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 Idle downchannel restriction 
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Joined: Tue May 06, 2014 5:34 pm
Posts: 44
Thank you very much, Tuner !

On another note, I've read thru some of your posts, that the idle system continues to play a role while the main system is at work...how long ? Do I understand they are working in parallel ? If that is so, I'm guessing the main system does a way better job of atomizing ? Now about the idle system working AT PART THROTTLE, is it discharging thru the lower idle air bleed only, or thru the transfer slot and idle screw discharge hole as well ? Do I understand that in a perfect world, the idle system would shut off once the boosters get enough signal to work, so that the only fuel available to the engine would be of better "quality", as in better atomizing ?

I'm sorry if I'm a bothering you with all my questions, I've been a huge fan of this carb for so long and can't stop trying to learn more about what I think is a masterpiece of engineering. I thought I was in heaven when I got my hands on the popular book that came out a few years ago but I now honestly think it's laking some important info and I'm so happy I was directed here, you seem to be the guy to go to regarding the Q-jet. Have you thought about writing a book ?


Fri May 09, 2014 6:44 am
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
Just wanted to report back on the results of the idle circuit mods.
The carbs' idle and off idle response have far exceeded my expectations. Gas mileage has also improved. Overall carb operation is much smoother too. My BBC has never ran this good! Haven't done the secondary mods yet other than re-locating the cap jets from above the air door to below and increasing their size from .052 to .055. A BIG THANKS to Tuner for his help and sharing his knowledge!


Sat Jun 07, 2014 6:51 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
You're welcome, thanks for the report. Exactly what mods did you do? What size upper and lower bleeds and restrictor orifices, IJ, IDCR, etc. did you use? Did you plug the upper IAB in the throat and move it to the lid with 6-32? Did you need to or did you try different size bleeds?


Mon Jun 09, 2014 6:39 pm
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
Plugged the upper idle air bleed in the throat with lead. Drilled and tapped the lid for a 6-32 brass set screws. Used a old accelerator pump checkball to reduce the size of the lower idle air bleeds to .0635 from .073. I also found and removed screws blocking the idle bypass holes in the main body. Opened the idle screw hole from .0625 to .081. Removed the IDCRs which measured .052. I suspect the previous owner did that too. Tapped the IDCR holes for 8-32 brass set screws. New IDCRs are now .0465. Reduced the Idle jet tubes to .031 from .033 by soldering the ends and re-drilling. Drilled and tapped both upper and lower main air bleeds for set screws. Both are currently at .052. Started with your suggestion of .040 upper idle air bleeds. Thought it might have been a little rich so I installed a set drilled to .042. Idle got a little rougher so I then tried a set of .041. That's where I'm at now. Great Idle quality. Started with .072 main jets and .044K primary rods. Ended up at .070 main jets. Plugs look real good!


Mon Jun 09, 2014 7:10 pm
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