Volkswagen Air Cooled 1949-1969 Repair Guide

Idle Speed and Mixture



Single Carburetor System

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Fig. Fig. 1 Idle speed and mixture adjustment screws found on 30 PICT-3 carburetors used on some 1968-69 models

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Fig. Fig. 2 Location of the idle speed (A) and mixture (B) screws on early carburetors

As a part of a routine tune-up it is necessary only to adjust the idle speed and mixture screws on the carburetors of most single carburetor Volkswagens. Before adjustment is begun, the engine should be at normal operating temperature and the idle adjusting screw must not be resting on the fast idle cam of the automatic choke. The following steps should be followed in setting the idle speed and mixture adjustments on single-carburetor Volkswagen engines:

  1. With the engine warm and running, turn the idle speed adjusting screw in or out until the proper idle speed is attained. The correct speed can be found in the Tune-Up Specifications Chart or on the sticker on the engine. If the Tune-up Specifications Chart valve differs from that given on the engine sticker, use the engine sticker valve.
  3. With the engine running at the proper idle speed, turn the idle mixture control screw slowly clockwise until the engine speed begins to drop, then turn slowly counterclockwise until the engine is running smoothly again. Now turn the mixture control screw another 1 / 4 turn counterclockwise.
  5. If necessary, readjust the idle speed. With the clutch pedal depressed, the engine should continue to run after the accelerator has been quickly depressed and released. If the engine stalls, either the mixture adjustment or the idle speed adjustment is incorrect and should be remedied.

The setting of the slow-speed (idle) mixture will have a great influence on the performance and economy of the Volkswagen at speeds as great as 50 or 60 miles per hour. If the mixture is too rich, the result will be excess fuel consumption, stalling when the accelerator pedal is suddenly released, and possible "running on" when the ignition switch is turned OFF. If the mixture is too lean (too much air, not enough gasoline), the result will be better fuel consumption, but exhaust valves may suffer burning or warping. The previously-given method for adjusting the slow-speed adjustment will give the proper mixture setting. Turning the mixture screw clockwise will lean the mixture, while turning counterclockwise will enrich it.

Dual Carburetor System

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Fig. Fig. 3 Use an airflow meter (as shown) to properly synchronize each carburetor in a dual-carburetor system

On certain type 3 models, there are two carburetors-one for each bank of two cylinders. While the current models are equipped with a fuel injection system, type 3 vehicles sold in the U.S.A. in 1966 and 1967 have dual carburetors and require slightly more sophistication in the tune-up operation. Adjusting the carburetors on the dual-carb models requires the use of a special instrument to measure air flow. This device measures the vacuum created by carburetor suction by means of a red piston which rides up and down inside a graded glass tube. The higher the vacuum, the higher the piston is raised.

Besides the synchronizing device mentioned above, a small frozen juice can will also be required in order that the device will fit on the air horns of the carburetors. Because of the screws that stick straight up from the air horn for the purpose of holding the air cleaner, the small can (open on both ends) is needed. By mounting it on top of the can, the test device will clear the screws without losing vacuum. Before attempting adjustment, the engine must be at operating temperature.

  1. Remove the right-hand connecting rod of the carburetor linkage system. This is the rod which connects the center bell crank with the right-hand carburetor throttle.
  3. Remove the air cleaner. It is held on by two wing nuts on each carburetor and one wing nut in the center. The connections to the air intake and to the crankcase ventilation system must also be removed.
  5. With the engine running, adjust the idle speed screw of each carburetor until the correct idle speed is attained. Each carburetor should then be flowing the same amount of air. When the synchronizer is moved from one carburetor to the other, the height of the red piston should not change more than 1 in., preferably less. Because of the fine adjustment possible with the use of the idle speed adjusting screws, there is no excuse for not being able to adjust the carburetor idle speeds so that the maximum variation is less than 1 / 4 of an inch. When checking the air flow through each carburetor, the disc on the device should be turned clockwise or counterclockwise until the piston rides approximately in the center of the range.
  7. In adjusting the volume control screw of each carburetor, slowly turn the screw clockwise until the engine speed begins to drop, then turn counterclockwise until the engine runs smoothly once again, then a further 1 / 4 of a turn counterclockwise.
  9. Recheck the idle speed adjustment, and, if necessary, increase or decrease the idle speed of each carburetor so that the correct speed is maintained. Make sure the synchronizer shows the same reading when it is moved from one carburetor to the other without moving the disc on the device.
  11. Recheck the adjustment on the mixture control screws. On 1600 engines, there is present, on each screw, a raised portion on the outside perimeter. This will enable one to feel the position of the screw when it cannot be seen. The correct position for the mixture control screw will be approximately 1 1 / 2 turns from the screwed-in position. When turning the screw fully to the closed position, care should be taken not to apply too much torque, because the seat or needle of the screw could be damaged.
  13. After the mixture adjustment has been rechecked, the idle speed and balance should also be checked again and corrected, if necessary.
  15. In checking the balance of the carburetors at an increased speed, it is necessary to once again install the right-hand connecting rod which was removed in Step 1. Block the front wheels, apply the parking brake and make certain the transmission is in Neutral. Then, have an assistant sit in the vehicle and, using the accelerator pedal, keep the engine running at between 1,200-1,500 rpm. With the engine running at this speed, check the carburetors for high speed balance.
  17. Install the synchronizer onto the left-hand carburetor and adjust the disc until the red piston rides in the center of the range. Now move the device over to the right-hand carburetor and, without moving the disc, compare the height of the piston here with the height achieved at the left carburetor. If the height of the piston is higher on the right side, the length of the right-hand connecting rod must be increased slightly. If the height of the piston is lower on the right side, the length of the right-hand connecting rod will have to be decreased. Changing the length of the right-hand control rod is accomplished by loosening the nuts on both ends and twisting the rod while leaving the ends stationary. The opposite ends have threads which tighten in opposite directions. The length of the right control rod must be adjusted until there is little or no difference between the readings of the test device when it is moved from one carburetor to the other.

For a really fine adjustment, it is possible to loosen only one of the control rod ends and turn it slightly in the desired direction. If loosening and moving this end is not sufficient to effect the adjustment needed, then the opposite end must also be loosened and moved slightly after the first end has been tightened. This is a step-by-step method which is guaranteed to be more accurate than simply loosening both ends and tightening them up again at the same time. It is quite impossible to avoid changing the setting when tightening the ends. Ordinarily the ends are offset by 90 degrees, but the step-by-step method takes into consideration that the ends may be offset slightly more or less if it will contribute to a more accurate balance.

  1. After low-speed and high-speed balance has been checked, the connecting rods should be lubricated at their ends with lithium grease and the carburetor's moving parts lubricated with a light oil.
  3. Reinstall the oil bath air cleaner, being careful to tighten the two outer wing nuts first, then the center wing nut. If the center wing nut is tightened first, it is possible that the adjustment of the right-hand carburetor will be altered when the air cleaner is fastened tightly to the screw protruding from its air horn. Replace the crankcase ventilation hose and air intake connections. In order to install the air cleaner it will be necessary to remove the right-hand carburetor connecting rod temporarily. Take care not to bend this rod.

Fuel Injection System

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Fig. Fig. 4 Set the idle speed of the fuel injection system by tightening or loosening the adjusting screw

The only tune-up adjustment possible on the Bosch electronic fuel injection system is that for idle speed. The adjusting screw is located on the left side of the intake air distributor. Early models have a knurled screw with a lockspring; newer models have a locknut on the adjusting screw. After adjusting idle speed to specifications, make sure that the throttle valve is completely closed at idle. See for further information on fuel injection.