Dwell angle is the amount of time (measured in degrees of distributor cam rotation) that the contact points remain closed. Initial point gap determines dwell angle. If the points are set too wide they open gradually and dwell angle (the time they remain closed) is small. This wide gap causes excessive arcing at the points and, because of this, point burning. This small dwell doesn't give the coil sufficient time to build up maximum energy and so coil output decreases. If the points are set too close, the dwell is increased, but the points may bounce at higher speeds and the idle becomes rough making starting harder. The wider the point opening, the smaller the dwell. The smaller the gap, the larger the dwell. Adjusting the dwell by making the initial point gap setting with a feeler gauge is sufficient to get the car started but a finer adjustment should be made. A dwell meter is needed to check the adjustment.
Connect the red lead (positive) wire of the meter to the distributor primary wire connection on the positive (+) side of the coil, and the black ground (negative) wire of the meter to a good ground on the engine. The dwell angle may be checked either with the engine cranking or running, although the reading will be more accurate if the engine is running. With the engine cranking, the reading will fluctuate between 0 dwell and the maximum figure for that angle. While cranking, the maximum figure is the correct one.
Dwell angle is set electronically on HEI distributors, requiring no adjustment or checking.
See Figures 1 and 2
Dwell can be checked with the engine running or cranking. Decrease dwell by increasing the point gap; or increase dwell by decreasing the gap. Dwell angle is simply the number of degrees of distributor shaft rotation during which the points stay closed. Theoretically, if the point gap is correct, the dwell should also be correct or nearly so. Adjustment with a dwell meter produces more exact, consistent results since it is a dynamic adjustment. If dwell varies more than 3 degrees from idle speed to 1,750 engine rpm, the distributor is worn.
- Open the metal window on the distributor and insert a 1 / 8 in. allen wrench. Turn until the meter shows the correct reading. Be sure to snap the window closed.
- An approximate dwell adjustment can be made without a meter. Turn the adjusting screw clockwise until the engine begins to misfire, then turn it out 1 / 2 turn.
If the engine won't start, check:
- That all the spark plug wires are in place.
- That the rotor has been installed.
- That the two (or three) wires inside the distributor are connected.
- That the points open and close when the engine turns.
- That the gap is correct.
- After the first 200 miles or so on a new set of points, the point gap often closes up due to initial rubbing block wear. For best performance, recheck the dwell (or gap) at this time. This quick initial wear is the reason the factory recommends 0.003 in. (0.08mm) more gap on new points.
- Since changing the gap affects the ignition timing, the timing should be checked and adjusted as necessary after each point replacement or adjustment.