I just rebuilt my Rochester quadrajet on my 70 454 and now the secondaries wont open which previously had no problem. The vacuum break is always pulled in and holding the secondary's from opening. If the car is off they release so it isn't frozen but apparently the vacuum to that part never releases. Can someone help me understand what might be wrong. Did i maybe put the wrong gasket on the carb? I dont think so but that is the only thing i can think of.

 

That is what the pull off is supposed to do. Once the vacuum to the diaphragm is released (ie WOT or nearly so) then the secondary air flaps are released and are allowed to open. If it was not that way then you would have a large bog going into the secondaries created by the temporarily lean condition of the secondary air flap opening too soon.

You may be able to observe it briefly by pulling off the air cleaner and manually applying full throttle and observing the secondaries opening just a little before you have to let off to keep from over reving.

 

Sometimes the secondary lock-out lever gets caught up on the pin on the secondary shaft(passenger side). Quick test is to tie it up or remove it completely and see if the problem goes away. The lever position is controlled by the choke lever and is there to prevent secondary operation when cold.

 

So what would prevent it from opening during WOT?

 

The Q-Jet secondary operation is commonly misunderstood. The following is an explanation of the Q-Jet secondary system, and a list of issues that may cause inoperable secondaries, copied and pasted from my Q-Jet Tuning Paper. This will address your questions. If you have questions, contact me.

Lars

Technical Information Bulletin Rev. CY 4-23-15

by Lars Grimsrud
©2015 Lars Grimsrud.
This article may not be published or distributed without the written permission of the author

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 mechanical carb with a secondary airvalve control.

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.

Contact me if you need further info.

Lars

 

It looks like there is a direct mechanical connection between the primary and secondary throttle shafts but really its a bit more indirect than that. On the secondary shaft there is a spring(driver side) that normally does nothing but if the lock-out lever is in play that spring winds up so that the throttle pedal still goes to the floor even though the secondaries aren't opening. With the engine off and cold go WOT while looking at the secondary shaft, youll see what I mean.

BTW, Lars was right on point saying that there should be no "kick in the butt" feeling when the secondaries open, it should be nearly imperceptible. Personally, I like a bit of the "kick in the butt" feeling so I usually set the secondary air valve wind-up as loose as I can and drill out the choke pull-off orifice(I actually have a few drilled out different amounts) to get as quick secondary opening as I can w/o a stumble. Its a fine line and and not everyone likes it but I do.

 

The above video is interesting-see the FOUR sets of secondary air valves momentarily open when progressing from idle speed to WOT when the vehicle is stationary . I am surprised the FOUR air valves will partly open when not under load. I have always seen this occur when doing a WOT in neutral on my vehicles though.

And four quadrajets? Over carburetion perhaps causing the vibrating secondary air valves under load ? Notice the vacuum gauge at and near WOT.

 

Pretty good example of intake reversion too. Lots of fuel in the air above the secondaries.

 

If they had designed the headers correctly to scavenge....they would not have had that problem.

As you were....

Jebby

 

Hmmm...can you explain that?

 

With so much overlap and the valve opening before TDC....the exhaust charge has to be strong enough to help pull the incoming charge in and get it started before the exhaust valve closes....the piston going down the bore will do the rest.....

Bad exhaust cam profile, restrictive exhaust will cause this.....

This is very old school theory......but it is valve overlap that causes choppy idle....as it is in efficient....exhaust scavenging off idle is critical to an engine with huge overlap....like the archaic W30 cam they used.....

I am sorry but these guys were idiots.....way overdesigned and underengineered....

Jebby

 

I'm following you on overlap and the exiting exhaust creating an 'atmospheric depression' to assist in pulling in the intake charge, just not so much as to pull it right out the exhaust port.

But... intake reversion happens on the latter portion of the induction cycle. So the effect of the exhaust valve is long over and the exhaust valve is closed.

Now the piston has passed BDC and is on it's way up the bore and the intake valve is still open. If it remains open long enough for the intake charge to reverse direction all the way back to the carb then that is what we are seeing in the way of mist above the carb.
Are you saying the events of the overlap period are responsible for the amount of intake reversion? or for intake reversion period. I would think intake port inertia (speed) and late intake valve closing, maybe cam profile on the closing side and amount of lift, would be bigger players.

 

They are...but this starts the bad things happening.....

If is reversing after BDC....that is just poor velocity....the air is not moving fast enough....it should still be trying to pack the cylinder even for a short while after BDC.

Lengthening the runners may have helped that.....

Jebby

 

Intake runners, right? Not exhaust primary lengths.

 

Yes.....that would help the BDC reversion.....runner length on the exhaust would help overlap reversion.....

Jebby

 

I thought that the reversion was caused by the extreme over carburetion at and near WOT.

The other tuning effects of cam,intake , timing and exhaust would be secondary--wouldn't it?

How would it be possible to tune an engine to accept 750 x 4 CFM carburetion at normal engine speeds?

 

The q-jet is a demand carb on the secondaries. So it's only going to provide what is demanded by the engine up to 750 CFM.
As far as intake reversion goes, imagine if the intake valve closed at 180* after BDC in other words TDC. How much reversion do you think would take place vs closing at 70* ABDC.

 

Not doubting you at all....cam timing and runner lengths could have fixed that car....this ain't Kansas anymore....

You are very knowledgable in your engine theory....hold on to that

Jebby

 

Thank you. Although I doubt I even scratch the surface in engine knowledge compared to the guys who really know.
I'm sure time and age will rob me of it in any case, but for now it's getting me by.

 

You already know more than 99% of people I talk to...
Stay humble and listen is the best advice....

BTW....Lars is spot on with the OP's concerns....

Jebby