Weird, sudden AFR issue Proform 750cfm!

Fuel and Air Systems

  1. KnuckleDuster

    KnuckleDuster Well-Known Member

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    Been tuning on this carb all weekend.

    Here is how it is set up right now.
    It is on an Airgap with 1" Edelbrock wood spacer.

    Jets
    Primary - 70
    Secondary - 84
    Power valve - 8.5
    PVRC - .037
    Squirters - 31/31
    Pink cams on #1 holes
    HSABs - 31/31 stock
    IABs - 70/70 stock
    IFRs - 31/31 stock setup in top of channel
    Emulsion holes are all empty.

    All this info is here to save questions, because I want to try to get straight to the problem at hand.

    Yesterday I measured all the restrictions, then changed the .031 air bleeds to .037 to try to clean up the highway cruise that was dipping into the 11's. That's when the problem started. Now at light cruise (20-50 mph) my AFR goes from 13.4 at idle to 10.0!

    I tried different primary jets.

    I changed back to the .031 bleeds.

    I double checked that the 8.5 power valve was sealing, checked two different ways for blown (it tested good), and then tried another 6.5 power valve, just to be sure.

    I changed metering block and bowl gaskets.

    I have NOT adjusted or messed with the accelerator pump arms at any point.

    I have had this carb apart five times this morning looking for a something I missed.

    What would cause it to start doing this suddenly, that none of this would fix??
     
    Last edited: Apr 23, 2019
  2. MobileCustoms

    MobileCustoms Well-Known Member

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    No clue but it might be worth taking the O2 sensor out and recalibrating it to clean air. Just a thought.
     
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    • KnuckleDuster

      KnuckleDuster Well-Known Member

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      It's an AEM meter, I don't think they do that like some of the others. Reads like normal all other times, new 02 sensor, too.
       
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      • Jadaharabi

        Jadaharabi FABO Gold Member FABO Gold Member

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        How does the wood spacer work compaired to aluminum?
         
      • KnuckleDuster

        KnuckleDuster Well-Known Member

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        Makes a world of difference from the aluminum or no spacer. Not sure how much it affects the tune, but you can feel the difference when you touch the carb!
         
      • mderoy340

        mderoy340 Well-Known Member

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        Needle and seat on the float may have crap in it. Same with air bleeds. A .031 to .037 is a huge jump and way out of the norm.
        Move the IFR to the bottom and readjust the idle circuit. 20 to 50 mph you are mainly on the idle circuit. Moving the IFR to the bottom stabilizes fuel flow in the Idle circuit which is where a street car mainly operates. 4 emulsion holes open, what size are the openings? 2 .026 is all you need. If you have 4 hole blocks run #1 & 3 .026 and block off 2 & 4.
        O2 sensors read O2 and can give false readings off what is really happening in a cylinder. As an example, a plug misfire allows more O2 to be seen at the sensor, you think you are lean by the AFR gauge.
         
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        • KnuckleDuster

          KnuckleDuster Well-Known Member

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          I was hoping you might chime in.
          Yeah, had to go get some stuff before I could pull it back apart to blow it out.
          I figured I must have dislodged something checking restrictor sizes, nothing else makes sense, because it was running good and I was getting closer to my target, up until yesterday.

          The emulsion holes are all open and empty as far as I can see, so I guess they are whatever diameter the set screws should be, unless they neck down. Hard to see in a black metering block, and I only have drill bits up to .037 to check with.I will look again after yardwork.
          I have been planning to take care of plugging them like you advised last yearI will, as well as moving the IFR to the bottom.
          The problem is I have been putting together an order with McMaster for the taps and blanks and pin vise, etc., but I'm not sure specifically what I need. (lengths, thread pitches, etc.) I'd like to make sure I get it all in one order.
          I don't have a drill press, so I also need to figure out how to set up to drill them.
          Any chance you could help me with a shopping list?
           
          Last edited: Apr 23, 2019
        • mderoy340

          mderoy340 Well-Known Member

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        • crackedback

          crackedback FABO Gold Member FABO Gold Member

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          Did the .037 IAB bottom out in the opening, so they aren't really .037 anymore?


          I get the 6-32 brass cup point set screws from mcmaster carr. They are about $5 for 50 of them.

          The drill sets are on ebay for cheap most of the time.
           
        • KnuckleDuster

          KnuckleDuster Well-Known Member

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          Not sure I fully understand your question. I used a .037 Quickfuel bleed with a shoulder from a kit, then changed back to the original .031.

          It's just weird to me that I can adjust the mixture screws to get low 13's at idle in gear ( the mixture screws are only about 3/4 turns out) , then by 10 mph it's going rich and by 20mph it's at 10.0 and stays there up to at least 50mph. I didn't drive it very far or get on it to see what it does beyond that.

          I ran late doing yardwork, and couldn't get back to it. I will pull it back apart and blow out all passages after work. Maybe some speck of something is in the passage, partially blocking the air when it starts sucking?
           
          Last edited: Apr 23, 2019
        • mopowers

          mopowers Well-Known Member

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          Sounds like it needs smaller IFRs - maybe .029"?
           
        • KnuckleDuster

          KnuckleDuster Well-Known Member

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          Closest bit I had to check emulsion holes was .027
          Dry, it barely fit, so I would say .028 is correct.

          I pulled, both squirters, needles, and all the bleeds, and blew out every passage in the main body and metering blocks with carb cleaner and then compressed air into it's own spot on a clean blue shop towel. I didn't see any specks, but it did barely discolor the towel on a few passages. I am now putting the carb back on the car for one test drive before I have to call it a day.

          20181101_075650.jpg
           
        • KnuckleDuster

          KnuckleDuster Well-Known Member

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          Same shit, meter goes to 10.0.
          At this point, I don't trust the meter/O2 sensor anymore.
          T-slots are just past square.
          Idle seems happiest about 13.0 in gear at 775 rpm, hardly any rpm drop from park, maybe 50.
          Car drives fine light to light, and burns them in second gear like a champ! As soon as I let off the gas, the AF reading goes back to normal.
          Exhaust smells good, not gassey.
          Pulled the #1 plug and it is sooty, but with all the fiddling these last few days, I think I will have to clean or replace the plugs to get a real read.

          If anybody has any ideas what else to check, I'd love to hear them. I just can't think of any other reasons for it to change so suddenly.
           
          Last edited: Apr 23, 2019
        • crackedback

          crackedback FABO Gold Member FABO Gold Member

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          Fuel dripping from somewhere? Out of the booster? too much fuel pressure, sinking float? Once off Idle, that should be on the IFR.
           
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          • 12many

            12many Well-Known Member

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            Fom all your posts it sounds like the car is running good by your seat of the pants meter, but the gauge is showing numbers that indicate otherwise? I'd disconnect it and go back to old school feel. Just drive it for a while, Short of a passage having an aluminum or brass thread shaving stuck or floating around somewhere, or perhaps an accelerator pump cluster experiencing some pullover, dribble? Keep your hands away from your hair, if you have hair that is! Good luck
             
          • mderoy340

            mderoy340 Well-Known Member

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            Try lowering the float level one flat at a time. IFR in the bottom will help.
             
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              The mixtrure screws just trim a portion of the idle feed. As soon as the throttles crack open, three things happen.
              a) More transfer slot exposure increases making the idle port less relevant.
              b) Less transfer slot is exposed to atmospheric pressure, so less air may be mixing into fuel air mix from the slot above the throttle.
              c) With a hot cam, vacuum under the throttle is increasing. More vacuum will pull harder on the idle circuits. This will increase flow until somewhere around 14"Hg when it chokes. Above that, more vacuum doesn't increase fuel flow. (That's why a stock setup is easier - its always above 14"Hg so doesn't change)


              Either x 4 is way too much.

              Increasing the air bleeds and e-holes can cause unpredictable results. Often increasing bleed pushes more fuel into the mains, especially at start up of flow. Only as the circuit approaches max flow does it typically reduce the proportion of fuel.
              Have you read Tuner's description?
              Post 2, and post 5 shows it graphicly although not extreme.
              Emulsion Tuning

              Sounds good.

              IFR relocation.
              Relocating the Holley Idle Jet

              Some Bits and taps links:
              Holley drill bits, etc.

              Building a Proform/Qft 750

              I usually use a hand held pin vise and place the set screw in a variable speed drill chuck. Go slow but steady. Some swear by nose grease. I use a little WD40 or such. The only set screws you really need to drill will be for the IFRs. The e-holes just plug two.
               
            • mderoy340

              mderoy340 Well-Known Member

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              One additional thing I see is you have a plug over the vacuum advance port. If you are running mechanical advance only on the street you need to get vacuum advance hooked up to dial in part throttle IMO.
               
            • yellow rose

              yellow rose Doctor of Thinkology.

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              Did you leave all 4 emulsion holes open? If so, you don't need all that emulsion. You have to fix first things first. Sounds like you have the T slots correct.

              I think I already mentioned getting the PV to open sooner. Now, you need to block off the number 2 and 4 emulsion holes. That is from the top.

              Again, you don't need all that emulsion.
               
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              • KnuckleDuster

                KnuckleDuster Well-Known Member

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                I am running vacuum advance off the full vacuum port on the front.
                I understand the tuning advice better now, and I am working my way there. Great advice and very much appreciated!
                The thing is, even with all that needing done, I had the fuel curve pretty decent, and then ALL OF THE SUDDEN, something changed!

                Accelerator pump? Too tight or maladjusted or a diaphragm issue, from something I did and didn't notice one of the 20 times I had this carb apart the last few weeks?
                Unfortunately, I have no way to see what if it is dribbling under vacuum while driving. I do feel like this is the most likely culprit.Is there some other way to test it?

                Trash in the carb? I THINK I eliminated that probability yesterday. Did I dislodge something checking restrictor sizes? Could I have been tuning to a blocked passage that is no longer blocked, allowing more fuel now?

                O2 sensor, or meter?
                How would it read accurate at idle, then go wild under flow?


                I will order the blanks and fix the IFR and emulsion. I have a whole other carb to fix after I get this one sorted out, also a Proform 750, but a race model, which brings me to my next question.
                Which rebuild kits are you guys using that come with the more ethanol resistant parts like the accelarator pump diaphragms?

                Thank you all for the input!!
                 
              • KnuckleDuster

                KnuckleDuster Well-Known Member

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                About ready to give up on this carb. I have been through it so many times, and can't figure out why it WAS fine, and now it is rich no matter what I do. I've tried everything I can think to try. Nothing is making sense anymore.
                 
              • MOPAROFFICIAL

                MOPAROFFICIAL Well-Known Member

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                Try what's been said.

                Make sure you didn't lose the check ball or pin that goes under the shooters... you can end up with siphoning... do you have a needle and seat leaking onto the blades at idle?
                 
              • KnuckleDuster

                KnuckleDuster Well-Known Member

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                DING! DING! DING!

                Had to climb up and squat on the inner fender with a light, but I FOUND IT!!!
                Front shooter is leaking around the base!
                 
                Last edited: Apr 26, 2019
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                • KnuckleDuster

                  KnuckleDuster Well-Known Member

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                  Aaaaand now the threads are stripped...fml.
                   
                • MOPAROFFICIAL

                  MOPAROFFICIAL Well-Known Member

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                  Ah man, take it slow...
                  Back when...they sold anti siphon shooters.
                  If you ever lose a ball/pin.. you can still buy them...as you can buy the ball too.
                  After mulling around tour story...it made me think back to my early yrs tuning and leaning holley carburetors.
                   
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