Lars,
The May 2007 issue of Hemmings Muscle Machines has a rebuild article on a Quadrajet. On page 76 it says;

"Want to increase a 650cfm Quadrajet to a 750cfm? Simply grind some material from the secondary stop lever. If your secondary air valve does not open perpendicular to the air horn, then the airflow is limited. Grind this tang to make sure the the air valve is fully open. There were no internal differences between a 650cfm and a 750cfm Quadrajet other than the factory setting that allows the air valve to open all the way on the 750 model."

After attending your "tuning for beer" session in Detroit, I am suspicious that this article is just plain wrong.

Hemmings Muscle Machines is one of my very favorite monthly magazines. Their tech articles are normally right on!

Jim

 

a reply from Not Lars:
the same info was published in the Ruggles book a while before the Hemmings article. a screw could also be added to provide adjustability.
i did the mod on current q-jet donned vette and at least it works (don't have before and after times); but i'm still working on getting the A/F at WOT right on.
so far no problems at all with air valve return, the only negative i've ever heard of with this mod.

 

When you say "ever heard of" is that from friends that have done this personally, or was that comment written in the magazine?

 

i didn't see the article in Hemmings, but what the "ever heard of" refers to are some q-jet reference books, i.e., the few published books that i have. off the top of my head they are a new Roe, an old Roe, and Ruggles, not counting internet resources. right now i can't think of whether i read that warning in one of the Roe books or an old auto magazine article. almost certain it's not in the Ruggles. i just remember reading it somewhere; this is not the overcenter rear butterfly and linkage issue. i also don't have any friends who have who have confessed to me having personnal experience with this mod causing problems. just thought i should mention it.
thx for reading . . .

 

Jim -
GM limited the cfm rating of Q-Jet carbs on certain applications using 2 methods:

1. The mechanical linkage to the secondary throttle lever on the driver's side was adjusted in such a way that full secondry throttle opening could not be achieved on some models. This was done, for example, on the 400 Firebird in '69. The Firebird used the same engine as the GTO, but the 'Bird was rated at a lower hp number. This was done for marketing reasons: The GTO was to be the "flagship" with the highest numbers. You could get the same hp numbers out of the Firebird by simply bending the secondary linkage tank to achieve WOT on the secondary side.

2. Chevy limits the cfm on some carb models by using a limiting tang on the secondary airvalve lever on the passenger side. This prevents full opening of the secondary airvalve, thus limiting flow. You can, in fact, grind a little material off of this tang to allow the secondary airvalve to open fully. You just have to make sure you don't grind so much that you allow it to open over-center.

During the TFB Tours, as you mentioned, I talk a little bit about the neccessary relationship between the various components on the secondary side of a Q-Jet. It is important that these relationships and angles be set correctly. Often, in the quest for more cfm, people get a little over-aggressive in "adjusting" things, and end up actually destroying airflow on the secondary side. But careful grinding and adjusting of the airvalve opening can produce better airflow on some Q-Jet models (not all).

 

i thought there was a 750 and 850 qj. Not a 650.

?

 

TV, you and me have the same recollection, due something to do with bumps in the primaries....limiting flow....was supposed to be the easy way to tell.....I know some carbs in some long forgotten apps had the secondaries limited due to trick best described by others, these daze....pix, memories, and words....comprehende'?....

so yes, for those seldom seen smaller engines....with a Q on board, it was limited to WOT was not an issue with fuel atomization....

 

That's what I always believed

 

i thought those were the approximate maximum values with all test assumptions for the two available castings. it appears the factory just detuned some

 

That's correct.