Rich idle is killing me

Fuel and Air Systems

  1. 91r/t

    91r/t FBBO Gold Member FBBO Gold Member

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    Ok all, I have a 500" stroker I just finished up this spring that has edelbrock RPM heads and intake, 10.6:1 compression, and a quick fuel SQ-850 mechanical secondary carb on it. The carb comes with 70 IAB from factory and a 4.5 PV. I am all square on the transition slots so it shouldn't be pulling fuel from there. I called holley and they told me not to mess with the IAB jets in the carb and just to use the Idle screw to adjust. This seemed a bit odd to me but whatever. They also said that maybe I should go down a couple sizes on the PV to ensure it isn't dragging fuel in from that circuit. I have about 9-9.5 in/mg of vacuum at idle so I was thinking my PV at 4.5 was ok but? I am having issues getting this thing to idle and not be pig rich. I am out on the idle screw 3/4 out from fully closed. If I lean it out any more, it idles and starts hard like its getting towards the lean side. Any other tricks to use before I say screw it and just run it as it. My timing is at 28 initial and 34 total as well so it has plenty of timing at idle I would say. I was thinking of maybe going to a 72 or 73 IAB and trying to calm down the idle circuit that way. HEMI-ITIS, LEWTOT, a few others hope to see you chime in with some ideas. I am fair proficient with the Holley's, but this is getting the best of me. Thanks in advance.
     
    Last edited: Jun 14, 2019
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    • lewtot184

      lewtot184 Well-Known Member

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      with that low of vacuum it tells me there's a bunch of valve overlap and that may let some of the intake charge escape out the exhaust at low rpm. not much of a fix for that but hopefully your not into the "burn your eyes" fumes. what i'd do is look at the idle jet. see if there's a small brass jet/set screw at the top inside of the metering blocks. whatever the diameter of this jet is double that size and use that as a starting point for the idle air bleed. double check float levels and fuel pressure; and check to make sure you don't have a soggy float.
       
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      • 91r/t

        91r/t FBBO Gold Member FBBO Gold Member

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        Shit, yea forgot cam specs.....was just going to type them. 244 @ .050 and .539 lift. Floats are a little less than half the sight glass which seems to be their happy place. I will pull the idle jet out tonight and see what the heck the size is for that brass jet.
         
        Last edited: Jun 14, 2019
      • ddpsnake

        ddpsnake FBBO Gold Member FBBO Gold Member

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        are you in gear when checking vacuum?
         
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        • 91r/t

          91r/t FBBO Gold Member FBBO Gold Member

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          4 speed.
          :thumbsup:

          If I was in gear with a stick I'd run myself over hahaha!
          :D
           
          Last edited: Jun 14, 2019
        • vance.dykes

          vance.dykes Well-Known Member

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          have you tried smaller main jets ? a 3.5 power valve might help and check the power valve to see if they are ruptured ?
           
        • 91r/t

          91r/t FBBO Gold Member FBBO Gold Member

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          The plugs look good running down the street so the main jetting is dialed in.....Plus the mains should not be pulling fuel in with the transition slots set correctly and being at idle. I have also swapped 4.5 power valves with a new one to ensure it wasn't ruptured. It will stall when I screw the idle screws in all the way so it can't be feeding from the power valve circuit at all I wouldn't think. That was just a suggestion from Holley to lower the number to a 3 or 3.5. Usually if it is pulling from the mains or power valve, it will keep running with the idle screws all the way in as it isn't running on the actual idle circuit.
           
        • ddpsnake

          ddpsnake FBBO Gold Member FBBO Gold Member

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          try to adjust the rear transition slots just past the slots,to clean her up.maybe that cam is a tad too big,for idle,we all know our engines love it it past 3 grand super clean than.what kind of breather ya running.
           
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          • 91r/t

            91r/t FBBO Gold Member FBBO Gold Member

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            What do you think of maybe dropping a jet size or two on the IFR bleed jet inside the metering block vs. messing with the IAB on top. That may take a bit of fuel out of the idle circuit and clean up my idle right?
             
          • 91r/t

            91r/t FBBO Gold Member FBBO Gold Member

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            Both sets of transitions are set to obtain an idle. Both are square. Air cleaner? 68 round unsilenced style. Breather is standard on one side and PCV on other.
             
          • ddpsnake

            ddpsnake FBBO Gold Member FBBO Gold Member

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            i still say open the rear a tad it will let some more air in
             
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            • 91r/t

              91r/t FBBO Gold Member FBBO Gold Member

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              I've also got the throttle plates drilled to help with that as well. There is NO way this engine will run without the plates drilled so before anyone says you ruined the carb the fact is it isn't ruined haha! Big cam equals big air needed. I will crack em a little more but then I am idling at 1100ish RPM's so we shall see. I still think if I drop a size or two in the IFR, I may help me problem out by restricting fuel allowed on the idle circuit. Up'ing the IAB jet only adds air to the equation and lowers it a bit, but I think that IFR jet may be my ticket. That is if my metering plate has jets there and not just predrilled holes. I need to check that out tonight.
               
            • tmaleck

              tmaleck FBBO Gold Member FBBO Gold Member

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              David Vizard has a book on Holley carbs. I also have 40+ years working and carbs and still learned a lot from this book. Lots of theory and detail.
               
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              • 91r/t

                91r/t FBBO Gold Member FBBO Gold Member

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                ....I appreciate the info.
                :thumbsup:
                 
                Last edited by a moderator: Jun 14, 2019
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                • vance.dykes

                  vance.dykes Well-Known Member

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                  you are right the holley books that are out there will give you a great head start on how and what works. this is the one i started with.
                  Super Tuning Holley Carburetors
                  by Alexander Walordy
                   
                • lewtot184

                  lewtot184 Well-Known Member

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                  i'd measure what the IFR is first; probably .036"-.038". I did notice in an older QF catalog a Q850 with .070" IAB's, but seen a 850-2 with .073" IAB. sometimes the dash numbers indicates a revision. .073" would be an easier change if you have them. I thought you might have been running a bigger cam than that. what type of ignition? my internet is out at home so reply's may not be too timely.
                   
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                  • 91r/t

                    91r/t FBBO Gold Member FBBO Gold Member

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                    I will measure those tonight or tomorrow in the AM at work. I have 74 air bleeds, but I didn't see a huge difference with those. I was hoping the IFR downsize of a jet size or 2 would maybe regulate the actual fuel coming in better instead of trying to mix more air with it. Holley website says the IAB is a 70 stock. This carb is maybe a year old or so. It is a SQ-850 and not a Q-850. Street model vs. Drag race model. Thinking pretty similar carbs by the spec sheet side by side. Just shooting some ideas out to those who have played with this setup before and had some success.

                    :thumbsup:
                     
                    Last edited: Jun 14, 2019
                  • Bigal75

                    Bigal75 Well-Known Member

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                    Check your vacuum when driving in high gear on a flat stretch. Then choose your PV.

                    And yes, install smaller idle restrictors! Your IAB is good. Also don't worry too much about squaring the transfer slot on the secondaries.
                     
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                    • hunt2elk

                      hunt2elk FBBO Gold Member FBBO Gold Member

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                      I would try downsizing the IFR's a couple of sizes first. I have had better luck with changing idle A/F with the IFR's than the IAB's. IAB's seem to affect everything whereas IAB's concentrate on just idle.
                       
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                      • Bigal75

                        Bigal75 Well-Known Member

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                        Try 0.035 to 0.40 for the idle restrictors or smaller if needed. Also do you have restrictors installed for the t-slots? It may or may not need them.
                         
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