Measuring Dwell Angle
NOTE: On modern engines, setting the points gap with a feeler blade is regarded by many as a basic adjustment. For optimum performance, the dwell angle can be checked and adjusted.
The dwell angle is the number of degrees (see Specification) through which the distributor cam turns during the period between the moments of closure and opening of the contact breaker points.
Recall that there is one best time to strike the match (timing) to ignite the fuel-air mixture in the cylinders and one optimum length of time for the match to burn. The crankshaft turns 360° each revolution. Dwell is 1/2 the number of degrees the crankshaft turns while the ignition points are closed. The Dwell Angle for Air Cooled Volkswagens (all models) should be 50°± 2° and this is equivalent to setting the points gap with a feeler gauge at 0.40mm / 0.016", when the nylon rider is on the high point of the cam lobe.
Remember : Top Dead Centre (TDC or DC, 0°) is the point where the piston stops dead still, changing its direction from toward the head to toward the crankshaft. At idle, the one best time for the spark plug to fire (the match to be lit) is somewhere around TDC or a little before but as the rpm's increase, the timing must be advanced, the match lit sooner, so all the mixture can be burned in the relatively less time allowed. This means that the burning starts before the piston reaches TDC and continues on past. Imagine this burning process at 3600 rpm happening 1800 times a minute in No. I cylinder alone. If we don't light the match faster-sooner, the mixture has no time to burn properly.
To Check the Point Setting with a Tach-DwelI Meter