Rescue Rogers

The QJet is a mechanical 4 barrel carburetor. The upper air valve can be locked shut buy the choke. Disregard the vacuum operation of the of the air valve or secondaries. It isnt pertinent to the operation of the carb when warm on the older non Pollution contolled elctric versions, I'm not talking about those models.

The air flow over the vavle opens the air flap. It is an on demand system. The more air the engine needs, the more the valve opens, and more air is allowed into the secondaries, as the valve opens it raises the secondary needles mechanically. there are small fuel channels in the walls of the secondaries that work off idle and part throttle air flow that creates a low pressure across the opening pulling out fuel until the secondary fuel needles are raised and the metering is contolled mechanically.

Now we talk about Quadrabog because it is connected to this subject. I did alot of research on this and it is mainly a problem of the air valve being improperly adjusted, PROVIDED that the needles and jet are correct. The secondary air flap opens too quickly creating a fluttering of the valve or a full openning and a sudden lean situation as the fuel tries to catch up.

slowing the openning of the valve is controlled by a screw that adjusts friction on the valve. You can adjust out the bog by incrementally tightening this screw.

SO....mechanical secondaries, mechanical locks, mechanical controlled air valve.
Vacuum controlled/ heat controlled choke. The choke has a vacuum pull off that works with the heat and unlocks and opens the choke. The choke opening unlocks the secondaires mechanically at the bottom and on the air vavle

jb78L-82

Thanks Gents....Very helpful to me understanding the Qjet!

I appreciate everyone's input. I think I have landed on a Qjet is dependent and requires vacuum to operate correctly but it NOT a spreadbore 650 Vacuum secondary to the the degree as my Holley 4175. I still do think that saying a Qjet is NOT a vacuum secondary carb is not quite accurate, either. The Qjet is a different tpye of vacuum secondary dependent carb from the traditional Holley but they are related like a second cousin..............

jb78L-82

Understand everything you described but it seems you are omitting that the upper flaps are controlled/dictated by vacuum. They do not just open dependent on airflow, according to what others stated, irregardless of vacuum. Decreased vacuum is a requirement for the upper flaps to be unlocked....That is a type of vacuum control, just like on my 4175, but different process. It is precisely this vacuum involvement step that seems to indicate in my original post that hemmings as well as wikipedia refer to the qjet in describing it as vacuum secondary circuit and the qjet acting like a vacuum secondary "HOLLEY" carb..branding again. There is definitely a vacuum component to the qjet operation from everything I read and described but NOT holley vacuum operation.

Rescue Rogers

They are just locked shut by vacuum while the choke is shut. thats it, its a choke pull off. You can remove it and it will operate the same when the engine is at operating temperatures.

7T1vette

jb78L-82:

The Q-Jet secondary air valve is NOT vacuum operated! It is opened by incoming airflow. The secondary throttle plates are opened mechanically. Once they are open, air pressure from above it attempts to open the airvalve. Since the pivot on the airvalve is NOT in the center of the valve, secondary airflow begins to open the airvalve, which is being resisted by the return spring. When there is enough AIR PRESSURE above the airvalve, it starts to open; and the more airflow into the carb, the farther it opens.

The Q-jet is a "demand" secondary carb. When properly set up, it supplies only the airflow required by the engine. Thus, it can be used on just about ANY engine and will not over-feed it with excess fuel/air mixture. I guess you can 'think' that the engine vacuum is sucking the airvalve open, but that would be a semantics issue on your part.

The Q-jet does not have a vacuum operated secondary system.

jb78L-82

So are you saying that if you have no choke that a Qjet will operate perfectly correctly with zero vacuum input?

jb78L-82

I get that now but then I misunderstood earlier when some stated that the upper flaps on the secondary side of the qjet uses vacuum to prevent the flaps from opening until the vacuum drops sufficiently to allow the flaps to open. Are you saying that the upper flaps which would be the gatekeeper to the secondary's opening are just opening based on airflow through the carb on the primary side ?

Rescue Rogers

IF yo look at your Holley, you can see the big vacuum canister that operates the secondaries at the bottom of the carb. No linkage tied to the accelerator. There is no air valve either. It looks like a big choke flap on the top for anyone not familiar with a Qjet. A Holley mechanical 4 barrel that has a choke, locks the secondaries the same way with a little arm that is moved by the choke when its fully open. There is an arm that also locks the upper flap when the choke is closed.

The qjet opens the secondaries at the bottom mechanically the same as a mechanical holley, but instead of having the fuel flow controlled by low pressure created by the openning of that bottom butterfly valve and air flow through the chamber, it is metered more precisely by the air forcing its way past the flap at the top. Just fluid dynamics of the air moving a flap that is connected to the metering rods. That is why some people say the Qjet is more sophisticated than a double pumper.

lars

I have a complete description of the Q-Jet secondary operation in my Q-Jet paper - available by e-mail request. The Q-Jet secondaries are not vacuum - they are mechanical. They operate as follows:

Technical Explanation: The Q-Jet Airvalve and Secondary Operating System

Lots of misunderstandings on the operation of the secondaries on a Q-Jet carb.

To understand the operation of the Q-Jet, first you have to understand what a “vacuum secondary” carb is and how it functions.

A “vacuum secondary” carb is a carb whose secondary throttle blades are opened by the force created by venturi vacuum in the primary side of the carb. The vacuum created in the venturi of a carb is directly proportional to the mass flow of air passing through the venturi. This venturi vacuum is completely independent of manifold vacuum, which is non-existent at wide open throttle (WOT). A vacuum secondary carb has a little hole drilled right into the venturi on the primary side, and this venturi vacuum is fed to a spring loaded diaphragm attached to the secondary throttle shaft. Once airflow on the primary side approaches the maximum flow capability of the primary venturi, the vacuum will be high enough to overcome the diaphragm’s spring pressure, and the secondary throttle is opened by the primary venturi vacuum. This is a vacuum secondary carb.

The Q-Jet does not have any vacuum holes drilled in the primary venturi, and there is no vacuum diaphragm attached to the secondary throttle shaft. The Q-Jet is not a vacuum secondary carb – it is a mechanical carb with a secondary airvalve control. There is hard, mechanical linkage connecting the primary throttles to the secondary throttles, and the secondary throttle plates are mechanically opened at a progressive rate by the throttle linkage. Purely mechanical.

But vacuum sucks the airvalve open, and the airvalve is connected to a vacuum diaphragm, so it’s vacuum operated, right?

Not really. Imagine this: Take a spring-loaded screen door and set it up right out in your front yard. As the wind starts blowing, the door gets pushed open. The harder the wind blows, the more the door gets pushed open. Do you have a vacuum on one side of your front yard sucking the door open..? Of course not – the pressure is the same all over your yard. The force opening the door is the mass flow of air pushing the door open. There may be a low pressure area in Texas that is causing the air to move, but Texas is not “sucking” the door open – mass air flow is pushing it open, and the door is responding to the actual total mass air flow being pushed through it. The Q-Jet operates the same way: At WOT, there is no vacuum in the manifold – the manifold is very close to atmospheric pressure (a correctly-sized carb will cause the manifold vacuum at WOT to be at about 0.5” Hg, which is nothing). So the force opening the airvalve is the same as the wind pushing your yard-mounted screen door open: It’s mass flow pushing it open. This is not a vacuum operated carb. There is no vacuum in the manifold at WOT, but there is plenty of mass airflow.

The diaphragm on the side of the Q-Jet “controlling” the secondary airvalve is actually the choke pulloff. It is also connected to the airvalve to hold it firmly closed when manifold vacuum is high. When the engine is placed in a power condition (WOT or low manifold vacuum), the diaphragm relaxes at a controlled rate to prevent excessively sudden opening of the airvalve: The longer the airvalve is delayed in its opening, the bigger “fuel shot” the secondaries get upon opening, thus preventing a secondary tip-in stumble. The pulloff merely allows a controlled opening rate of the valve, and is not a vacuum-operated control of the secondary throttle in any way. Think of the pulloff as the damper cylinder on the screen door: The damper cylinder does not open the screen door – it merely controls and dampens its opening rate.

Thus the Q-Jet is not a vacuum secondary carb. It is an airvalve-controlled mechanical secondary carb with a damper. The airvalve is not operated by vacuum – it is operated by mass flow. The airvalve’s opening rate is controlled and dampened by the loss of vacuum signal – not by the creation of any vacuum.

For some interesting reading on the 3 different types of engine vacuum, feel free to drop me an e-mail request for my “Engine Vacuum Explained” tech paper.

Rescue Rogers

Yes, at operating temperature.

Rescue Rogers

No , its air flow through the secondary side. Its just held shut by a spring and friction on the axle rod of the flap.

jb78L-82

Awesome Lars.....Thank You!

I will read and re read what you are saying so I can get it through my thick head about the Qjet....I just want to understand how it works for my own edification.

Lars, why did GM spend so much time and money developing a qjet when emissions and fuel economy were distance thoughts for the future in the 60's? They could have just used Holley or Carter carbs. Why didn't they invest more in mechanical fuel injection which was being mastered by the foreign car companies, especially the Germans?

You are the expert.

ToomRaider

Rescue Rogers

Probably cost and planned obsolescence. Problem was it was a great design

vince vette 2

vince vette 2

Not playing Lars here, but emissions requirements were not a distant thought in the 60's

"As early as 1953, Los Angeles County Supervisor Kenneth Hahn inquired of Detroit automobile makers as to whether research was being conducted to eliminate emissions. The response was vague. With the threat of mandatory federal regulations, the auto industry began to install crankcase blowby devices (which returned unburned gases to the combustion chambers) on their cars. This was a significant advance because crankcase blowby produced 25 percent of the engine's hydrocarbon emissions. This equipment became mandatory on all cars sold in California beginning with the 1963 models."

"The 1963 Clean Air Act for the first time gave the federal government limited enforcement power over interstate pollution. The Motor Vehicle Air Pollution Act of 1965 produced national standards comparable to California law for the 1968 model year. Also in 1967, the Air Quality Act was the first piece of federal legislation designed to control lead emissions."

http://www.autolife.umd.umich.edu/En..._Overview4.htm

REELAV8R

jb78L-82

Yes very true for at least California especially since they were attempting to minimize smog/haze even in the early 60's although it turns out that smog/haze was not directly attributable to HC and later found to be a by product of the geography, temp, ocean, mountain etc. Another words, despite the almost complete cleanup of the auto exhaust issue, the smog/haze never really abated, even today.

As for the clean exhaust issue, that challenge was never really reduced aggressively until the advent of electronic fuel injection starting in the 90's which was a cool 30 years after the first discussions......

jb78L-82

That is funny........but ah, no! LOL

I love my Holley 4175 to be honest and especially its simplicity, tuneability, reliability, and ease of rebuilding. I had the L-82 355 out today, holy crap January, no salt on the roads yet, coming tonight, last chance) and the 4175 served me flawlessly for 25 years on the OEM L-82 and unbelievably, with a minor primary jet change, the built L-82 355...It is simply a GREAT carb with good MPG as well.

I am intrigued by the complexity of the Qjet and how it works...pretty fantastic for the time. I wish GM had invested some of that ingenuity into mechanical fuel injection BUT I digress..........

Big2Bird

The inversion layer effect you describe pertains only to the Los Angeles basin. It was described by Native Americans hundreds of years prior during summer fires.

As for smog efforts, in 6th grade the smog was so bad, it would make you cry with eyes burning. Although we still have smog, today, with perhaps 10 times the vehicles, the air is better than it was then. I have lived here 64 years, and I for one am glad the EPA was created.