Did a rebuild/freshen up of my Quadrajet on my 71 350 270hp, all stock. Did not drive the car prior to the rebuild. I am questioning the position of the choke plate when the engine is hot. Full open is about 75 degrees, which I would expect to be about 90 degrees. The choke plate will physically not open further as it is stopped by the linkage, (not the choke pull down).
is this normal or should it be straight up 90 degrees???




Also, when the engine is hot or cold the secondary throttle plates only slightly open when you use the linkage to go WOT. Comparing this to a 73 Quadrajet I have, the 73 will open the secondary throttle plates wide open. I am sure the linkage went back together as it was when I purchased the carb but it could have been wrong when I got it. As far as performance it’s responsive but not great. It doesn’t bog but I don’t hear that distinctive sound of the secondaries opening when I go WOT while driving. I am new to Quadrajets, I have the Cliff Ruggles book and I have ordered parts from him…..Thoughts?

Thanks for any help

 

Before lars comments, here's Alan 71's 350/270 carburetor for reference.

Choke closed


Choke open

 

The choke plate should be at a full 90 degrees (completely vertical) when the engine is hot. If it is not, you do not have your choke intermediate rod correctly adjusted. It's also possible to have the fast idle cam support lever incorrectly adjusted to prevent the linkage from rotating to the full "down" position.

The secondary throttle plates, at WOT, should go to just short of vertical: The top edges of the throttle blades should point to, and be in line with, the lower edge of the secondary airflow baffle inside the secondary venturi. If it does not fully open, your secondary linkage is incorrectly adjusted.

Lars

 

It has been a long time since I worked on a Q-jet but isn't there something in the linkage that prevents the secondaries from opening when the choke is on? If I understand your description correctly that is the first thing I would look at.

 

All Q-Jets have a secondary lockout lever that prevents the secondaries from opening when the choke is not fully open. It locks the secondaries completely closed - it does not even allow a "slightly open" condition. If the secondaries are only opening "slightly" when manually actuating the throttle lever, it sounds like the lockout lever is not engaged, but there is a linkage problem. When manually operated using the throttle lever, the WOT position of the throttles should be like this:


Carb shown with the secondary lockout lever engaged (choke closed) and preventing the secondary throttle shaft from rotating:


Lockout lever retracted just enough to allow rotation of the secondary throttle shaft:

 

Someone please correct me if I'm wrong (and I have no doubts they will): I don't think the QJet with its mechanical secondaries has that whoomp sound that vacuum secondary carbs like the Holley have. My understanding is that mechanical secondaries are on a continuum of opening throughout the acceleration range whereas the vacuum secondaries require a specific level of air flow to activate the secondaries and open all at once.

 

Thanks everyone for the overwhelming response. It is obvious there is an issue with the linkage. I will start with the accelerator pedal and work back from there. My carpeting/sound deadener/insulation is quite thick so I will make sure this isn’t impeding the amount of travel I am getting on my cable. Next it will be a detailed look at the linkage/choke operation. The secondary throttle plates in the current linkage configuration just don’t open…..back out to the garage….I will keep you posted…

Thanks again

 

Quadrajet has the air valve above the mechanical secondaries which are also air flow dependent. A friend once described the sound as similar to flushing a toilet.

 

Here is an extract from my Q-Jet paper that may assist you. "Cut-n-Paste" from the formatted paper causes some formatting errors, so use your imagination with the numbers and formatting below:

Lars

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.

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

1.  
   1. 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.

1.  
   1. 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.

1. 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:

1.  
   1. Remove your floor mat. I call this the “25 horsepower floor mat tuning trick.”
   2. 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.
   3. 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.”
   4. 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.
   5. 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.

1. Finally, there is a common linkage adjustment problem that can disable the secondaries if the linkage has been altered (very common). All carburetor throttle blades are installed on a carb’s throttle shaft “over-center,” meaning that one side of the throttle blade “hangs out” further from the shaft centerline than on the other side – they are not symmetrical. This causes engine vacuum to try to pull the throttle “closed” at all times. This is a safety measure that has been used since carburetors were invented. The Q-Jet, with its small primary throttles and very large secondaries, can see a lot of manifold vacuum trying to keep the secondaries closed when you “tip into” the secondary side at elevated rpm. For this reason, the secondary linkage has a special feature that puts a force on an elevated “tang” on the secondary throttle lever to produce the “leverage advantage” required to force the secondaries open. If this tang and the associated linkage has been bent or altered to prevent the “crack-open tang” from engaging, manifold vacuum will hold the secondary throttle blades firmly closed, and the throttle linkage will simply rotate around the spring connections without ever opening the secondary throttle. Verify that the secondary throttle linkage hits and moves the tang at the top of the secondary throttle lever first. When it does, the secondary throttle blades should be cracked open just enough to release manifold vacuum. The lower part of the linkage should then engage and slam the secondaries open.

 

Thanks Lars..... I have your complete paper in my files and have read it over a few times...big help.

i have started looking things over and right off the bat I have found there is no lockout lever on the secondary throttle plate. As well the small rod on the secondary throttle lever is resting on the choke coil spring cover. Something isn’t right... My diagnosis continues.

 

Missing lever won't prevent secondary opening. The choke cover is not interfering with the lockout rod on the shaft - you have no problems there.

 

Quick update on my progress. I again reviewed Lars paper and started looking at the throttle linkage. First thing I found was the linkage was attached to the upper bolt hole on the throttle lever……..changed it to the lower hole and the travel improved. I found that once I disconnected the throttle cable the throttle lever moved all the way back to fully open the secondaries. I then moved the cable in the bracket, and again the travel improved, but not enough. Last thing is the pedal, and it looked like I had too much carpet/sound deadener and insulation directly under the pedal limiting it’s travel. The devil (fix) is in the details…

 

Good job... you just did the "25 horsepower floor mat removal trick!"

Lars