jb78L-82

I am not a fan of the Qjet (750 CFM and Some later 800 CFM ones) but replaced my OEM L-82 Qjet in 1985 with a Holley 4175 650 CFM vacuum secondary Replacement Qjet carb in 1985. I still use this same Holley 4175 today on the rebuilt L-82 355. I have always been confused when folks on the forum say the Quadrajet is NOT a vacuum secondary carb with the secondaries only providing air and fuel on demand, plus the Qjets other big advantage is that the Qjet has very small primaries and BIG secondaries which gives it enhanced throttle response and fuel economy?

To the latter point, my Holley 4175 650 CFM vacuum secondary carb also has VERY small primaries and BIG secondaries that only operate on demand by the engine so I remain confused to this day why folks say the Qjet is different than other vacuum secondary carbs like my Holley 4175 650 spreadbore and has a fuel economy/throttle response advantage with small primaries?

To the primary point, The Qjet is controlled by vacuum on the secondary side BUT the vacuum effects an airhorn metering the fuel and air into the engine? Unless I am missing something here, my Holley 4175 uses vacuum to move a diaphram which moves the big secondaries on demand as well which controls the air/fuel metering? What am I missing here or are both carbs truly using vacuum to meter air and fuel and we are splitting hairs with semantics that on one the vacuum effects an air horn and the other the vacuum moves a diaphram? Neither carb can open the secondaries under no load just operating the throttle....both require sufficient airflow and load to move the secondaries.

Here are 2 independent sources about the Qjet that BOTH clearly call the Qjet a vacuum secondary carb?:

Wikipedia:

"Most Quadrajets use a vacuum operated piston to move the primary metering rods to control the air-fuel ratio, allowing the mixture to be lean under low load conditions and rich during high load conditions.
​​​​​​Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage; the primaries open before the secondaries, and use on-demand air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. Thus the Quadrajet acts like a vacuum-secondary carburetor and only delivers more fuel as it is needed."

Hemmings Motor News:

First released by Rochester in 1965 and used extensively in GM cars from late 1967 on, The Quadrajet was the evolution of the earlier Rochester 4GC carburetor; however, it also incorporated ideas from the other manufacturers, such as the spread-bore design and vacuum secondaries. They are stingy on gas at lower speeds, which makes them more fuel efficient than their competition, but their vacuum secondary circuit can give your car the added performance boost it needs, once they are engaged." The smaller primary bores give the carburetor better idling and low-load fuel economy. The larger secondary bores are regulated by tapered metering rods which set the air to fuel mixture.

It seems that vacuum has a role in both carbs secondaries with the Holley 4175 using a more simplistic approach and the Qjet using metering rods versus jets/metering plates on the secondary side to control air/fuel mixture. The bottom line both use vacuum in some respect to control the secondaries. What Am I missing here? I am trying to finally convince myself that a Qjet is NOT a vacuum secondary influenced carb.

Both carbs use vacuum in a different way but it appears that vacuum controls the secondary circuit to some degree in both? Trying to learn something here.

cooper9811

The way I think about it is that the secondary throttle plate is mechanically actuated. The distinction I make is that the secondary throttle plate is mechanically opened via the linkage to the primaries, and the air valve just helps manage he transition into the secondaries - but it does not actuate the secondaries.

From observation on my old QJet, when the primary throttle is about 1/3 of the way open, the rod/linkage between primary and secondary throttles starts push the secondary open - this action is completely driven by your foot on the gas pedal.

The air valve above the throttle plate simply opens as vacuum disappears during acceleration (which is what I believe leads people to call the QJet a vacuum secondary carb).

So I view the QJet as a mechanical secondary carb that uses the drop in vacuum to manage a smooth transition from cruise to WOT.

Jim

jim2527

Base plate throttle blades are mechanical. Upper flaps do NOT open from vacuum. There's a vacuum line attached to the rod but that's kinda like a release lever that allows the upper flaps to open mechanically.

At idle vacuum pulls on the upper rod which 'locks' the upper flap closed. Increase engine speed and as vacuum drops the upper rod loses vacuum an 'unlocks' the upper flaps. As air demand increases that same demand air applies 'air pressure' on the flaps. If air pressure on the flaps exceeds the spring pressure that keep the flaps closed then the flaps will open. As the flaps open the secondary metering rods are raised which in turn increases fuel .

There is NO vacuum source attached to the secondary's. Its impossible to test them with a Mityvac type device.

jb78L-82

Ok That Helps Jim...Thank You.

So on the Qjet the secondary butterflies are mechanically opened by the throttle position, which is not the case on my Holley 4175, and the the vacuum operates the air valve. As the vacuum drops, like on a traditional vacuum secondary carb, the drop in vacuum causes the air valve to meter gas through the carb unlike the Holley 4175 spreadbore 650 in which the vacuum drop actually opens the secondaries on the carb and the airflow pulls fuel to mix with the air?

Pretty much what I was thinking that both are influenced by vacuum in the secondary circuit but in a different way. Looks like there are some semnatics in play here since both carbs like I stated control the secondaries with vacuum so saying a Qjet are not vacuum secondaries, technically is true since the secondaries butterflies do not open with vacuum BUT the fuel metering IS influenced by the carbs vacuum state.

jim2527

I edited my post. Qjet vacuum drop 'unlocks' the upper flaps. Air flow open the flaps.

jb78L-82

Ok I get that the secondary plates are mechanical BUT the air valve uses vacuum to meter the gas which is a vacuum component to the secondary circuit. The Qjet is using vacuum to operate/meter gas through the secondary circuit, just not the operation of the secondary butterflies.

What I have so far is that the Qjet has mechanical secondaries BUT uses vacuum to control the air valve which meters the gas based on vacuum and the 4175 Holley uses vacuum to open the secondary butterflies which pulls gas through the carb by gravity in the case of the 4175.

Either way there seems some semantics here since both use vacuum to operate the secondary circuits (not the butterflies per say only). What appears to have happened here over the years is that Holley managed to BRAND the concept of a vacuum secondary carb to mean the vacuum operating the secondary butterflies when in reality a Qjet is using vacuum to operate the secondary circuits as well but ONLY the gas metering function, not the secondary throttle plate operation.

As for the gas saving function/crisp throttle response of the Qjet using small primaries, I knew, that advantage was a fallacy 35 years ago when i replaced the Qjet with the small primary 4175 Holley with big secondaries.

cooper9811

Jim 2527 is right in correcting me that there is no actual vacuum source pulling the air valve open - It's a change in pressure that opens the air valve, allowing more air into the secondaries.

While I used the word "vacuum" it might be more accurate to say "pressure differential" on either side of the air valve, but that may be an exercise in semantics. Maybe we'll get Lars to better explain...

jb78L-82

ugh...so a vacuum source allows the upper flaps to open..either way the control is vacuum operated on the qjet. What about the air valve that meters the fuel? Is that controlled by the vacuum drop OR the amount of air through the secondaries after the flaps open?

I thnk what I have now is that the Qjet does NOT have mechanical secondaries but the upper closed flaps are allowed to open when the vacuum drops enough to allow the lock to release the flaps to open. The subsequent airflow through the secondary barrels allows the air valve to meter gas based on the volume of air or on the vacuum level at that point?

jim2527

Correct. Upper flaps are offset on their pivot. Air flow hitting the edge of the flap provides leverage to over come the spring pressure which keeps them closed.

jim2527

https://i.imgur.com/KYkBngA.mp4

https://i.imgur.com/JFiHxqs.mp4

vince vette 2

That's exactly it.

From Cliff Ruggles book "When the primary shaft reaches about 2/3 of its full-open position, the secondary throttle plates plates begin to open and achieve full-open position at the same time as the as the primary side." That is strictly a mechanical operation. The vacuum "control" comes manages the air flaps which sit above the throttle plates. Again, from Ruggles book, "Located in the air horn are two air flaps on a common shaft. They control all of the air that is allowed to enter the carburetor whenever the mechanical secondary throttle plates in the base plate open." ... "The air flap is used to provide control of the air that enters the carburetor. Its progressive operation ensures smooth engine operation during full-throttle application. The outer portion of the linkage is connected to choke pull-off, assisting the spring to ensure the flap does not 'whip' open too quickly. The choke pull-off holds the secondary air flaps tightly shut during high-vacuum situations and allows the flaps to open when the vacuum falls off at full throttle."

So basically the vacuum has no direct role in the secondary throttle plate movement, but it does control when the air flaps above them open to let air get into the secondary half of the carb so the throttle plates can do their thing. The drop in vacuum results in the choke pull-off freeing up the air flap linkage so it can open and the the spring on the air flap linkage regulates how quickly they open to avoid the engine going lean which would occur if those massive secondaries were free to dump in all they air they could be sufficient fuel starts flowing into the secondaries.

jb78L-82

Ok guys but there is a vacuum control component to the secondary circuit operation, the opening of the upper flaps on the secondary side. Once the flaps open and allow air to flow, the pressure differential control the amount of gas meter by the air valve. Good making progress here.

Either way, though, if you remove the vacuum source from the Qjet, there would be no secondary circuit operation on a Qjet, just like if I disconnect the vacuum hose from the 4175 secondary diaphram, no secondary operation, as well.

The Qjet does rely and have vacuum control of the secondaries BUT the gas metering is not controlled by the vacuum. HOWEVER, on the Holley 4175 the secondary plates only are controlled by the vacuum, the actual gas metering is gravity based on airflow through the secondaries.

This discussion seems to indicate there are definitely some semantics involved here and the fact the Holley BRANDED the vacuum secondary operation term, a long time ago. From I what understand so far, the Qjet definitely has vacuum control of the secondary operation BUT it is different than the Holley. I am not sure it is accurate to say a qjet is not vacuum operated as well, certainly to some degree.

jb78L-82

Very good...Understand now..the bottom secondary throttle plates ARE Mechanically operated BUT the top upper flaps are vacuum operated. In a way though, it does not matter what the bottom plates are doing if the uppers are not open..seems redundant and not necessary..overly complicated?

Either way vacuum is controlling the secondaries on the qjet ...back to the holley BRANDing of a vacuum secondary carb......Am I the only one questioning the terminology here?

cooper9811

I think of it more as a refinement that complements the mechanical secondary action, in order to control incoming air and prevent poor performance like bogs.

Conceptually not unlike using vacuum-controlled timing advance (in addition to your initial/mechanical advance) to get better overall street performance and mileage....

jim2527

Think of the vacuum as being a 'lock'. High vacuum = locked Low vacuum = unlocked. Vacuum only locks, unlocks. Disconnect the vacuum and secondaries will always be 'unlocked'. Flaps only open from air flow. Gas metering is controlled by the rate of opening, height opening and rod size/shape.

Rate: Loosen the spring and the flaps will open with very little pressure differential. Tighten the spring too much and they'll need a huge pressure differential
Cam: Theres a 'cam' under the hanger which control progressivness
Height: Rods are attached to hanger. Hanger shape determines how high/far the rods are pulled up
Size/shape: Thickness of rods, taper, tip length. Thick rods = more lean. Thin rods = more rich.

Clock my links above. I cant get the videos in embed.

derekderek

also, the vacuum that opens the holley is venturi vacuum, not manifold vacuum. it is a pressure differential caused by airflow thru the venturis-just like the q-jet. but the q uses the actual airflow, not pressure drop pulling a diaphragm. but the end result is the same. except the q also varies the fuel available to the secondaries. as does the AFB.

jim2527

No, NOT vacuum operated. Vacuum controls when they're allowed to open. Correct, bottom plates can be fully open while the top plates are fully closed/non-functional.

jb78L-82

Ok guys helpful..everyone! Appreciate your answers.

So vacuum determines when the flaps can open but does not open them mechanically and venturi vacuum for the Holley versus manifold vacuum for the Qjet.

However, without vacuum the holley 4175 and Qjet will not operate or will operate incorrectly. A mechanical secondary Holley or any mechanical carb has no dependency on vacuum.

Both the Holley 4175 and the Qjet are both controlled/dependent on vacuum for proper operation one more than the other but the fact remains the qjet needs vacuum for proper operation, just different vacuum. The Qjet is probs better/more accurate at automatic fuel metering and the holley is more crude/jet dependent for fuel metering.

Either way, I do think Holley branded the vacuum secondary carb concept to their advantage since the Qjet must have vacuum to operate correctly as well to control the secondary circuit..its different but the qjet is controlled/allowed/dependent on vacuum as well to operate correctly. It is a vacuum carb to some degree, JUST NOT how a holley vacuum secondary carb operates.

stingr69

Just a semantic argument.

Holleys are easy to describe one way or the other. On a vacuum secondary Holley, the vacuum is fully committed to the secondary opening process. On a mechanical secondary Holley, vacuum is not involved in the secondary opening process.

Qjet secondaries "involve" vacuum in the secondary metering process but not to the same way that it is used by Holley.

If it makes any difference you could say Qjets are a "hybrid".

jim2527

Qjet secondarys will operate without Vacuum. But NOT correctly.