Application

The Turbo Trans Am was produced in 1980 and 1981. The 1980 carburetor was a Rochester M4ME electric choke Quadrajet # 17080274.

The turbo carb differs from the regular Pontiac 301 4 bbl carb in a couple of respects. The most important is that the vacuum for the power piston in the turbo carb is 'externally referenced' meaning that there is a vacuum port at the center front of the throttle body that is connected to the intake plenum after the turbocharger.

Normal QJets have the power piston actuated by vacuum right under the carb. When engine load is light, vacuum is high and the piston is pulled down, seating the metering rods and giving a lean mixture.

Under load, manifold vacuum drops and the piston is pushed up by the power piston spring giving a richer power mixture.

In a pull through turbo, under heavy load, vacuum under the carb is high. Under light load, vacuum is relatively low. This is the opposite of the case in a normal carb.

So, the turbo carb has a vacuum port which draws from the plenum after the turbo. When the engine load is high, the vacuum after turbo is low, and so the power piston moves up to give a power mixture. At light load, vacuum after the turbo is high, so the piston seats giving a cruise mixture.

Restoring this carb

Like all Quadrajets, the biggest problem is the wear around the throttle shaft. This is fixed by installing bronze bushings to bring the throttle shaft hole back to size. Usually, the shaft itself, being steel, doesn't wear too much.

Electric choke Because of the placement of the carb on the manifold next to the turbo, the electric choke connector has to be placed higher than on a regular electric choke Quadrajet. This part is no longer available because it was only used on turbo Trans-Ams. However, a standard electric choke coil can be modified to fit.

Finishes The float bowl and airhorn are made of zinc and are finished in a olive-green chromate conversion coating, which prevents the metal from oxidizing to form 'white rust'.

>>More about the chromate finish

For a correct restoration, the main body screws should be Fillister head slot screws in a black finish; the choke and airhorn screws are also black but use a binding slot head

Parts for this carburetor from www.carburetor.ca