How to properly drill carb jets?

Fuel and Air Systems

  1. MopaR&D

    MopaR&D Nerd Member FABO Gold Member

    Messages:
    3,852
    Likes Received:
    698
    Joined:
    Jan 4, 2008
    Location:
    Fort Collins, CO
    Local Time:
    8:28 PM
    I have a tuning issue with my 750 Street Demon where the tuning range I need can't really be had with the primary jets and metering rods available from Holley. The primary jets jump from a #82 up to #87 and after lots of messing around with both sizes and all of my metering rods (which are basically all the ones they make, got the full kit) I could really use a #85 jet. I have other primary jets from the tuning kit which I will never use (far too small, like #76-80), is it possible to carefully drill them out and not mess them up? I've heard just using a normal drill bit will leave burrs on the inside surface which can screw up the fuel flow? I would just buy the correct jet if I could but they aren't available and I have at least 2 spare pairs of jets to mess with.

    Basically is there a special drill bit that can drill out carb jets properly?

    EDIT: And no regular Holley or Edelbrock jets won't interchange, only the secondaries use regular Holley jets. Primaries are specific to the Street Demon only.
     
  2. yellow rose

    yellow rose Doctor of Thinkology.

    Messages:
    16,600
    Likes Received:
    12933
    Joined:
    Jun 19, 2015
    Location:
    Living on the razors edge
    Local Time:
    7:28 PM
    Don't drill them. Drills are about as accurate as measuring you cylinder bore with a ruler.

    If you are going to modify an existing jet, you need a reamer. Drill the jet to ~.010 under the size you want and ream it to finish. Only run the reamer through the hole forward, and use only one pass.
     
    • Like Like x 1
    • brian6pac

      brian6pac Well-Known Member

      Messages:
      3,660
      Likes Received:
      2167
      Joined:
      Nov 15, 2016
      Location:
      N.E.Ohio
      Local Time:
      10:28 PM
      I believe it's the length and the diam of the hole not just the diameter.
       
    • MopaR&D

      MopaR&D Nerd Member FABO Gold Member

      Messages:
      3,852
      Likes Received:
      698
      Joined:
      Jan 4, 2008
      Location:
      Fort Collins, CO
      Local Time:
      8:28 PM
      So if I have a jet that's 0.076" I can just ream it to 0.085" and not use a drill at all? Is that something I could do in my garage or should I just perhaps take the jets to a machine shop?
       
    • mderoy340

      mderoy340 Well-Known Member

      Messages:
      2,074
      Likes Received:
      798
      Joined:
      Aug 2, 2012
      Location:
      Melbourne FL
      Local Time:
      10:28 PM
      I drill mine all the time and go in small steps using a hand drill. Diameter and depth determines flow. Using an AFR meter and reading plugs after 1/4 mile WOT works for me. All four jets on my car are different sizes.
       
    • krazykuda

      krazykuda Well-Known Member FABO Gold Member How-To Section Editor

      Messages:
      47,607
      Likes Received:
      12581
      Joined:
      Aug 20, 2007
      Location:
      Orland Park, IL
      Local Time:
      9:28 PM


      They radius the hole on the inlet and outlet to blend it when they make the jets, it's nearly impossible to duplicate that at home in our garage... Best practice is not to drill jets, but buy the proper size from Holley...

      Holley rates the jets after the hole is drilled and blended according to how it flows, not the hole size...
       
      • Agree Agree x 3
      • Demonic

        Demonic Well-Known Member

        Messages:
        459
        Likes Received:
        258
        Joined:
        Mar 21, 2015
        Location:
        Maryland
        Local Time:
        10:28 PM
        • Agree Agree x 2
        • Like Like x 1
        • MopaR&D

          MopaR&D Nerd Member FABO Gold Member

          Messages:
          3,852
          Likes Received:
          698
          Joined:
          Jan 4, 2008
          Location:
          Fort Collins, CO
          Local Time:
          8:28 PM
        • Mtrhead

          Mtrhead Ace of Spades ♠️

          Messages:
          1,202
          Likes Received:
          293
          Joined:
          Sep 19, 2018
          Location:
          S.C.
          Local Time:
          10:28 PM
        • yellow rose

          yellow rose Doctor of Thinkology.

          Messages:
          16,600
          Likes Received:
          12933
          Joined:
          Jun 19, 2015
          Location:
          Living on the razors edge
          Local Time:
          7:28 PM
          You can probably remove .011 with the reamer but be damn careful. Brass is grabby shit and you can stick a reamer, although that small a reamer with that short length of engagement should be doable. Just use a good lube.

          Again, you also need to account for the radius at the entrance and exit of a jet. If you drill out that out, you can have a jet with a bigger hole that flows LESS and you will go wild trying to sort it out.

          In the late 1980's I was working with mechanical FI and alcohol. I was buying jets and nozzles that were said to be flowed. I was at the track and it was hot Hot HOT and the DA was so high the birds were wearing oxygen masks. Anyway, I increased the return jet .010 (on mechanical injection the jet, or pill is in the return line so a bigger jet is LEANER which is opposite of a carb but the principal is the same) and the car went slower. And the plugs looked like the engine was a bit more fat. I said W in TF is going on. Threw another .010 at it and it slowed down more. I said piss on it and went home. If I was so stupid I couldn't pull fuel out and my tuning was slowing the car, I should pack it in.

          So I called a buddy and he hooked me up with a local guy who had a flow bench for fuel injection. I ran down there after work and I watched him flow all 20 of my pills. On 2 or 3 actually flowed as marked. The pills I was using that should have been leaner were in fact, flowing LESS than they should have because the company that made them had ZERO standards. All that junk went in the shitter and I order Kinsler nozzles and jets.

          The moral of the story is drilling jets is a bad idea. BUT, you have to do what you have to do. Actions have consequences. Once you remove materiel it's near impossible to put it back.
           
          • Thanks! Thanks! x 1
          • Agree Agree x 1
          • Krooser

            Krooser Reform School Graduate

            Messages:
            1,229
            Likes Received:
            882
            Joined:
            Jul 19, 2015
            Location:
            Land of Cheese
            Local Time:
            9:28 PM
            Used to be SOP was to drill jets since aftermarket stuff had not been invented.

            I can't imagine Gartlis, Mickey Thompson, The Summers Bros. and thousands of others waiting for Jegs and Summit to be available so they could get their carbs.

            Like YR said...do what you have to do.
             
            • Like Like x 2
            • Agree Agree x 2
            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

              Messages:
              6,222
              Likes Received:
              3446
              Joined:
              Feb 7, 2013
              Location:
              Phila. Pa
              Local Time:
              10:28 PM
              What's the threading on Carter and Eddy main jets?
               
            • mderoy340

              mderoy340 Well-Known Member

              Messages:
              2,074
              Likes Received:
              798
              Joined:
              Aug 2, 2012
              Location:
              Melbourne FL
              Local Time:
              10:28 PM
              Wax works great as a lube. You are metering fuel in a carburetor not a main rocket engine booster. You have multiple unused jets so go at it IMO.
               
              • Agree Agree x 1
              • krazykuda

                krazykuda Well-Known Member FABO Gold Member How-To Section Editor

                Messages:
                47,607
                Likes Received:
                12581
                Joined:
                Aug 20, 2007
                Location:
                Orland Park, IL
                Local Time:
                9:28 PM
                I wouldn't recommend that for sex, hot wax can burn your private parts.... :eek:
                 
                • Like Like x 1
                • Agree Agree x 1
                • straightlinespeed

                  straightlinespeed Sometimes I pretend to be normal Staff Member FABO Gold Member

                  Messages:
                  9,530
                  Likes Received:
                  2781
                  Joined:
                  Oct 21, 2012
                  Location:
                  Oakdale, MN
                  Local Time:
                  9:28 PM
                  I bought a bunch of brass blank plugs from McMaster Carr that fit my Holley carb. Then bought a pin vise and the numbered drill bits to drill it by hand to the sizing I needed. I was able to make any arrangement of jetting I needed at the time. Might look at their site to see if they off brass blanks in that thread pattern and go that route?
                   
                • MopaR&D

                  MopaR&D Nerd Member FABO Gold Member

                  Messages:
                  3,852
                  Likes Received:
                  698
                  Joined:
                  Jan 4, 2008
                  Location:
                  Fort Collins, CO
                  Local Time:
                  8:28 PM
                  I think for now I'll get my fuel system sorted out, I'm starting to feel like my fuel pump is giving out... it is just a parts-store stock replacement mechanical piece. If I do try to modify my carb jets I'll take them to a machinist I know of in town who does more oddball misc. stuff.
                   
                • gzig5

                  gzig5 FABO Gold Member FABO Gold Member

                  Messages:
                  409
                  Likes Received:
                  236
                  Joined:
                  May 3, 2018
                  Location:
                  Mequon, WI
                  Local Time:
                  9:28 PM
                  The pin vise and GOOD QUALITY numbered drills is a good way to go. A good finish can be had from a drill using the right technique and good lube. I'd consider polishing the entrance and exit of the holes with some 600 grit valve compound on a toothpick or formed hardwood dowel when you are done.
                   
                  • Like Like x 1
                  • Mark2205

                    Mark2205 Well-Known Member

                    Messages:
                    97
                    Likes Received:
                    39
                    Joined:
                    Aug 7, 2005
                    Location:
                    Tn
                    Local Time:
                    9:28 PM
                    I’ve been drilling Carter jets for years. If you’re worried about polishing after drilling, just run some string soaked in brass and silver polish back and forth through the jet. Works great.
                     
                    • Like Like x 1
                    • MopaR&D

                      MopaR&D Nerd Member FABO Gold Member

                      Messages:
                      3,852
                      Likes Received:
                      698
                      Joined:
                      Jan 4, 2008
                      Location:
                      Fort Collins, CO
                      Local Time:
                      8:28 PM
                      After doing more driving and testing it turns out my primary part-throttle is way lean to the point of bogging out (can see it with my wideband O2 AFR readout), I thought it was a tad lean but close to good... I just ordered the #87 jets from Summit they were only $6, also a Carter stock replacement pump. I'm more inclined now to add an electric fuel pump at some point too.
                       
                    • 12many

                      12many Well-Known Member

                      Messages:
                      656
                      Likes Received:
                      789
                      Joined:
                      Dec 14, 2016
                      Location:
                      South Carolina
                      Local Time:
                      9:28 PM
                      Somehow missed your thread when it mattered, but I have that same carb that I was using on my 410, had the same deal as you. You mention primary jets sized from 76-82 That is the kit for the 625 carb. You need the kit for the 750 which comes with 87/89/91 primaries, as well as 54-38, 56-40, and 58-44 rods. What rod were you running with the 82's?
                       
                    • MopaR&D

                      MopaR&D Nerd Member FABO Gold Member

                      Messages:
                      3,852
                      Likes Received:
                      698
                      Joined:
                      Jan 4, 2008
                      Location:
                      Fort Collins, CO
                      Local Time:
                      8:28 PM
                      I got the kit for the 625 because I'm at high altitude and it needed to be leaned out from factory. Now I'm running #87 primary jets and 62-54 metering rods, I had to richen up the secondaries "back" to the factory #86 jets and now it's pretty much spot-on. I could possibly even go one step leaner on the primary side, I have 64-56 rods for that.
                       
                    • 12many

                      12many Well-Known Member

                      Messages:
                      656
                      Likes Received:
                      789
                      Joined:
                      Dec 14, 2016
                      Location:
                      South Carolina
                      Local Time:
                      9:28 PM
                      Ahh, well that explains that! I got to a point where I needed to consider drilling bleeds etc for overly rich off idle cruise and either plugging or restricting the secondary bleeds to eliminate a hesitation when going wot quickly. Not really the carb for that sort of tuning. Still a nice carb though.
                       
                    • MopaR&D

                      MopaR&D Nerd Member FABO Gold Member

                      Messages:
                      3,852
                      Likes Received:
                      698
                      Joined:
                      Jan 4, 2008
                      Location:
                      Fort Collins, CO
                      Local Time:
                      8:28 PM
                      Seems like the rich off-idle cruise is just the way these carbs work, lots of people complain about that online. However it always seems to be "I don't like the numbers my AFR gauge is showing but it runs fantastic" so I'm just gonna roll with it. Haven't checked my spark plugs but I'm not too worried about it, others have said their plugs looked fine even though their O2 sensor showed it was rich. Not sure what exactly is going on there but I'm not going to lose sleep over it, also not going to spend any money on another carb, that's going to go towards a full MPFI conversion.

                      Even the owner's manual that comes with the carb says to target a 13.5:1 mixture at cruise using a wideband, seems pretty rich but that's what they say. I have a feeling it's designed to have a lot of "crossover" between the idle and primary circuits to give that excellent response.

                      It would be quicker, faster and more tune-able with a 750 double pumper but I'm not drag racing. I'd rather have a few extra MPG and be able to "set-it-and-forget-it".
                       
                      • Like Like x 1
                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

                        Messages:
                        6,222
                        Likes Received:
                        3446
                        Joined:
                        Feb 7, 2013
                        Location:
                        Phila. Pa
                        Local Time:
                        10:28 PM
                        You're using your noodle. :) Tuning for performance is right. Shooting for a number on a meter only works if every assumption involved is correct. And the fact WBO2 AFR numbers are themselves interpretations dependent on assumptions about the combustion doesn't help. You're right. They're reference tool, not a targeting tool.

                        Or maybe whomever put together the manual didn't really know their stuff.
                        Like this graphic that shows up in several of the Eddy instructions.
                        upload_2019-5-23_17-38-15.png
                        It's just plain wrong.
                        Mislead many of us for many a years.
                        Part throttle acceleration actually should go leaner than cruise at pretty much all speeds.

                        Go back to the text books based on testing and experimentation for the automotive and aircraft industries.
                        A little synopsis from Larew's here:
                        https://www.forabodiesonly.com/mopa...g-to-compare-to.395945/page-3#post-1971911709

                        More here:
                        https://www.tapatalk.com/groups/rac...r-characteristics-at-differen-t421.html#p3408

                        Some links to various papers (getting much more technical):
                        https://www.tapatalk.com/groups/racingfuelsystems/metering-characteristics-of-carburetors-t14.html
                        https://www.tapatalk.com/groups/racingfuelsystems/air-bleed-characteristics-t80.html

                        Carter type calibrations discussed here, including the "economizer" and "bypass air bleed"
                        https://www.tapatalk.com/groups/rac...ry-of-the-carter-idle-restricions--t2055.html
                         
                        Last edited: May 23, 2019
                        • Like Like x 1
                        • Thanks! Thanks! x 1
                        1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
                          By continuing to use this site, you are consenting to our use of cookies.