Bryan,

With your distributor, dwell variation with engine speed is most likely caused by shaft bushing wear inside the distributor housing. The bushings can be replaced if you can find them, and can find someone to install them. An electrical rebuilder maybe can help, and I think others have posted information here on that topic.

A couple of degrees of dwell change as you raise the engine speed to 1500-1800 RPM is not of concern, but if you get more than 3 or 4 degrees change, you should look into rebuilding the distributor- bushings and drive gear. Four degrees change for your distributor is getting pretty high, bearing in mind that a degree of dwell change results in a degree of spark timing change. That amount of timing change can cause light throttle hesitation, roughness, poor throttle response, and detonation, depending on how much the timing changes and whether it advances or retards.

Before you get too far involved with dwell change, disconnect the vacuum advance and see if the change (if any) is consistant with, and without the vacuum advance movement- it should be, since your breaker plate does not move with vacuum advance; the whole distributor turns. Also, make sure your vacuum advance bracket is tight on the engine, the clamp on the distributor is tight and the whole mechanism is not worn and loose. The vacuum advance mechanism should not affect dwell change in your distributor, but check those items anyway. Some distributors have a natural dwell change when the vacuum advance operates, but yours does not.

A couple of points not related to dwell change, but may be helpful:

When the engine is stopped, you should be able to turn the rotor a small distance, a quarter inch or a little more, against spring tension. It should return to the original position when you release it. That movement gives important information:

1)The direction of movement indicates the direction of shaft rotation when the engine is running, which normally you don't need to know, but you should know.

2)The movement against spring tension tells you that the mechanical advance springs are functioning, and the weights are moving. Or at least one of the springs is functioning. If you don't feel movement and spring tension, either the mechanism is stuck, or the springs are weak or broken. The breaker cam should not be loose and floppy when it moves.

Hope this helps.

Harvester