Any years Quadrajet questions

[Cadman-iac]

  Has anyone ever noticed that the Quadrajet has three different lengths for the lever mounted on the primary shaft for the secondary linkage?

 During the rebuilding of a Quadrajet, I was going through my primary shafts looking for the best one when I noticed the difference in this lever between the various shafts and carburetors.
 
 What this does basically is change the point at which the secondaries begin to open in relation to the primary and the rate at which it goes from closed to fully open.
 
 A short lever, (1" C/L to C/L), causes the secondaries to begin opening when the primaries are open approximately 45%, and then it opens slowly until both are open completely.

  A long lever, (1.5" tall C/L to C/L), on the primary shaft allows the primaries to open appropriately 75% before the secondaries begin operating, which it does at a much faster pace, basically snapping open from fully closed to fully open during the last 25% of travel of the primaries.

 The other length, (1.25" C/L to C/L) of lever splits the difference between the long and the short levers, giving you a moderate opening rate for the secondaries beginning at approximately 60% of primary opening.

  Can anyone explain the advantages of having either of these opening rates for the secondary throttle plates?

 What would you gain from having the ability to open your primaries 75% before you get into the secondaries?

 On the other hand, what would you gain by having the secondaries begin to open before the primaries are even open 50%?

 Which one would work better for say towing?

 Which is better for everyday use?

 Thanks in advance for shedding some light on this.

  Rick

[TJ Hopland]

I think it depends on where the power curve is on the engine its installed on and things like the gearing and intended use.  I would not change anything unless you really want to get into it and learn how each adjustment effects something else. 

[Cadman-iac]

  I'm not positive yet, gotta do a bit more research, but I've noticed that the carburetors used for some big block Chevys, Olds, Buicks, Pontiacs, and Cadillacs that have the bigger primary venturis also have the longer secondary lever on the primary throttle shaft. The actual primary throttle blade size is the same for all Quadrajets, the difference is in the float bowl portion of the carburetor. The diameter of the venturi opening is larger for ones used on bigger engines.

Just speculation here, but I think because of the bigger primary venturi, the primary throttle can be opened farther and still be able to supply the engine with enough air before the secondaries begin operating, and once they do, they engage quickly if you continue to apply more throttle.
And because of the larger displacement, the secondaries opening quickly doesn't cause a stumble or hesitation.

These seem to be carbs from the mid 70's through the 80's from the numbers I've been able to look up so far. These are all carburetors that I've been collecting for decades.

I don't know for sure if my theory applies to all large displacement engines, and I'm curious to know how a carb with the longer lever would perform on a smaller displacement engine, something  around a 350 or smaller.

I have multiple books on Quadrajets and they go into great detail on how to tune one and explain the various designs over the years, but none of them have once mentioned this difference in the lever length and what effect it has on the carburetor and engine.

If I hadn't been building one from all the parts I've got and looking for the best pieces, I wouldn't have ever noticed the difference. There is a reason why the factory did it, and I want to know why.

Anyone have any experience with this, or an idea as to the specific effects this lever length change makes in the performance of the carb/engine?

I'm thinking the 425 from 77 to 79 would be a good example, or a 402, 427, or 454 Chevrolet, or 455 Olds, 430, 455 Buick, or 400, 421, 428, or 455 Pontiac from the same time frame.

 Your thoughts are much appreciated.

 Rick

[klinebau]

I am not sure that it really matters.  Don't forget that the airhorn has to open for the secondaries to come online.

[Cadman-iac]

  Here's the parts I've been referring to. In addition to the lever on the primary throttle shaft, the mating lever on the secondary shaft is also different.

These are the 3 different length levers used on the primary shaft.


 Here's the complete 3 piece linkage between the two shafts.
 The center link on the top set is an early version that uses a clip to hold the front end. These are so much easier to remove if you have to.


  The lever on the secondary shaft has two versions, one used for the shortest primary lever, shown here with a straight edge showing the alignment between the lever and the mounting ear for the shaft. Notice the straight edge matches the lever.


 This shows the second design lever for the secondary shaft. Note the straight edge is at an angle to the lever here. This one is used with the medium and long primary levers.


 One thing I like to change on my Quadrajets is the accelerator pump linkage. I prefer the early design with the retainer clip on the top end because you don't have to drive the pivot pin out of the pump lever in order to remove the air horn. The early linkage is thin like most Quadrajets use, but when the electronic Quadrajet came out the linkage diameter was increased, making it more difficult but not impossible to use it on older Quadrajets.
 Here's a picture of the thicker linkage and the pump lever with the holes enlarged for the linkage. The hole on the throttle shaft for the pump linkage needs to be drilled out too, or the piece on the shaft swapped with a newer one.

  You can identify the  longer primary shaft lever by the angle of the linkage between the two shafts.
 
 On the longest one the linkage looks like it runs downward from front to rear.

 On a carburetor with the short lever, the linkage looks like it runs upward from front to rear.

 And if the carburetor has the medium length lever, the linkage is pretty much level from front to rear.

  I forgot to take a picture of the linkage angles for the different lengths. I'll post some later.

 Rick