How To Make an Updraft Carb Straightening Jig

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How To Make an Updraft Carb Straightening Jig

Postby Rudi » Wed Sep 26, 2007 1:43 pm

When Em and I attended Cecil and Priscilla's CubFest Northeast 2006, I was priveledged to not only watch but to assist side by side with Don McCombs in straightening a few IH 3/4" Updraft carbs.

Don is hard at work straightening carburetor castings :!:
Picture courtesy of my friend "Lurker" Carl Kurnas.

After working with his copy of George Willer's jig, I knew that I had to have one. Don graciously let me borrow his Jig so I could make a few before returning it to him. What follows is what transpired in actually making this jig. So right up front I would like to thank Don and my young friend Johnny Stevenson (Jostev) who helped me navigate through AutoCad. Also thanks to Rick (Redman) Neuman for his help (and his Dad Elmer in spirit) and his invaluable skills in photography. Each of the photo's of the Jig Fabrication are courtesy of Rick Neuman, RLN Photography.

How To Make an Updraft Carburetor Upper Body Straightening Jig

First step is to measure and then draw it out.


This was one of the hardest parts of the project as I am a drafting board and head kind of guy, not at all used to Auto-Cad. I learned a lot, but I am missing one key measurement and I will have to work that one out.

Thanks to Phil Lenke, I now have that missing measurement and a new CADD drawing to add to the article.

You can also see the upgrade that was made to my own jig after trying a few different carburetors on the jig. Seems the idle tube is not perfectly aligned so some leeway had to be allowed. By removing the jig material between the venturi and the idle tube hole the problem becomes moot.

Bill of Materials
  • 3/16" x 4.00 x 4.13" Steel Plate
  • 4 ea 1/2"x 10-24 NC Allen Head Cap Screws
  • 8 ea 1/4" Stainless Steel thin Flat Washers
  • Sprayable Contact Cement
  • 1/4" x 4.0 x 4.13" Masonite
  • 1 tub Rapid Tap

Tool List
  • Drill Press and Vise
  • 1/16", 5/32", 3/8" HSS Twist Drills
  • Morse Master Cobalt Hole Saw Arbor Model M34
  • Morse Master Cobalt 15/16" Bi-Metal Hole Saw
  • Morse Master Cobalt 1-1/8" Bi-Metal Hole Saw
  • #10-24 NC Tap

  • Print out a template on your printer of the full scale drawing.
  • Transfer the center locations of all holes to a Masonite template blank. Verify fit by predrilling all holes in the Masonite template and fitting it to the upper Carb Body with sprayable contact cement.
  • Using your drill press and a adjustable vise, begin boring the 4 cap screw mounting holes and the Idle Tube Relief Hole. Start with a 1/16" HSS drill bit pilot hole and then graduate to a 5/32" HSS drill bit for the 10-24NC Tap.

    You will notice that there are actually two steel plate Jig blanks in the vise. This was to ensure a truly flat surface and also to help in transferring the locations to the second blank. This step was repeated for 5 of the 7 jigs that I made.



  • Drill successive holes from 1/16" to 5/32" to 3/8" dia holes to accept the Idle Tube. This will have to be modified as per individual carburetors as the Idle Tube sometimes does not line up for all carburetors.

    The next step is a little more difficult. But proceeds the same way as the previous steps. While you are drilling the 1/16" holes for the Cap Screws and the Idle Tube, you should also drill the centres of the two Carb Body Relief Holes. These will then be upsized in this step. During my project I skipped the pilot hole and went directly to the Morse Hole Saw step. However, the pilot hole is a useful step to utilize.

    Bore the 1-1/8" Relief Hole.

    Bore the 15/16" Relief Hole.

    Surprisingly these Cobalt Hole Saws are pretty nice bits to work with. Nice clean holes, no muss no fuss and no bother.. I like em.. a lot 8) :D Looks good to me.

    Now to Tap the Cap Screw Mounting Holes. Use Rapid Tap or a good Cutting medium to assist the tap.



    Now Test Fit the Carb to the Straightening Jig. Pay attention to clearance on the sides for the Carb Body Relief Holes. You may need a power reamer/rasp to enlarge the size where required.

    Tada :!: All done.... If I can do this in a Cabinet Shop, anyone with a reasonable workshop should be able to make one of these jigs. Do enjoy. Now, we need to talk Don into doing a How To Straighten Your IH 3/4" Updraft Carb on the Jig
:idea: :!: 8) :D
Last edited by Barnyard on Tue Mar 06, 2018 8:41 pm, edited 1 time in total.
Reason: Added Missing Files
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