Late ‘70s Q-Jets have an externally-accessible adjustment screw (through a small hole in the carb air horn) for adjustment of the power piston height. Many people refer to this as the “mixture screw” on a Q-Jet. Over the years, people have screwed these, 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.
Power Piston Metering Adjustment Screw allows adjustment of height of primary metering rods within the primary jets. Great for fine tuning your Quadrajets APT cruise Air/Fuel metering. Note: Spring #2021C is required under adjustment screw. (Sold separately), Adjust the float, set it and the needle in place, then insert the primary needles/power piston in place. BE CAREFUL TO INSERT THE NEEDLES IN THE JETS AS YOU INSERT THE PISTON OR YOU WILL BEND THEM! Drop in the float chamber baffle as it keeps the fuel in the float well and has no affect on the amount of fuel in the chamber!, 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.
APT Power Piston Metering Adjustment Screw, USA made power piston retainer bushing and power piston tuning spring kit are two of our most popular products. A secure, tight fitting power piston retainer will protect your primary metering rods from getting bent when installing the airhorn. Save your rods, replace your PP-retainer today!, 10/10/2016 · The first round of Quadrajet improvements came in 1968 with the APT (adjustable part throttle) feature, which enabled more finite tuning and a smoother transition from idle to power circuit.
3/31/2018 · Technical Procedure #2: How to Adjust a Q-Jet Adjustable Power Piston To adjust a Q-Jet with an adjustable Power Piston , proceed as follows: The Q-Jet uses a power piston with metering rods to lean out the fuel mixture at cruise and at idle, and to richen up the mixture at wide open throttle (WOT). When engine vacuum is high, the power piston …
The stock Q-jet is an emissions carb as most Q-jets in the early to mid 80’s are. These carbs aren’t computer controlled as many people think. … The power piston has a pin made on it that hits the APT screw and stops it from going any farther down into the jets. … You don’t want to go too far with this adjustment , because it will just waste …
1/4/2019 · A carb with Adjustable Part Throttle makes tuning cruise mixture easy. My Edelbrock 1902 Qjet has a screw in the base plate that adjusts the down position of the power piston to change how much rod taper is in the jets at part throttle. Jet makes a kit to convert later carbs to easily adjust this.
