Q-jet Confusion
 

While looking through an old January 2003 Rod and Custom Magazine, I saw an article on "Q-jet Calibration", page 53. I know, magazine articles are notoriously wrong and lots of time just plane stupid. This was written by Doc Frohmader and was based on info from Eric Weinrich of Dyno-Motive and Curt Hooker of Edelbrock.

Anyway, on page 58 there is a picture of the APT adjusting screw. The caption says, "Not all Q-jets have this, but if they do, it is used to limit the amount the power valve can lift. If you remove it, reinstall it the same number of turns from the bottom as it came out. Adjust it only if you find you need more enrichment under hard acceleration."

This is not the way I had thought this system works. I had thought that the screw limits how far the needles can go DOWN under high vacuum, low load conditions, such as at cruise. Lowering the screw would allow the needles to go farther down, for a leaner cruise. Raising the screw would hold the needles up higher, for a richer cruise.

I didn't think they had anything to do with hard acceleration.

Please clear this up for me. Thanks, Billk

The APT stands for "Adjustable Part Throttle". It was used on quite a few production Quadrajets, and allows for fine metering control of the part throttle A/F ratio only.

This means cruising at light load, when the engine vacuum is high. The metering rods will be all the way down and the APT will control the height of the power piston.

Tapered metering rods then control the amount of orifice available to allow fuel past the jets.

There were quite a few different metering rods teamed up with APT systems. Some provide much greater control than others. For example, many later models APT carbs used "K" metering rods, which only have .005" range of adjustment on the upper section with .026" tips.

A "P" metering rod will have .010" of taper on the upper section, provided a wider range of control, they also used .026" tips.

Most very late q-jets used "M" series metering rods, which have .010" worth of tapered section, but larger .036" tips.

APT systems are not limited to the later model carburetors, some early models used it, but in most cases the factory loc-tited the screw in place, and they can be very difficult to move.

The odd-ball 1975 and some 1976 Q-jets used an aneroid located APT system, which would put extra fuel past a single jet and tapered metering rod to richen things up some if/as needed. Below is a pic of an early style APT system, we've added an external screw to it......Cliff

Q-jet confusion
 

Cliff, thanks for you reply. I have rebuilt 2 Q-jets so far with your book, kits, parts and great advice. While the carbs are initially complicated, after several readings of the "Book", and rebuilding even just one, the systems get more easy to understand. Thanks for clearing this up for me. Billk