Trying to understand idle slots and main circuit

Discussion in 'The Venerable Q-Jet' started by guyver002, Jul 10, 2018.

  1. guyver002

    guyver002 Well-Known Member

    Been reading a lot on the Quadrajet as Im going to roll up the sleeves and try to tune mine. I needed a little help understanding where all the fuel comes from up to part throttle.

    So blades closed only my idle discharge hole (hole controlled by mixture screws) should be my only source of fuel ?

    Then confusion kinda starts

    As blade opens the off idle slot now provides fuel but is the idle mixture hole still providing fuel?

    And as i continue to open blades now the lower idle air bleed provides even more fuel but are the lower 2 still also providing fuel?

    Then i believe past this point the main system provides fuel through the venturies but are all the idle slots still providing fuel?

    Just tryin to wrap my head around what provides fuel or doesnt to zero in on tuning the carb.
  2. techg8

    techg8 The BS GS

    At idle you want a small square of the slot exposed.

    And yes everything that provides fuel continues to provide fuel.

    They just add fuel one on top of the other.
  3. carmantx

    carmantx Never Surrender

    Idle fuel starts in well under jets, comes up through idle tube restriction then across to down channel restriction. Upper idle air bleed adds air to fuel at that point, then more air is blended when it passes to bottom of down channel before getting to base plate mixture screws.
    You idle air bleeds are bringing air in to mix with fuel, not letting fuel out.
  4. Cliff R

    Cliff R Well-Known Member

    With few exceptions some transfer slot will be exposed at idle so some idle fuel comes from them, and why you may not be able to completely kill off the engine with the mixture screws, but you should be able to slow it down with them showing control of the idle A/F ratio.

    The transfer slots are key players in transition from idle to the main primary system. The LIAB's are placed high enough they are really not players there, at least not until you get into the throttle pretty heavy and uncover them.

    The Q-jet (and TQ's) have an excellent venturi/booster arrangement that provides multiple areas to increase signal to the jets and also to very effectively atomize fuel pulled from the nozzles. The main airbleeds play a pretty big role in how sensitive and how early fuel will flow from the main system, as does fuel level in the bowl and jet size vs metering rod diameter in the jets.

    It's all a very well designed system and why these carbs work so well and are still seeing widespread use today despite the availability of "modern" technology, ie fuel injection and electronic TB systems.

    What cripples the Q-jet is than all but a very few were emission calibrated and delivered on "wimpy" low compression engines with tons of quench and retarded cam timing, not to mention bogged down with CAT's, air pumps, EGR and other devices to retard/control ignition timing to get them to look clean out the tail pipes.

    Once we get past all that and get the idle/off idle system up to par things really start to come around for using the Q-jet for high performance use. I don't care what they tell you anyplace, once you get one dialed in exactly for what you are doing there isn't any improvements waiting for you anyplace going to something else.

    I use a plain old 1977 Pontiac Q-jet on my 455, makes over 550hp/600tq and is flawless and has been for many years. I've had it back to back on the dyno and at the track against every other type of carb out there and nothing will out run it in either place. Some of that testing is documented in HPP magazine and the Popular Hot Rodding "Engine Masters" editions so I can't come on here and mislead folks about it.

    The cool part is that the old/ugly Q-jet has never had a grinder or sanding roll touch it, just a slight recalibration to the idle system and a few simply modifications that are outlined in my book.

    As far as the tuning the idle, off idle and main system, like anything else it's a "recipe" used depending on what the engine is sitting on. Very few engines these days are still "stock" or even close to it, so you've basically got to look at the engine parameters, which carb number you are using, how it is set-up, then dial in each area exactly for what you are doing. I'm pretty good at that sort of thing these days, and know just about every single part number out there and how they are set-up by the factory, and what they would need done to them for any particular application.

    These days in lieu of building carburetors I spend most of the day putting together custom rebuild/tuning kits for them and at times it's overwhelming as to how busy we get with that sort of thing. I've hired some good folks to do the shop work so we are still offering rebuild/restoration and custom tuning services.

    The only bad news here is that I'm considering getting out of the restoration business. WAY too time consuming, materials are extremely expensive, and there is plenty for us to do in other areas......Cliff
  5. guyver002

    guyver002 Well-Known Member

    Thanks for the responses guys! I feel pretty good about where I need to look to address my carb's issue. WOT is 12.3 to 12.5 so i feel the jetting and secondary rods are pretty spot on specially combined with what I have learned with my research on the lean at cruise and little idle mixture control. I'll try some of those screw in idle tubes to see how she reacts. This all because of some overheating issues ive been battling and now that ive methodically checked and rechecked ive come to the carb tuning with how lean it is.
  6. guyver002

    guyver002 Well-Known Member

    So I still have to get some idle tubes ordered but I realized I ran aground because I don't know what size is normally in my carb. It is a 7040240 that is all stock but now on my 455 that a previous owner warmed over. It has long tube headers with a strait though exhaust (Dynomax ultra flows) 2 1/2 inch, an Edelbrock dual plain performer and from what I can tell when peering in with a bore scope some flat top pistons with a raised edge on one side with some valve reliefs and a cam that revs the motor between 4500 to 5000 rpm. What size Idle tubes did this carb come with stock? When I take AFR readings with the idle screws screwed waaay out it is idling at 16 AFR and screws almost in is 16.5. What size should I step up to ? 1 size, 2 sizes 3 sizes more? Lately someone told me that I should drill out the the idle screw orifices to the max size that the screws still seat at and then drill the passage the same size as that that leads to it (not the upper part but the lower part where the carb body combines with the lower butterfly plate). Is that also something I should consider to get a non stock 455's q jet some control over it's idle?
  7. carmantx

    carmantx Never Surrender

    Idle tubes were originally about .032 and down channel restrictions about .049.
    You could go to .035” idle tubes and possibly to .055” dcr’s for richer idle.
    You can go to .090 on idle screw holes, they are probably .086 or so stock. but you need to make the other changes for richer idle calibrations.
  8. fine threads

    fine threads Member

    Another option is you can reuse your old idle tubes just solder the tips closed and then drill them to whatever size you like.
    Last edited: Aug 26, 2018
    HotRodRivi likes this.

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