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 Q Jet Secondary 
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Here are some suggestions for secondary modifications.

In actual fact, the QJet secondary is a “Compound Nozzle” carburetor. The patents for the compound nozzle were issued to Zenith Carburetor Co. over 100 years ago.

This link is to a copy of a page in an old Zenith Carburetor Co. handbook which describes the operation of this type of carburetor metering.

http://www.oldengine.org/members/diesel ... enith2.htm

The “compound nozzle” principle combines parallel fuel circuits, a Main Jet circuit and a Compensation Jet circuit, each with different fuel flow vs. air flow characteristics, so the combined flows provide the desired A/F at each end of the extreme range of venturi air flow, from low to high engine speed.

As applied in the QJet secondary, the Main Jet is obviously the metering rod controlled orifice.

The Compensation Jet (Comp Jet) is the orifice feeding the well(s) at the rear of the float bowl.

The Capacity Jet (Cap Jet) is the discharge orifice in the forward wall of the secondary throat or the tube added (in the pictures below) to discharge the fuel in the center of the air stream between the forward side of the air valve and air valve shaft.

Because the well fed from the bowl by the Comp Jet has a 3/16” vent to atmospheric pressure (inside the air cleaner) the venturi vacuum has no effect on the fuel flow from the bowl into the well and through the Cap Jet discharge nozzle. The flow from the bowl through the Comp Jet is by gravity only, therefore the flow through the Comp Jet circuit is constant after the small quantity of fuel standing at float level in the well is discharged through the Cap Jet when the secondary is opened.

The rate of discharge of the initial volume is controlled by the size of the Cap Jet discharge nozzle.

Because the fuel flow into the well is constant, the A/F provided by this circuit is progressively leaner with increasing air flow, compensating for the (potentially) increasingly richer A/F of the Main Jet circuit.

Because a QJet secondary main circuit has an air bleed and can have more or less air bleed correction of the A/F, combining the main with air bleed correction with the comp jet provides the QJet secondary control of A/F over a very wide range of air flow, much more than is possible with other carbs.

Drill 4 x .024” holes in each tube. Drill through both sides of the tube at 90°. For the main well tube, solder the large - originally upper - end of the tube closed and install the tube with the .026” restriction at the top.

In conjunction with inverting and adding bleed holes to the main well bleed tubes, enlarge the secondary main well with a #1 drill. Stop when you feel it hit the bottom. It should be about 1.250” to where the drill point stops. Be careful with this because it is possible to break through into the secondary throat.

The total amount of fuel which is standing in both the Main and Comp Jet wells before the secondary is actuated is discharged when the secondary is initially opened. That volume of fuel provides acceleration transient enrichment and after it is discharged the following continuous fuel flow is metered by the metering rod and metering rod jet and the .040” (or thereabout) Comp Jet orifice feeding into the Comp Jet well.

A 6-32 brass setscrew can be installed in the location of the Comp Jet so the size may be adjusted as necessary. However, in conjunction with the other modifications described here, most generally .040" is a good choice for the Comp Jet and larger than .043" will make the early part of secondary opening too rich.

It is real easy to make a QJet too rich in the initial part of the secondary opening (act as if it has too much secondary accelerator pump) if you get greedy with the diameter of the main well, or the Comp Jet (the feed from the bowl to the Comp Jet well), or the Cap Jet (which is the discharge tube for the Comp Jet circuit added to the secondary throat front wall).

#1 drill bit for the Main Well, .040 Comp Jet, and for the Cap Jet, .093” OD brass tubing which is no larger than .067” ID is a combination which seems to work well in most applications.

You will find the metering rod will not need to be as small (rich) at the tip and it can be larger (leaner) in the upper 1/3 to ½ of the taper, so the hanger will not need to set the rod so high in the jet with the air door closed. The M through V hangers will be more often used than the A through L which are usually necessary with the “normal” QJet secondary without these mods.

These secondary modifications eliminate the need for the vacuum diaphragm and link to dampen the opening rate of the air door.

Also, the air door spring will not need to be as tight. Some engines will be happy with only 1/4 turn.

These fuel circuit modifications, when combined with the secondary lever and linkage modifications described below, make a QJet feel like a very large single barrel. There is no sensation of even having a secondary, no lag of any kind and very progressive control of the engine power.

Image


Plug the original Cap Jet discharge hole in the bore wall by driving lead in the vertical passage in the carb lid after you remove the 1/8" diameter brass tube and qualify the hole above the tube up to the discharge hole with a .096” drill bit so a .093” flat tip punch won’t get stuck.

The lead wire available in rolls for making weights for fishing tackle is handy for this. Squeeze it between the smooth inner jaws of needle nose pliers to make a length of it small enough diameter to fit in the hole.

Use a long enough piece of lead it will block the hole in the bore wall. Drive the lead in the .096" hole using a .093" flat-tip punch. If the lead is dead soft, some will extrude out the discharge hole.

To drill the holes for the discharge tubes, use a center punch to make a drill starter dimple in the casting at the location in the pictures. With the lid inverted, align the punch with it nearly or just resting on the air valve shaft. The starter dimple will hold the 1/16” bit to make the initial hole.

With the lid inverted, drill with the bit only very slightly above the air valve shaft. Use some lube, WD40 or such.

Drill the starter hole with a 1/16” bit. So the hole will be a press fit for the .093” brass tube, finish the hole with .089” #43 bit. Chamfer the hole so the tube will start in easily.

The holes can be drilled by hand with an electric drill motor using a regular small hand pin-vice to hold the drill bits and act as an extension. Use some lube like WD40 or similar.

After the hole is drilled for the discharge tube, use suitable sized bits in a pin vice to clean up the lead and where the .089” drill bit entered .125” vertical passage for the siphon tube. Take care not to upset the press fit of the siphon tube. Use smaller drill bits to scrape away any burrs remaining from the lead plug and the drilling.

A driver can be made with a pilot stem for support inside the tubing and a square shoulder to push the tube using a nail or length of 1/8" or 3/16” steel or brass rod. Use a lathe, or a file and a drill motor, to make a 1/16” pilot on the tip, or whatever fits well in the tube used for the nozzle.

The 3/32” (.09375") brass tubing “Cap Jet” discharge nozzles for the “Compensation” acceleration or ‘tip-in’ circuit are installed just below the air valve. Larger tubing than 3/32" will make the initial tip-in part of the secondary opening too rich (for gas) on most engines. The tubes are above the flap in this picture of the upside-down lid.


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Mon May 26, 2014 6:03 pm
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Secondary throttle lever and linkage.

The object of this modification is to make a three-point pick-up of the secondary throttle, and to open the secondary earlier in the primary travel and proportion the motion range of the secondary over a larger percentage of the primary throttle lever motion.

Some early QJets have a secondary lever and linkage that does this. This is just a way to make it with existing parts which are already on the carb you have.

When the large QJet secondary throttle is opened a large percentage of the primary air flow is diverted to the secondary, resulting in a large change in primary metering signal. If the change is large enough and the A/F metering is close to the edge of the functional range the result can be a sag or stumble, the infamous “Quadra-bog”.

Modifying the lever and linkage like this makes it so the primary is not as far open when the secondary begins to open. This reduces the severity of influence the flow change has on primary metering. Because the A/F is not disrupted or changed as much the overall carb operation throughout the throttle range is much smoother.

This linkage deal with the modifications to the tubes shown in the post above this one are the keys to the QJet acting like a very large single barrel with no sensation of primary-secondary, just as smooth increase of power as the throttle is opened.


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Q Jet secondary lever 1st step.jpg
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Q Jet secondary lever 3rd step.jpg
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Mon May 26, 2014 8:06 pm
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Before disassembling the linkage, scribe a line over the top of the large outer spring from above the shaft to where it latches under the hook on the throttle lever. Use a small round file to make an elongated slot following the scribed line. A 5/32” chain saw chain sharpening file is ideal for this.

The roll pin or split pin used for the intermediate pickup point is the one that comes in most QJet carb kits for the accelerator pump lever pivot. The hole drilled in the lever for the pin needs to be very close to the rear edge and near the location in the picture, near the mid-point between the upper pickup point and the slot.

To achieve correct operation it is necessary to adjust as required by bending the secondary link and the actuation tangs on the primary throttle lever.

Another example of the secondary lever.


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Secondary Lever.jpg
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Mon May 26, 2014 9:08 pm
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Joined: Tue May 06, 2014 5:34 pm
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Excellent information. I'm wondering, does this apply to lower powered street cars as well ? I'm talking 325 hp range street cars. What is the metering size on those tubes ? (cap jet discharge nozzles).


Tue May 27, 2014 8:46 pm
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frank350 wrote:
Excellent information. I'm wondering, does this apply to lower powered street cars as well ? I'm talking 325 hp range street cars. What is the metering size on those tubes ? (cap jet discharge nozzles).

I added some more text to the first post. I will be adding and editing text in this thread as I add more pictures.

Like a many people surely have, when Doug Roe's QJet book came out in the early 70's I started with the secondary modifications in it and just kept tinkering. I've been doing close variations of this scheme detailed above for nearly 25 years and it works very well on every engine so far, from a 4 cylinder Iron Duke, 250 cu.in. 6 cylinder Chevys, 502 BBC crate motors, Goodwrench crate motors of all variations, Olds, Pontiac, SB Fords from 302 to 427, etc. etc. ......

No matter the motor, they all need their individual appetite satisfied as far as idle circuit, main jet, rod and spring, secondary rod and hanger, etc. but this secondary set up is universal. In other words, the same basic secondary mods but tune the carb to each engine.

The single common denominator in the feedback from users is how smooth and progressive the throttle feel is with this setup when opening or pedaling the throttle. This makes a QJet into a superior circle track and road racing carb. The secondary linkage modification is a very important part of this.


Tue May 27, 2014 11:11 pm
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Joined: Wed Apr 24, 2013 10:03 pm
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Tuner, I'm trying to get a better understanding of your text, "flow through the Comp Jet circuit is constant after the initial emptying of the well".

As I understand it (drag race application) when the secondary throttle plates open the air rushing by the compensation jet restriction opening, near the secondary air valve causes fuel to flow and thereby begins emptying the compensation well. The velocity near the compensation jet opening will go from a high velocity (as the secondary air valve begins to open) to a somewhat lower velocity (as the air valve becomes wide open).

Let's assume that the compensation well drains to a level very near the bottom of the compensation tube, which sits in the well.

The well begins to fill at a rate that is allowed by the size of the orifice (0.40" - 0.042" ??) separating the main well from the compensation well.

Here is where I am unclear about what happens and my observations.

When I take and empty fuel bowl and suddenly fill the bowl, it takes about 7 or 8 seconds for the fuel to completely fill the compensation well.

With this in mind, I can't grasp how the fuel can meter at a constant A/F ratio through this circuit unless the orifice (0.040") was opened up to a very large value.

Also, as the manifold vacuum changes throughout the RPM range the signal to the compensation jet restriction changes.

Any help will be appreciated.


Thu May 29, 2014 5:56 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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Did you watch this video?

http://www.ryanbrownracing.com/video/Qjet_dyno_vid.wmv

.....and read this....? http://www.oldengine.org/members/diesel ... enith2.htm


Thu May 29, 2014 6:00 pm
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Joined: Wed Apr 24, 2013 10:03 pm
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I watched the video and read the Zenith document.

I have still not convinced myself that the there is constant flow through the compensation discharge hole.

Please view the crude sketches below and comment on which of the bottom row sketches you believe actually occurs or if all three can occur (which is what I believe).


Image


Thanks again for your helpful insight.


Fri May 30, 2014 4:37 am
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Greenlight wrote:
I watched the video and read the Zenith document.

I have still not convinced myself that the there is constant flow through the compensation discharge hole.

Please view the crude sketches below and comment on which of the bottom row sketches you believe actually occurs or if all three can occur (which is what I believe).


Image


Thanks again for your helpful insight.


Quote:
I have still not convinced myself that the there is constant flow through the compensation discharge hole.

You may as well just believe it, because it is so.

The Compensation Circuit discharge orifice, whether a hole in the bore wall or a nozzle (like the tube added in the pictures above), is the “Capacity Jet” or “Cap Jet”. I guess they old boys called it that because the size of the Cap Jet determines how rapidly the capacity of the well is initially discharged as the circuit starts, when the static, or standing at rest, volume of fuel in the well is drawn down to the bottom of the tube.

The capacity of the well certainly does add a brief enrichment but the circuit continues to flow all the time the secondary is in operation.

If there is some confusion about the semantics of “constant” and “continuous” I apologize and I will edit for clarity or a better choice of words.

Nice pics. Do another with the fuel level in the Comp Jet Well at the bottom of the Cap Jet tube when the secondary is open, because that is where it is after the initial volume is discharged and the flow has become only that which can flow by gravity through the Comp Jet.

At 1 g horizontal acceleration, the vertical g and horizontal g are both 1 and the liquid angle is found by arcTan x Accel g/g = angle, so 1/1 x arcTan = 45°.


Fri May 30, 2014 7:40 pm
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Not as nice as Tuner's job, but.


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Thu Jun 05, 2014 8:21 am
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