My new carb concerns

Discussion in 'The Venerable Q-Jet' started by Lucy Fair, Aug 9, 2015.

  1. Lucy Fair

    Lucy Fair Nailheadlova

    I am trying to put back carb for my Riv and i wanted to ask guys who now more than me on this subject.
    Engine is 425 Nailhead bored ,030,
    cam is:
    @.050 intake :218,
    @.050 exhaust :224,
    122LSA
    and 108ICL,
    making 18 inchHG (with almost no sealing at valves, i know i should do it first..)
    heads, intake and exhaust port matched and just a little touch on runners.
    Carb is Q-jet 7041305, looks like service replacement for 67 Riviera. Stock setup is as follows:
    jets 73,
    rods 41,
    idle tubes 0,026,
    idle channel restriction 0,040
    idle air bleed 0,046875
    main circuit air bleed 0,0625,
    high speed bleeds 0,040

    So i am puting it all back together and i stopped at idle tubes. New tubes from Cliff are 0,036 in comparison to stock tiny ,026. And here starts my concern if i put there Cliff's ones than what should i do to the rest of carb?

    Thanks in advance for any help on this one.


    Peace,
    Kacper.
     
    Last edited: Oct 24, 2016
  2. carmantx

    carmantx Never Surrender

    I would say use Cliffs idle tubes at .036 so it will idle correct. You will probably need to enlarge your down channel restriction some also to between .046-.050.
    That carb should also have an idle bypass air in the baseplate of about .080.
    Jets at 73 may be a little large, but you will need to get it together and running to see how the combo works out for you.
     
  3. Lucy Fair

    Lucy Fair Nailheadlova

    Tomorrow i will check base plate and carb for idle bypass but i don't think there is any. The only bypass i can think off is that little screw in air horn just above the float.

    Peace, Kacper.
     
  4. Gary Bohannon

    Gary Bohannon Well-Known Member

  5. Lucy Fair

    Lucy Fair Nailheadlova

    As i thought this is an old design so it doesn't have air bypass, but that's not a problem. I can even make it adjustable:).
    I have also measured lower air bleed which is 0,0625 and idle fuel discharge hole which is close 0.085.
    Hopefully Cliff will chime in and maybe he will have some more answers.

    Peace, Kacper.
     
  6. Cliff R

    Cliff R Well-Known Member

    What size are the lower idle airbleeds?

    Upper idle airbleeds? Are they located in the main casting or airhorn?

    What size are the main air bleeds?

    ....Cliff
     
  7. Bigpig455

    Bigpig455 Fastest of the slow....

    I have a similar carb at similar spec (there's a thread out here somewhere) but I like a 43 or 44 rod for it. Cause rods are my solution to everything!

    Seriously, the jet size is probably fine ( Ive got 73s in all my nailhead and 455 Q-Jets) but your rod/ jet ratio is pretty rich..keep us posted.
     
  8. Lucy Fair

    Lucy Fair Nailheadlova

    Lower air bleeds: 0,0625
    Upper air bleeds in Horn: 0,046873
    Main air bleeds: 0,0625

    Thanks, Kacper.
     
  9. Lucy Fair

    Lucy Fair Nailheadlova

    Cliff??
    Is there any hope for me with that carb?:pray:

    Peace, Kacper.
     
  10. Bigpig455

    Bigpig455 Fastest of the slow....

    Bumping this thread, see if it catch's Cliff's eye - I sent you my specs to compare too, and you could also contact Ken at Everyday Performance via PM, he goes by TechG8.

    I can only tell you what I am running in my 66 7041304 Service replacement carb on my 65 nailhead- Upper idle air bleed is .050, the lower is .060, and both main air bleeds are .060. The carb is very quick in all ranges - off idle, part throttle and full throttle with awesome throttle response, but I'm not wild about idle and off idle mixture. I does idle very smooth though, so I haven't tried to fix it. . I also lied about the jets in that carb though - I've got 74's in it.

    I'd consider opening up your upper idle air bleed, slowly and incrementally, and bring your idle channel restriction up to maybe mid .040's - go slow there too. I made too many changes at once to follow Cliff's recipe (which is still overall a great starting point or build target), and I should have made one change at a time.

    Keep us posted......
     
    Last edited: Aug 28, 2015
  11. Cliff R

    Cliff R Well-Known Member

    Sorry, been completely BURIED in work and haven't had a chance to get back here. I need to know the size of the following items:

    UPPER idle airbleed:
    LOWER idle airbleed:
    LOWER main airbleed:
    UPPER main airbleed:

    You have indicated that the upper idle airbleed is in the airhorn, and not above the DCR in the main casting, is this correct?......Cliff
     
  12. Lucy Fair

    Lucy Fair Nailheadlova

    Hi,
    good to hear that You are alive and working and thanks for having some time for us little here who wants to learn.
    Here is what You needed:
    UPPER idle airbleed: 0,047
    LOWER idle airbleed: 0,062
    LOWER main airbleed: 0,039
    UPPER main airbleed: 0,062

    and yes Upper idle airbleed is in the airhorn.

    Peace,
    Kacper.
     
  13. Cliff R

    Cliff R Well-Known Member

    That helps. 73/41 is going to be a tad "fat" in the "normal" driving range, you may want to lean it up some after testing.

    With idle bleeds that small you woln't need real big idle tube, should be fine with .034" or so for that engine/cam combo......Cliff
     
  14. Lucy Fair

    Lucy Fair Nailheadlova

    So i shouldn't alter anything, just enlarge existing idle tubes, i will start at 0.030. What about idle bypass as Mark suggested and idle down channel restriction?

    Peace, Kacper.
     
  15. Bigpig455

    Bigpig455 Fastest of the slow....

    I think you should also try a 43 or 44 rod too - that will take care of the rich condition Cliff describes. Then see if you can tune out any lean stumble with the mixture screws, if not, then start looking at the ICR and idle tubes. With tubes being something that are extremely difficult to go back and change, I'd start a little bigger than .030 - maybe .034 - .036, and you can tune the ultimate idle fuel with the ICR.

    Keep us posted....
     
  16. Lucy Fair

    Lucy Fair Nailheadlova

    Maybe this weekend i will try that carb finally, i think i'll just throw in idle tubes from Cliff. It also happend that i have correct 43 and 44 rods so i can try them out. I am also thinking of adding APT to the baseplate would it be beneficial? Carb also have that little screw in air horn so i can actually enlarge upper air bleeds without drilling them if i understand it correctly?

    Thanks for all the help so far, i'll let you know how it's working,soon, i hope.

    Peace,
     
  17. Bigpig455

    Bigpig455 Fastest of the slow....

    that sounds like a good plan, although I'm not sure about the bypass, my car runs fine without it.

    Just make the changes slowly, and one at a time....yes, you'll be taking the carb off and apart about every ten minutes, but in the end you'll be glad you went slow.
     
  18. Cliff R

    Cliff R Well-Known Member

    Since the carb has small idle bleeds, it will put down a lot of idle fuel without opening things up much. You are also using a pretty small cam for the CID and compression, so the engine is going to make good vacuum at idle, and woln't want or need a lot of idle fuel to the mixture screws, or idle bypass air.

    I'd go a couple thousandths over stock for the idle tubes and larger primary metering rods and see how it works?........Cliff
     
  19. Lucy Fair

    Lucy Fair Nailheadlova

    I just checked 67 manual and carb which this one was replacement for had 70 jets with 37 rods...and 71 jets with 40 rods with A.I.R.. So maybe i should put 71 jets with 41 rods as i bought this carb with and leave stock idle tubes with ,026 oirfice ?Rest of the internals are really small when i compare them to let's say first recipe from Cliff's book.
    That 73 jets have to be my mistake, i had to mix some stuff from other carb...:Dou:.
    I remember that when i have put ,036 idle tubes in my original 66' q-jet it went reallly rich. Engine is making 18 inches of vacuum.

    So many little things of big importance:Smarty:...

    I have also question about power piston spring, i have a set from Cliff and there are 4 springs, blue, green, red and black. Which one would be best for 18''hg?
    Thanks,
     
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    From Greg Gessler's site,

    http://quadrajetparts.com/quadrajet...ing-assortment-color-coded-springs-p-210.html

    "You want to use the strongest spring possible that will NOT begin opening at idle.

    Spring.... Inches of Vac....Length
    Yellow:........(2-7)...........0.950"
    Silver:.........(4-9)...........1.020"
    Black:......... (5-9)...........0.950"
    Blue:...........(8-12)..........1.450"
    Note:
    Inches of Vacuum are to be used for estimates only. Condition of power piston and bore can affect and change inches of vacuum."
     

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