r16678

I have a 1979 L48 with quadrajet, the choke is operated by hot air coming intake. I was trying to understand how the secondary butterfly valves open on the carb. I let the car run idle for 20 minutes then started revving the motor but did not see the secondaries open up. Been told that it only operates when the car is being driven but I am not sure I understand that, any guidance is greatly appreciated.

MelWff

with the engine off did you watch how the linkage works as you slowly open the throttle?
at a certain point the linkage causes the secondary throttles to start opening but the air valve on the top will not open till a certain volume of air is passing through the secondary throttles, you cannt accomplish that by revving the engine.

bobs77vet

on some carbs they are mechanical on others they open by a combination of valving and vacuum and air forces. i dont think you will ever see it opening

Dusky

You can see them open, but need to rev it a lot.
vids :
Edelbrock
https://youtu.be/gOr2OlKcOD8
Qjet
https://youtu.be/FoqQkXteiRM

calwldlife

lars

Here is my paper, posted several times here on the Forum, about how the Q-Jet secondaries work. You cannot get the secondaries to open by "winging" the throttle in neutral - there is not enough airflow in a no-load condition to open the secondaries. Drop me an e-mail request if you want a copy of the full Q-Jet paper:

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

by Lars Grimsrud
©2018 Lars Grimsrud

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.


Technical Explanation: Help! My Secondaries Don’t Work!

Here’s one of the most common “complaints” I hear. “My secondaries don’t work. I don’t get that jerk when they open, and the diaphragm is holding them closed!”

There is almost nothing that can prevent the secondaries from opening on a Q-Jet. As described in the previous section of this paper, the secondaries are purely mechanical. If the throttle cable pulls them open, they will open. The airvalve will open if there is mass flow demand to open the airvalve. The choke pulloff will relax and control the airvalve opening rate upon loss of manifold vacuum and allow the airvalve to open. Very few things can prevent this sequence of events.

When “testing” for secondary operation, many people make a couple of basic mistakes. First, many will flick the throttle momentarily wide open with the engine running in neutral. In this condition, the secondary airvalve will seldom open since manifold vacuum never drops enough, or long enough, to allow the choke pulloff to relax: The engine must be under load with a high air mass flow rate through the carb in order for the secondary airvalve to open. You cannot “wing the throttle” in neutral to get much, if any, indication of secondary operation.

The second presumption of secondary operation is a “kick in the butt” upon secondary opening. Actually, if the secondaries are operating correctly, the transition into the secondaries will be so smooth that there should be no “kick” feeling at all – only smooth acceleration. What many people regard as “that secondary kick” is an improperly set up secondary airvalve which actually causes a momentary hesitation before “catching” and pulling. This incorrect operation will first throw you forward, and then throw you back in the seat. Although you may be able to impress your teenage son with this, the car is not running as fast as it should with such an incorrect setup.

There are 3 issues that can actually prevent secondary opening:

1. Secondary lockout lever staying engaged. There is a lockout lever on the secondary throttle that will prevent the secondaries from opening before the choke is wide open. I.e., the secondaries are not allowed to open when the engine is cold, thus preventing engine damage from the engine being placed under excessive load before being properly warmed up. The secondary lockout lever can lock out the secondary throttle if the choke is not opening fully, or if the choke is incorrectly adjusted. It is very easy to observe if the lever is retracted and if the throttle is operating – simply look at the lever and verify that it is allowing movement of the secondary throttle shaft when the choke is open. Some carbs, such as Pontiacs and pre-’68 Chevys, use a lockout lever on the secondary airvalve rather than on the throttle shaft. Same principle applies – simply look at the lever and make sure it is retracted when the choke is open.

2. Mechanical jamming of the secondary airvalve. There are 2 things that can commonly cause this:

a. Distortion of the airhorn casting at the rear airhorn attached screws. It’s common to see these two, long, rear attach screws grossly over-tightened, and this will actually bend the airhorn around the screws. When the airhorn bends in this area, it can jam the secondary airvalves and prevent them from opening. If you see that the airvalves are touching & “catching” the casting in this area, you can do 2 things:
i. Take a fine file and gently re-shape the distorted area. You need to remove the carb to do this to avoid getting filings down into the intake manifold.
ii. If the distortion is not really bad, you can loosen the secondary airvalve screws about ¼ turn and slide the airvalves forward as far as the clearance in the screw holes will allow. This will usually free up the jammed airvalves.
b. Bent secondary airvalve shaft. If the airvalve is not operating smoothly, or is binding and jamming as the airvalve opens, it’s possible that you have a bent secondary shaft. Unhook the secondary airvalve windup spring from the lever and then operate the airvalves by hand – they should be perfectly smooth with no binding. If the airvalves need to be forced through their rotation without the spring hooked up, you have a bent shaft. You need to remove the airvalves by grinding off the staked back side of the attach screws and slide the shaft out of the airhorn to straighten it.

3. A more common and subtle problem is the issue of inadequate throttle cable travel. This is very common on Corvettes and Camaros. With a helper in the car and with the engine “off,” have the helper fully depress the gas pedal (with the engine hot and the choke wide open to assure that the lockout lever is disengaged). Observe the carb on the driver’s side and see if the secondary throttle moves to the wide open position: You can grab the throttle lever once the assistant has the pedal to the floor and see if you can move it further. If the gas pedal does not fully open the secondary throttle, you have one of several methods to fix this:

a. Remove your floor mat. I call this the “25 horsepower floor mat tuning trick.”
b. Install the throttle cable in the correct position on the throttle lever. The Q-Jet has an “upper” and a “lower” cable attach hole: The upper hole was used for trucks and station wagons. The lower hole was used for Corvettes, Camaros and Novas. If you install a Corvette throttle cable in the upper hole, you will never achieve wide open throttle since there is not enough cable travel to open the throttle from the top lever location.
c. Straighten your throttle cable attach bracket. The bracket that supports the cable at the carb is often bent slightly forward. If the bracket is bent or moved forward, it will not allow enough cable travel to open the throttle fully. Simply bend the bracket back just a tad: I use a big hammer and a steel rod to give it a whack. I call this the “25 horsepower hammer tuning trick.”
d. Fix your gas pedal linkage. Very often, the actual steel linkage coming off your gas pedal will be bent a little “flat.” This will cause the pedal to hit the floor before the carb is wide open. You can give the pedal more travel by simply grabbing the gas pedal and bending it up off the floor. Care should be taken not to damage any plastic pedal rod bushings when doing this: Support the plastic pivot points when bending the pedal rod so the plastic bushings do not break.
e. Fix the slop in the gas pedal attach lever. Many C3 Corvettes have a bit of wear on the lever that attaches to the gas pedal rod: The rod is "D" shaped, and the attach lever has a "D" hole with a screw tightening feature. If this "D" hole is worn, you can remove the lever, grind some material off the locking feature surfaces so it snugs up tighter, and re-install it to gain some travel

r16678

Thanks I sent you an email for the paper, appreciate it.