* BTC at 1000 rpm. In which case adjust it for 14* to 16* BTC at 1000 rpm. Then after its warmed up, reduce the speed with the throttle stop screw. Say to 800 rpm and recheck the timing. It may have dropped back a couple degrees.
    That done, readjust fine mix screws. There should be response to changes - especially when closing them off (too lean is always more obvious than too rich).

    Repeat as required. Test driving (low speed and steady only -NO wide open throttle) to confirm results in gear.

    Another way to do all this is take the carb off, observe the primary side transition slot openings and make sure only .020 to .035 shows beneath the bleadees when shut. Holley says .040 but QF t-slots may be wider or longer than old school Holleys.

    #3 Modify the carb. This is more involved and does not change the procedure above. Relocate the idle feed restriction so its always submerged and get rid of excess e-bleeds. This simply will make the way the fluid moves more consistant.
     
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    • rigger3006

      rigger3006 Well-Known Member

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      floats are spot on, fuel pressure is 4.5 as suggested, t slots are also correct.just fiddled some more, when air/fuel screws are all the way in,gently, vacuum increases.initial timing is 20, all in by 34
       
    • dano

      dano Evil Handy Man

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      Manual or electric choke? If manual, wire it open and verify your air cleaner isn't messing with it. If electrical, make sure it has power and is opening fully. I've seen the linkages for the choke be bound, so look it over and be sure it's all free. How is your PCV set up? Also be sure your throttle cable isn't allowing the blade to shut all the way.
       
    • rigger3006

      rigger3006 Well-Known Member

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      electric choke, fully opens.pcv worked fine with the smaller holley i had on before. throttle cable works fine. ive used this carb off and on over the last 2 years, always something with it.soon to be wall art i think
       
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      • mderoy340

        mderoy340 Well-Known Member

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        make sure the idle air bleed passages are clean. Are they removable? If so take them out and see if you get control with the mixture screws.
         
      • dart_68

        dart_68 Well-Known Member

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        If you turn the idle mixture screws all the way lean the motor should die. In fact you should hear the idle change or stumble before you get that far. If not, chances are your throttle plates are too far open uncovering too much of the transfer slot.
         
      • Aaron65

        Aaron65 Well-Known Member

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        All QF carbs (I believe) have replaceable air bleeds and IFRs...I might try smaller idle feed restrictors first and see how it runs (maybe start with .002 smaller).
         
      • RustyRatRod

        RustyRatRod Weenie idiot loser. FABO Gold Member

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        What's the combination of engine? Cubes, cam, you know those things called details everybody leaves out? Including you. Where's initial timing set? What's the vacuum signal at idle?
         
      • yellow rose

        yellow rose Doctor of Thinkology.

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        Or you can open the idle air bleeds up and not pull the fuel bowl off. Either way.
         
      • rigger3006

        rigger3006 Well-Known Member

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        360, 302 heads, 267/268,214/220,480/489 bullet cam,108 c/l plus 4 degrees adv ground in, air gap, headers, 3000 stall, 355s, 20 initial, all in at 34 with fbo plate, 13 vac at idle, turn in mixture screws and it goes up to 15
         
      • Aaron65

        Aaron65 Well-Known Member

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        There should be 4 idle air bleeds on that carb (right on the top)...they're the outermost bleeds. Doing a quick Google image search will show you where they are. I'd probably open them all up .004 and see where that gets you. If you have pin drills, you can try it that way, but remember...you can't put material back. :)
         
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        • rigger3006

          rigger3006 Well-Known Member

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          ok, new to me, theres 4 over the primaries and 4 over the secondaries. would i just open up the 2 outermost ones over the primaries?
           
        • Aaron65

          Aaron65 Well-Known Member

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          You'd open up all four, because Holley-style carbs feed some idle fuel to the secondaries, even on the 2-corner idle carbs. The idea is to keep the secondary float bowl fuel level refreshed, even for people who never use the secondaries.
           
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          • rigger3006

            rigger3006 Well-Known Member

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            so the primaries are 70s, secondaries are 39s, should i open them up accordingly, or all 4 the same size?
             
          • Aaron65

            Aaron65 Well-Known Member

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            You'd open up the primaries to roughly 74 and the secondaries to 43. Looking at the specs, I'd say this is a Slayer carb, right?
             
          • rigger3006

            rigger3006 Well-Known Member

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            Yes it is
             
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