One of GM's better ideas was to include a way to fine-tune the part-throttle fuel mixture with the Adjustable Part Throttle (APT).  The APT is an adjustable lower stop for the metering rods, located beside the power piston on newer Quadrajets.  A pin sticks out of the side of the piston, which hits the APT stop at high manifold vacuum.  Having the metering rods sit a bit higher above the jet effectively enriches the part-throttle fuel mixture as the APT is turned out from the bottom.

The APT has a rectangular head, which requires a special tool.  Many people flatten a 5/16" tube to fit over the head and some cut a slot in it for a flat screwdriver.  SC made me a tool in their shop.  There is plug in the Quadrajet's cover that is removed to access the APT without having to disassemble the carb.  SC drilled a cone into this plug so that it can be removed with an ordinary wood screw.  I've also seen a kit for replacing this press-fit plug with pipe plug but I think that this is way more work than necessary for the few times the APT needs to be accessed.

 

Quadrajet APT

 

APT Adjustment

Although a Corvette club has posted the write up How to Adjust a Q-Jet Power Piston with LG as the author, LG recommends against its use.

According the above document , the top of the economy step of the metering rods should line up with the bottom "lip" of the jet.  Using my digital vernier caliper, I measured the length of my 52K metering rods to be 2.41" and my jets to be approximately 0.087-0.088".  The length of the non-metering portion of the metering rod was 2.01" and the length to the top of the power step was 2.026".  With the power piston pulled down all the way into the jet (economy zone), the length from the top of the metering rod to the top of the jet should be approximately 1.922-1.923" (ie, 2.010" - 0.088").  After screwing in my APT all the way, I found that the top of the metering rods were about 1.95" above the top of the jets.  This confirmed my expectation the factory used 0 turns of the APT adjustment starting point.

Since no specification exists for the number of APT turns of any carburetor, I presume that the factory set each carburetor individually on a chassis dynamometer when each car reached the end of the end of the assembly line.

My understanding is that Specialty Carburetors sets each Quadrajet they rebuilt to 3.5 APT turns as their rule of thumb,. From my internet survey of APT adjustments, I believe that the APT may be turned out to a maximum of 8 to 8½ turns.  If turned out this far, the metering rods would be sitting very high out of the jet so that the economy fuel mixture would be fairly close to the power fuel mixture.

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