I would certainly agree on some were different for improvements:
(A) elimination of the internal dashpot
(B) replacement of the umbrella fuel valve with conventional
(C) replacement of the pressed-in well plugs with spun-in versions
(D) location change on the position of the float hinge pin
(E) others
But there were also many internal differences, especially in the placement and size of air bleeds, the size of idle tubes, bypasses, etc.
There were three different sizes of Q-Jets: 750, 800, 850.
1965~1969 all 750
1970~1975 most 750 (see exception list below)
1976~up mostly 800
Exceptions to 750 size 70-75:
(1) Most Buick 455 were 800
(2) Pontiac SD 1973 and 1974 were 800
(3) 1971 Pontiac H.O. and R.A. were 850
(4) 1971 Pontiac SOME S/T late production and S/R 7041263 were 850
The CFM difference was all on the primary side. The secondary side were basically 600 CFM variable depending on engine demand.
The 750 CFM have the small primary venturii
The 800 CFM have the large primary venturii
The 850 CFM have the small primary venturii with the idle booster venturii removed.
The discussion on how much CFM is necessary has disturbed MANY pixels on the internet. I have an 850 CFM on my own Pontiac 350 (and it uses it all, plan to try a 1000 CFM if I have the time) however, my 350 is no longer mild.
The change to the 800 (slightly larger primary) improved city and cruise driveability over the 750 for engines of 350 CID and above.
The book on Q-Jets by Cliff Ruggles is one of the top three (opinion of mine) carburetor books that has ever been written (at least in English). I would highly suggest this book for understanding the Q-Jet to any one, even experienced carburetor rebuilders.
And Bart, I wasn't trying to pick on you at all, merely to offer additional information.
One last comment: maximum CFM only comes into play at sustained W.O.T. Idle and cruise driveability and fuel economy are the result of holding at constant as possible A/F ratio at these RPM's. Those that understand the metering rod design and use it in tuning may see a large increase in fuel economy.
Jon.
