A paper by Lars Grimsrud
OverviewThe 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 is pulled down into the carb against spring pressure, and this inserts the “fat” part of the primary metering rods into the jets for a lean, crisp, economical fuel mixture. When engine vacuum is lost, such as occurs under high power settings, the piston pops up from the spring pressure, and the “skinny” part of the primary rod is all that remains in the jet. This increases the metering area of the jet and richens up the fuel mixture for good power and performance. 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 adjustment screws every way possible in an attempt to “tune” the carbs, and I now frequently see people asking about what the “spec” is for this adjustment. Fact is, there is none. But here’s how you can get your carb set up so it’ll run right again.
This tech paper will discuss how to adjust the adjustable power pistons found on late-model Quadra-Jet Carbs for optimum street performance and drivability.
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