Volkswagen Air Cooled 1949-1969 Repair Guide

Ignition Timing

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DESCRIPTION





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Fig. Fig. 1 Typical ignition advance curve from a Bosch distributor equipped with a vacuum advance canister-a corresponds to the mechanical advance and b to the vacuum advance



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Fig. Fig. 2 On upright engines the TDC timing mark is stamped into an edge of the crankshaft pulley

Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5°BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

ADJUSTMENT



It is only after adjusting the breaker points properly that the ignition timing should be adjusted. It is most important that the ignition timing adjustment be carried out only when the engine is dead cold, because rising engine temperature causes the setting to change.

If, in exceptional cases, it is necessary to adjust the timing with a warm engine, not exceeding 122°F, the timing should be advanced about 2.5° beyond the normal setting. The timing must then be rechecked at the first opportunity with the engine cold.

VW engines have had several different arrangements of crankshaft pulley timing marks. On early type 1 engines, the pulley bore two timing marks, 7.5° Before Top Dead Center (BTDC) and 10° BTDC, reading clockwise. Later, with the introduction of emission controls, a 0° TDC mark was added. The 7.5° and 10° marks were subsequently removed, leaving only the 0° mark. Newer engines have only a 5°After Top Dead Center (ATDC) mark. Type 2 engines are generally the same as type 1 models. Early type 3 engines have marks at 7.5° and 10° BTDC; later engines have marks at 7.5°, 10°, and 12.5° BTDC. Fuel injected type 3 engines have marks corresponding to 0°, 7.5°, 10°, and 12.5° BTDC. Refer to the Tune-Up Specifications Chart and the sticker on your particular engine for the correct timing setting.

  1. Turn the engine by hand until the appropriate mark on the crankshaft pulley is lined up with the crankcase dividing line. (On type 3 models, the mark is to be lined up with the timing setting surface, or pointer, on the fan housing. At the same time that the appropriate mark is opposite the dividing line the rotor must be pointing to the spark plug wire of cylinder No. 1 (the cylinder toward the front of the car on the passenger side). No. 1 position is indicated by a mark on the rim of the distributor. If the rotor is not pointing toward the No. 1 cylinder, the crankshaft must be turned one more revolution clockwise until it is. On recent models, No. 3 cylinder is retarded about 4° compared with No. 1 cylinder and only No. 1 cylinder is to be used in setting the ignition timing.
  2.  
  3. Loosen the clamp screw at the base of the distributor.
  4.  
  5. Attach the lead of a test lamp (6 volt for 1966 and earlier models, 12 volt for 1967 and later) to terminal 1 of the ignition coil and ground the test lamp.
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  7. With the ignition switched ON, rotate the distributor body clockwise until the contact points close and the test lamp illuminates, then rotate it slowly counterclockwise until the points begin to open and the test light turns off.
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  9. Without moving the distributor body, tighten the clamp screw at the base of the distributor.
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  11. Recheck the timing setting by turning the crankshaft pulley counterclockwise one-half turn, then engine block timing turning clockwise until the timing mark is within 1 in. of the line. At this point, proceed more slowly by tapping the right side of the fan belt with your hand. Such tapping will cause the fan belt to move in either moderate or very small jumps, depending on the strength of the tap. Slight taps toward the end of the check will ensure the finest possible check on the accuracy of the adjustment. If, upon rechecking, the lamp lights before the mark gets to the dividing line, the timing will have to be retarded slightly by loosening the clamp screw and rotating the distributor body in the clockwise direction. Rotating the distributor clockwise retards the timing, while rotation in the counterclockwise direction advances the timing.
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Adjustment of ignition timing on 1967 and earlier engines must always be done with a test lamp. A stroboscopic timing light should not be used, as it will alter the entire setting range. However, it is recommended that exhaust emission controlled engines (1968 and later) and type 3 fuel injected engines be timed with a stroboscopic light. These engines should be timed at idle speed, with the distributor vacuum line disconnected and the engine at normal operating temperature.

 
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