Accelerator Pump Adjustment

When Dave's son purchased the 1973 Super Beetle that Dave has since restored, it was equipped with a Bosch 009 distributor, which has no vacuum advance; the spark is advanced on acceleration by a centrifugal mechanism. The car is equipped with a Solex 34 PICT/3 carburetor. Many, including Rob and Dave, believe that the 009 centrifugal advance distributor is incompatible with the 34 PICT/3 carburetor. An inherent problem with the 009 distributor, when used with the 34 PICT-3 carburetor, is that it has an acceleration "flat spot;" that is, the engine "hesitates" on acceleration at low speeds (see the separate treatise on this hesitation problem).

Note: Because of the problem of hesitation on acceleration from a stop and when turning corners, Mike and Dave finally replaced the 009 distributor with a single-vacuum dual-advance (SVDA) model, which helped resolve the hesitation/stumbling problem (the biggest contributor to the hesitation problem was found to be air leakage into the intake manifold around the carburetor throttle shaft).

If you are using the 009 distributor, it is essential that both of the vacuum ports on the carburetor are plugged. Any vacuum port below the carburetor on the intake manifold must also be plugged if the stock air cleaner is not being used. If you are using the preferred SVDA disributor, the single vacuum line attaches to the vacuum port on the left side of the carburetor; the port on the rear of the carburetor must be plugged.

If it is necessary to use the centrifugal advance 009 distributor with the 34 PICT-3 carburetor, there are a number of modifications that can be made to the carburetor to compensate for the acceleration flat spot to a degree. One of these is to adjust the accelerator pump for maximum stroke; others are discussed in other procedures.

Adjustment of the accelerator pump is done as follows:

The 34 PICT-3 carburetor has a bell-crank with an adjusting segment on the right-hand side. The quantity of gasoline that is injected upon acceleration can be measured. With the float bowl filled with fuel, attach a length of tubing to the discharge end of the accelerator pump injector(s) so that the expelled gasoline can be caught and measured in a 25-ml graduated cylinder. Hold the graduate under the end of the tubing and operate the throttle valve rapidly exactly ten times. Measure the amount of gasoline caught and divide by ten to get the average quantity of a single injection pulse. The average quantity should be 1.45 to 1.75 ml for the 34-PICT-3 carburetor.

To adjust the injection quantity, loosen the retaining screw on the bell-shaped adjusting segment and turn the adjusting segment clockwise to decrease injection quantity or counterclockwise to increase the injection quantity. Then tighten the retaining screw and recheck the injection quantity.

To help compensate for the flat spot inherent with the 009 distributor, adjust the accelerator pump injection quantity to the maximum; i.e., move the adjusting segment counterclockwise as far as it will go.

While you are there, check for any interference between the accelerator pump mechanism and the body of the alternator. On our '73 Super Beetle the pump mechanism was rubbing against the alternator body in two places; this interference prevented the pump mechanism from operating through its entire cycle and thus prevented the proper amount of fuel from being injected into the carburetor on acceleration. We found it necessary to file a couple of small notches in the alternator body to provide clearance for the accelerator pump mechanism.