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  • Carb Tuning- whats first?

    I have an old truck with a Quickfuel carb on it. I also have an AEM wideband that just got installed. The engine is running rich (11.8-12.5:1) by just glancing at the readout while driving, but I have no numbers written down to correlate to RPM or vacuum. I want to tune the carb to the best performance I can get both on the track and off. So what should I work on first? I have it idling at 1100 RPM fairly decently, but I am not sure what I should work on first. This truck will be on Drag Week this year (hopefully). However, I have not done any track testing yet either. So- should I work on road manners or track? And what circuit should I attack first? What is the strategy that you use?
    Why think when you can be doing something fruitful?

  • #2

    Re: Carb Tuning- whats first?

    heh....that's what my holleys read when I'm driving down the road....and it's running perfectly.

    I'm eagerly awaiting some answers to you question.
    My fabulous web page

    "If it don't go, chrome it!" --Stroker McGurk

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    • #3

      Re: Carb Tuning- whats first?

      for a big engine, those numbers are normal.

      I do the same for boxers...until low rpm.
      the big engine needs pretty fat all the time.
      Previously boxer3main
      the death rate and fairy tales cannot kill the nature left behind.

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      • #4

        Re: Carb Tuning- whats first?

        Make sure you have no exhaust or vacuum leaks. Adjust valves (and make sure you don't have any burnt ones, especially if you did find an exhaust leak). Set timing (sounds like you may have to set idle speed and then check timing again after you get your idle circuit dialed in). After you know all of that's good then it's generally pretty straightforward.

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        • #5

          Re: Carb Tuning- whats first?

          The engine is in good shape- no leaks, all adjustments are good, just wanting to fine tune the carb for a little better street fuel economy ( ha! From a 489 BBC with a 950 DP!)

          My reading has led me to believe that I should get the idle mixture set first, then move on to the rest. I have watched some Holley videos and read about setting the idle mixture. However, my idle mixture screws have no affect. Originally, I have adjusted the curb idle screw to get the RPM at idle. Has this negated the effect of the idle mixture screws by revealing too much of the idle transfer slot? What are the alternatives to the curb idle screw in an effort to achieving the desired idle RPM? I did read a suggestion to close the primaries and open the secondaries slightly to get more air thru- any thoughts? Should I even be worried about the idle mixture and idle transfer slot since the engine will not work below 1100 RPM?
          Why think when you can be doing something fruitful?

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          • #6

            Re: Carb Tuning- whats first?

            If it idles ok (does't cough or die when you put it in gear, or goose it a little) then you probably don't need to worry about the idle. holley carbs have a flaky idle system as far as I can tell, as long as it runs ok don't worry about it.

            You can lean out the main jets on the primary side and see if it drops bumps up the AFR, but watch that it doesn't lean it too much when you open the throttle partway under load (like going to pass someone on the highway).
            My fabulous web page

            "If it don't go, chrome it!" --Stroker McGurk

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            • #7

              Re: Carb Tuning- whats first?

              If you can't kill it by screwing in the idle mixture screws, then it is idling on the transition circuit. The transfer slots are probably overexposed. They should just appear as a "square" below the throttle plates.

              What carb is it?

              If it needs more air to idle, some guys drill the throttle plates and others install a small ball valve into a CENTRAL vacuum port as a controlled vacuum leak to meter a little more air.



              Last edited by BBR; June 19th, 2013, 12:11 PM.
              Life is short. Be a do'er and not a shoulda done'er.
              1983 Mustang GT 302/T5 http://www.bangshift.com/forum/forum...485-bbr-s-83gt
              1973 F-250 BBF Turbo Truck http://www.bangshift.com/forum/forum...uck-conversion
              1986 Ford Ranger EFI 545/C6 https://bangshift.com/forum/forum/ba...tooth-and-nail

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              • #8

                Re: Carb Tuning- whats first?

                Make sure you have a hot enough plug so you can adjust the carb and not be having issues because you cant keep it clean
                2007 SBN/A Drag Week Winner & First only SBN/A Car in the 9's Till 2012
                First to run in the .90s .80s and .70's in SBN/A
                2012 SSBN/A Drag Week Winner First in the 9.60's/ 9.67 @ 139 1.42 60'
                2013 SSBN/A Drag Week, Lets quit sand bagging, and let it rip!

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                • #9

                  Re: Carb Tuning- whats first?

                  The carb is a QuickFuel Race Q950

                  http://www.quickfueltechnology.com/c...q-950-cfm.html

                  It has screw in idle air bleeds, can these be changed to make the idle mixture screws functional or am I missing their purpose?

                  I will have to check on the plugs.
                  Why think when you can be doing something fruitful?

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                  • #10

                    Re: Carb Tuning- whats first?

                    No. I'd square the transition slots (primary & secondary), set the mixture screws 1.5 turns from fully seated and see if it will idle. It might want more initial timing instead of more curb idle screw. I ended up just locking mine out at 32 because it wanted to much initial. If those transition slots are too exposed, the idle mixture screws will not really do anything.

                    Here's a fantastic forum for carb related stuff. Sign up and pick their brains. They have helped me tremendously. http://racingfuelsystems.myfunforum....forum.php?f=25
                    Life is short. Be a do'er and not a shoulda done'er.
                    1983 Mustang GT 302/T5 http://www.bangshift.com/forum/forum...485-bbr-s-83gt
                    1973 F-250 BBF Turbo Truck http://www.bangshift.com/forum/forum...uck-conversion
                    1986 Ford Ranger EFI 545/C6 https://bangshift.com/forum/forum/ba...tooth-and-nail

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                    • #11

                      Re: Carb Tuning- whats first?

                      I am also curious about this as I have the same carb on mine and need help tuning. It idles with an afr closer to 14. Since I changed the intake, it has been backfiring when I let off the throttle. Am I safe to assume I need to start off by adding more jet in the primaries to give it more fuel?

                      Other than the wisdom of our experienced tuners on here, can anyone recommend reading material on this subject?
                      Rich

                      Drag Week Survivor 2009, 2011, 2012, 2013 - 2nd Place - Pro Street N/A, 2017

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                      • #12

                        Re: Carb Tuning- whats first?

                        First thing is float levels. If the float is too high you will run rich all through the RPM range. If its too low you get a bog because the accel shot doesnt keep it running long enough for the fuel to flow to the boosters. A good place to start finding what works is just under the sight gauge, and I usually drop it from there and see what it does. Sometimes it is higher, sometimes lower, but it has a large effect on how the carb works. Everything else will be a bandaid trying to fix the float level if you dont have that right. Where the carb wants it depends on the vacuum the engine produces, sometimes its higher, sometimes lower, you have to find out by trial and error.

                        Next is timing and idle speed. You want the timing to be where it makes best power, and then you set the idle speed to where it idles good and doesnt overheat. Some trial and error works here too. When you change lots of components, like cams, heads, intakes etc, it moves things around a bit. You dont want the transfer slots exposed if you can help it. If you do it will run rich. Only really large cams need holes in the throttle blades.

                        Since you have an O2, you can see where the curve is real well. That allows you to jet up or down, play with the pump shot, Holleys and E carbs are really easy, setting up a Qjet to run good all over takes more time.

                        really though, start with basics, float and idle speed, then move on to the rest of it.

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                        • #13

                          Re: Carb Tuning- whats first?

                          At 1100ish rpm it isn't idling, fuel is coming thru your main wells at that point. With those holes in the throttle blades you won't be able to get it to idle because the holes are 4 vacuum leaks. Depending on the cam and etc i would experiment (i haven't tried this but in theory...) and replace the secondary blades with solid ones of the same thickness and see if that will give you your idle adjustment screw back to you so's you can actually set the idle speed and idle richness.
                          If you replace all 4 blades you'll definately regain your idle adjustment but the throttleblade angle might tip the blades a tad high into the transfer slot - the way around that is to open the secondaries to the tip of the transfer slot is exposed and that will let you roll the primaries down a little.
                          This is just dealing with the idle, nothing else.
                          Don't mess with the bleed screws etc, in every carb i have 'sorted' out after the guy 'adjusted it myself!' all i had to do is restore it to the factory settings and then make a minor adjustment. I have got carbs in my shop that are over 100years old and the holes in them are the same as the holes in your carb, they figured this stuff out ages ago and nobody has changed the physics of how fuel burns since then.
                          A Carter Carb Shop, sales and service

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                          • #14

                            Re: Carb Tuning- whats first?

                            I got out last night and did a couple of things. The fuel pressure is 6.5psi at the carb. The fuel is halfway up the windows in the bowls (per Quick Fuel). I pulled the carb and adjusted the blades so that the transition slot is exposed .02-.04" front and back. I did get it back on but that is as far as I got. I plan on trying changing the timing per BBR since that is easy first, then look at changes to the carb.

                            A little more info on the carb and engine: the carb came with holes in the front blades from QF. The engine is 489, 10:1 compression, hyd. roller cam- .625" lift @ 250 deg intake at .050" lift, .605" @ 261 deg exhaust at .050" lift. LSA=113 degrees. This is a nitrous cam custom from Bullet.

                            Thanks for the help so far.
                            Why think when you can be doing something fruitful?

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                            • #15

                              Re: Carb Tuning- whats first?

                              Im using a 950 proform redone because the idle was bad does it have the billet base on it... had biggs carb shop put a holley base on and all is good now... Quick fuel I hear is a proform and a Chinese holley knockoff... most carb shops wont work on them pro system, AED told me they wont... I would also get the initial timing up to 20 to 24 and total around 35
                              Drag week 2009 Quickest street rod
                              Drag week 2010 Quickest street rod

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