Jerry is correct about the early issues:
In 1966, the internal dashpot was discontinued. The 1965 units may be bored, and custom sleeves made and inserted.
In 1967, the "umbrella" fuel valve was discontinued. The earlier units may be converted by plugging the transition channel, milling and tapping the body for the conventional fuel valve, and installing the new fuel valve. The aftermarket "press-in" valves would work, but only if all of the planets were correctly aligned
As soon as the planets lost alignment.........
In 1968, the well plugs were redesigned, and spun rather than pressed.
There were two different changes to the fuel inlet fitting, from 3 to 5 threads, and from 5 to 8 threads.
In 1969, the plastic fast idle cam was discontinued, and steel service replacements issued for earlier units. The plastic units (90 different) are in demand by the numbers-matching folks.
Virtually all Q-jets by now will require throttle bushings to be installed on the primary side. MEASURE FIRST! If the clearance does not exceed 0.009 inch, don't re-bush!
By 1968, the Q-Jet was an excellent street carburetor.
By 1969, the Q-Jet was a great street carburetor.
The 1965~1967 units are best left for the numbers-matching folks, although ALL of them can be made to perform WELL. It is a matter of money, and the experience of the rebuilder.
Over a few years, from about 1971~1974, the float pin placement was changed. This change allowed better fuel fill for highly modified race engines (450 HP or more). This change makes the Q-Jet an excellent race carburetor for straight-line racing. Racing with corners is possible, IF REQUIRED BY THE SANCTIONING AUTHORITY. Other carbs are easier to make go fast around corners.
The upper limit (it is continually pushed) for horse power for the Q-Jet is currently about 550.
Anyone seriously considering the use of a quadrajet, especially for racing, should consider the three books in my previous post in this thread. All are in paperback, all are inexpensive, and all WILL help with Q-Jet understanding.
Finally, there are a LOT of CHEAP SHORTCUTS to fix issues with the Q-Jet. These were designed to separate the owner from his/her income, but not to work properly!
Jon.
