Date registered: Jul 2005
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RE: adjusting points
They're two sides of the same coin. The point gap determines the dwell angle.
Dwell, in a nutshell, is how many degrees of rotation that the points remain closed. In an 8-cylinder engine, each cylinder has 45 degrees of rotation to use; therefore a dwell of 30 degrees means that the points are closed for 30 degrees, then open for 15 before closing again. (The coil fires when the points open.)
A wider point gap means the points are open for longer, resulting in a shorter dwell. Conversely, narrower gap means a longer dwell.
When you change the point gap, you change the location of the point opening, which changes the ignition timing. (To be precise, changing the dwell by 1 degree changes the timing by 2 degrees, since the distributor rotates once for every two rotations at the crankshaft.) That's why you always need to check and adjust timing after touching the points.
To measure dwell, you need a dwell meter (usually combined with a tachometer). One lead attaches to ground, the other attaches to the distributor side of the ballast resistors mounted on the side of the engine bay near the coil. Then, to properly set it, it's a simple matter of:
1. Start engine, measure dwell. If it's 30 +/- 3 degrees, you're done.
2. Pop cap, then bump the starter until the moving point either sits on a lobe or just past it (you can rotate the point plate backwards a bit if necessary to measure).
3. Tweak points the appropriate direction (closer for more dwell, farther apart for less dwell).
4. Replace cap.
5. Go to 1.
It's not difficult, just a pain in the butt. That's why I've got a Pertronix on order -- I'll never have to set my dwell again. :)