Carburetor Rebuild - Rebuilding The Electric Q-Jet

Performance Mods
SMI's performance feedback Q-jet upgrades are intended to improve driveability and throttle response. Keep in mind that only very small changes are necessary to see big gains in part-throttle feel. These mods are part of SMI's Stage 2 upgrades, if you would rather have SMI do the honors. Keep in mind that as far as secondary jetting is concerned, the electric Q-jet still allows the tuner the full range of metering rods and hanger adjustments, just like the older Q-jets.

Murphy drills the idle downtubes from 0.032 to 0.034 inch to add a little more fuel to the idle circuit using a jobber-length drill bit. He also drills the accelerator pump discharge holes in the carb lid from 0.026 to 0.029 inch.

The choke pull-off is also the diaphragm that controls how quickly the secondary air valve door opens at WOT. Murphy grinds the tip of the nipple to expose the restrictor located inside and then drills this restrictor to 0.016 inch, which allows the air valve door to open faster.

You can also change the opening point of the secondary air valve door by loosening the small Allen locking screw and then using a small, straight-blade screwdriver to adjust the door-spring tension. The proper tension is one that will open the door the quickest without causing a bog. Too loose an adjustment created the undeserved Quadra-bog nickname.

During reassembly, Murphy coats the TPS plunger with grease to ensure it operates smoothly. Often the plunger sticks, which causes driveability problems. Also lightly stretch the spring underneath the plunger to ensure it has the proper tension.

When reassembling the primary metering rod assembly, make sure the rods touch the horizontal part of the mixture control assembly baseplate when at rest. As you can see (arrow), they don't in this photo. If not, lightly stretch the two small, primary metering rod springs until the rods touch. This will eliminate a possible part-throttle stumble.

This is the orientation of the idle air bypass valve to the primary mixture control solenoid plunger plate. The solenoid moves up and down, which also moves the air corrector. The air corrector screws into the top of the carb. Murphy says this idle air bypass valve is the proper way to adjust the carb for idle quality, not with the idle-mixture screws. The stock adjustment is to bottom the air corrector and then turn it four times counterclockwise. Turning the air corrector clockwise (down) is richer, while counterclockwise (up) is lean. There is also a dwell adjustment once the carb is back on the car, using this same idle bypass valve. Dwell is adjusted to 30 degrees using the six-cylinder scale.

With the Q-jet back together, Murphy set the idle-mixture screws at the factory spec 33/4 turns out (the needles have a very fine thread pitch). It's now ready to bolt back on the Monte.

By Jeff Smith

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