Volkswagen Air-Cooled 1970-1981 Repair Guide

Throttle Valve Positioner


All 1970-71 Type 1 and 2/1600 models equipped with manual transaxles, and 1972 Type 1 models with manual transaxles sold in California use a throttle valve positioner to hold the throttle butterfly slightly open during deceleration to prevent an excessively rich mixture from reaching the combustion chambers. The throttle valve positioner consists of two parts connected by a hose. The operating part is mounted on the carburetor, connected to the throttle valve arm. It regulates fast idle speed. The control section (altitude corrector) is located at the left side of the engine compartment. It controls throttle valve closing time.


See Figure 1

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Fig. Fig. 1: The throttle valve positioner, located at the left rear of the engine compartment, can be adjusted by loosening the lockscrew and turning the adjusting screw-1970-72 Type 1 and 2/1600 models

The car should first be warmed to operating temperature (122-158F) for this adjustment. Make sure the choke plate is open.

  1. Attach a tachometer (0-3,000 rpm sweep minimum) to the engine with the positive lead to the distributor side of the coil and the negative lead to a good ground. You will also need a stop watch or a good wristwatch with a second hand.

Keep yourself, any clothing, jewelry, hair, tools, etc. well clear of the engine belts and pulleys. Make sure you are in a well ventilated area.

  1. Start the engine and let it idle in Neutral. Make a check of the fast idle speed by pulling the fast idle lever back so that it contacts the lever stop on the carburetor. The tachometer should read 1,450-1,650 rpm. If the fast idle is not within specifications, turn the adjusting screw (on the lever stop) as required. Disconnect the tachometer.
  3. Take the car for a warm-up drive. Recheck the fast idle as in Steps 1 and 2. The tachometer reading should not exceed 1,700 rpm.
  5. Now, make a check of the throttle valve closing time. Pull the throttle lever away from the fast idle lever until the tachometer reads 3,000 rpm. While keeping an eye on the second hand of your watch, release the throttle lever and check the time elapsed until the engine reaches idle speed (800-900 rpm). The closing time should be 2.5-4.5 seconds. If the closing time is not within specifications, adjust the control section at the left side of the engine compartment. After loosening the lockscrew, turn the adjusting screw (1) clockwise to increase closing time and counterclockwise to decrease closing time. Recheck the adjustment. If it is within specifications, tighten the lockscrew and disconnect the tachometer.
  7. Take the car for another test drive. Once again, recheck the closing time as in Step 4. The closing time now must not exceed 6 seconds. If it does, go back to Step 4.

If, after several attempts, the positioner does not consistently operate correctly, check the hoses for looseness or cracks, then check the diaphragm unit for clogging or diaphragm malfunction.

If the diaphragm unit (the part that is attached to the carburetor) is replaced, the pull rod on the new unit must be adjusted. After installing the new unit, adjust the rod by loosening both locknuts on the rod and turning the middle part to either extend or shorten the length. When the throttle valve is closed, the fast idle lever (inner lever) must not touch either the carburetor body or the throttle valve lever (when the choke is fully open).