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 Idle downchannel restriction 
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
I have noticed that the idle downchannel restriction size in all my Qjets (1971,72,76,77,82.85) is always .046". I suspect it has to do with the length and diameter of the idle channel and the amount of vacuum signal it provides on the idle tube. Am I anywhere near correct? Also, did the earlier pre-emission Qjets have that same .046 size idcr or were they smaller?


Sat Mar 29, 2014 1:42 pm
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Joined: Wed Oct 23, 2013 8:01 am
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I suspect you are measuring the idle feed restriction, that has a tube approx. 13/4 inches long & that's the ID of the tube. The restriction on that is at the bottom & is smaller. The down channel usually start out at .05 or bigger.


Thu Apr 10, 2014 7:09 am
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.046” to .048” Idle Down Channel Restrictor (IDCR) is common in many Chevy carbs but not all QJets are so. Several less common calibrations range from .040” to .052” or more, however the larger size is uncommon in OE carbs, but sadly, way, way, way too common in custom “Stage XYZ” carbs which also incorporate a grossly too large idle jet and so combined with the too-large IDCR are grossly over-rich in the low speed circuit.

The idle circuit in a QJet feeds all the time, unlike a Holley in which the idle circuit reverses at large throttle opening and high air flow. Because the idle circuit discharges continually, if the idle circuit is not carefully tuned to avoid unnecessarily overlapping the main in the throttle opening range larger than the point at which the main begins to discharge, the fuel consumption will be significantly more than necessary in the low HP range of level ground cruising and moderate acceleration metered by the QJet’s primary.

From 1965 to 1970 emission control engine modifications focused on reducing CO and UHC. In comparison to pre-emissions carbs, the general effect on carburetion was every year engines were tuned leaner at idle and part-throttle until by 1970 they were as lean as engines would tolerate and in many cases the WOT was leaner also. Leaner AFR accomplished compliance with those early low CO smog regulations until it became obvious Nitrogen Oxides were the major cause of smog. At that time it became obvious lean AFR, in particular AFR leaner than stoichiometric (14.7/1 or 1λ), could not be tolerated because excess Oxygen remaining after the HC combustion is complete produces NOx.

The result of this is carb calibrations were changed so AFR in cruise and moderate acceleration stayed on the rich side of 1λ. This is why mileage went in the crapper for cars and trucks in the 70’s and the phrase you heard often about PU trucks was, “It gets the same terrible mileage loaded or empty, towing a trailer or not.”

Old school, pre-smog, carb calibrations were basically: (A) as rich as gave the best slowest smooth idle (12.5/1?), (B) then above idle as lean as would not misfire and gave least fuel consumption at part-throttle (12.5 ~16.5?), and (C) increasing leaner as load is increased (12.5~17+?), (D) up to the load percentage where very high or full power is desired, (E) at which point it swings rich to the maximum power AFR or however rich it needs to be to protect the engine from damage (back to 12.5/1?).

This is the carb behavior described in these graphs below.

Image
Image

I remove the pressed-in brass cup Idle Down Channel Restrictor and use an 8-32 brass setscrew to make the IDCR adjustable. Warming the carb body with hot water and using a .110” drill bit with a twist and pull motion will usually remove the little brass cup without destroying it. You can use the cups for restrictors in the 1/8” main well bleeds in the 1970 and later Chevy QJets.

If the 1st (upper) idle air bleed is entering the bore at a downward angle right there above the IDCR, I plug that bleed orifice with lead and move the 1st IAB to the lid by drilling through the vertical passage cast in the lid above the idle tube and IDCR. That is the location of the IAB in several early carbs and marine carbs. Drill with a .104” or .106” bit and tap for 6-32. Using a 6-32 brass setscrew provides an easily accessible adjustable idle air bleed which substantially simplifies tuning the low speed cruising AFR.

The range of 1st IAB size for a .026” to .031” idle jet (in the idle tube) will be between .040” and .052” (usually).

The second idle air bleed, (or idle down channel air bleed, below the IDCR, down in the main body bore wall), is best at .063” max. Some 2nd IAB’s are large in later carbs and can be closed up some by peening or swaging with a 3/16” ball or flat tipped punch of the correct size and then qualifying the hole with a 1/16” drill bit.

The situation just is what it is with the different length and location T-slots and different size curb idle discharge ports. You can enlarge the curb idle ports if necessary.

Sometimes, on the carbs (mostly the very late emissions units) which have excessive T-slot below the closed blade, I bend the half of the throttle blade forward of the throttle shaft downward some so it closes off more of the T-slot. After they are bent re-fitting the blades to the bores by loosening the screws and filing and scraping the edges of the blade and bore wall is usually necessary to achieve good fit.

Pinching back the QJet idle circuit in this way will improve the mileage considerably. About all a QJet idle circuit needs to do is idle and provide enough fuel for another couple hundred RPM above idle because the main starts so early due to the double boosted venturi.

The earlier (60’s and 70’s) carbs are easier to work with because they don’t have the fine threads and long taper mixture screws like the carbs produced near the end of production.


Attachments:
QJet Down Channel Restrictor_1.jpg
QJet Down Channel Restrictor_1.jpg [ 34.4 KiB | Viewed 5099 times ]
Q Jet Idle Air Bleed.jpg
Q Jet Idle Air Bleed.jpg [ 53.68 KiB | Viewed 5124 times ]
Thu Apr 10, 2014 5:25 pm
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77cruiser wrote:
I suspect you are measuring the idle feed restriction, that has a tube approx. 13/4 inches long & that's the ID of the tube. The restriction on that is at the bottom & is smaller. The down channel usually start out at .05 or bigger.


Most original carbs were .048” or smaller, not very many were larger. However, many rebuilders and ‘custom carb builders’ drill the down channel restrictor larger, I guess because Doug Roe suggested doing so with a picture in his book “Rochester Carburetors”, in spite of the clear admonition in the text that a tiny change in the IDCR size has a large significance in terms of fuel consumption. Doug Roe was a Rochester carb engineer for GM so he likely understood what he published.

Now, another book about modifying QJets has several recipes for Stage 1, Stage 2, Stage XYZ, all which advise drilling the IDCR grossly over-size, and now it is common to see them grossly over-rich, fouling plugs and getting terrible mileage.

This is a crying shame because the QJet’s double-booster primary venturi will vaporize fuel so thoroughly most engines can be tuned to run extremely well significantly leaner at low power load than with other carbs, brand H or brand C (except perhaps a T-Quad which has the same venturi type).

I find it hard to believe any of the people who are hacking the idle circuit modifications I see in the mail-order carbs I repair have ever driven a car with the carbs they sell people, because if they did they would know the error of their ways. The mail-order QJets from every source I have seen lately have the idle circuit modified like the popular book (not Doug Roe’s) and on a WBO2 they cruise richer than 10/1 in some cases. This is sad because a QJet can cruise at 16/1 and accelerate at part-throttle leaner than 17/1 on nearly any engine, even with radical valve timing.


Thu Apr 10, 2014 6:13 pm
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
Tuner,
Thank you very much for those replies. It answered a lot of my questions. A couple more if you will...
1- I noticed in the pic that the idle tubes are recessed some below the casting. About how far down max can they be? Reason I'm asking is that I am trying to make my own screw-in idle tubes.
2- In tuning Qjet idle for a mild performance application( 226-236@050 cam 11" vac idle in drive) should I start with ij, dcr, iab about the middle of the range and then depending on results go either up or down in size or should I start towards the larger sizes?
Yeah, I admit I fell for the "Drill baby Drill" methodology from that book. Learned my lessons the hard way. You and Mark sure have enlightened a lot of us. Thanks!


Sun Apr 13, 2014 10:12 am
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Joined: Fri Feb 08, 2013 2:37 pm
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Papastoy wrote:
Tuner,
Thank you very much for those replies. It answered a lot of my questions. A couple more if you will...
1- I noticed in the pic that the idle tubes are recessed some below the casting. About how far down max can they be? Reason I'm asking is that I am trying to make my own screw-in idle tubes.
2- In tuning Qjet idle for a mild performance application( 226-236@050 cam 11" vac idle in drive) should I start with ij, dcr, iab about the middle of the range and then depending on results go either up or down in size or should I start towards the larger sizes?
Yeah, I admit I fell for the "Drill baby Drill" methodology from that book. Learned my lessons the hard way. You and Mark sure have enlightened a lot of us. Thanks!

The IJ and IDCR depend on what size the 2nd idle bleed/discharge hole is in the idle down-channel in the body, and the length and location of the T-slot. I like that lower bleed/feed to not be larger than .1/16”. If the T-slot is low, extends a lot below the butterfly, you can bend the butterfly down on the forward half but you will need to loosen the screws to reposition the butterflies and the probably will need to file and scrape to fit if you do that.

I would use .029” or .031” IJ with .043” to 046” IDCR and start with .040” 1st IAB. Move the IAB to the lid like in the picture above. You drill from the bottom side of the lid. There is a hole die-cast in the lid extending vertically from the kidney shaped area over the idle tube and IDCR. Use a .104” or .106” drill and follow the existing hole, it will come out in the right place to tap from the top with 6-32, as shown in the picture above.


Attachments:
QJet Idle Circuit IJ & IDCR Influence_1.jpg
QJet Idle Circuit IJ & IDCR Influence_1.jpg [ 44.72 KiB | Viewed 4937 times ]
Sun Apr 13, 2014 9:27 pm
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Joined: Wed Oct 23, 2013 8:01 am
Posts: 113
I might try that calibration. I have a hyd roller with similar specs going in a 421.
Thanks for the tips Tuner!! :thumbr:


Mon Apr 14, 2014 2:08 pm
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
That chart explains why the carb I modified was so thirsty! :( Old man winter has paid us an unwanted return visit so its off to the shop. Thanks again for all the help :thumbl:


Mon Apr 14, 2014 4:10 pm
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Joined: Tue May 06, 2014 5:34 pm
Posts: 44
Tuner, I can see that you made some custom "screw-in" IDCRs out of brass set-screws, I love the idea and intend to do the same for ease of tuneability, but does it matter that it's higher (closer to the top) than the original brass cups ?

Also, any suggestion for making idle tubes smaller when you can't find the size you want ? Would shutting off with epoxy and then drilling be a good way to do it ? If so, would JB Weld be a good choice ?

Thing is, I'd like to change the recipe in a late model quadrajet (1979) I built for a friend, following a recipe from a popular book which called for larger idle tubes and IDCRs...I'm so glad I found this forum/info. I was directed here by jmarkaudio from the yellow bullet forums.

Frank.


Thu May 08, 2014 6:22 am
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Joined: Fri Feb 08, 2013 2:37 pm
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Solder the tip of the idle tube and drill a new orifice. Use one of the new lead-free solders.

Plug the 1st (upper) IAB (the one just above the IDCR entering into the venturi bore at a downward angle) with a bit of lead before you tap the threads for the setscrew which is the new IDCR. I use lead wire commonly used to make “pencil lead” fishing sinkers, available locally in 1/8”, 3/16” and ¼” diameters. For this I start with a 1/8” diameter piece and squeeze the end of it between the smooth flat part of needle nose pliar jaws while twirling between thumb and fingers to gradually mash it to a smaller diameter that fits the air bleed I want to plug. Squeeze and rotate a little each time, repeat, etc. until it just fits through the bleed and protrudes through into the venturi bore, cut it off and squeeze it with needle nose pliar tips to expand the lead to secure it in the hole. Squeeze with one pliar tip in the venturi bore and the other in the idle down channel so as to expand the lead in the bleed hole. Rotate the tapered tip of the closed pliers, or something else tapered appropriately, in the down-channel to swage the lead to conform to the down-channel diameter, which is already the correct size to tap for 8-32 thread. That way the threads are cut in the lead and the setscrew supports the lead plug. Tap just enough thread depth that the set screw is flush when installed.


Thu May 08, 2014 11:33 am
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