Look at the vacuum brake system in the distributor. It's supposed to retract at light throttle, and allow the centrifugal spark advance to operate. The flathead distributors don't have a vacuum advance- - -the brake retards the timing during acceleration, then the centrifugal advance is supposed to come back in at light-throttle cruise. They're notorious for getting sticky and not allowing the advance to work. There's a leather disc in the end of the brake plunger that wears out or falls out and jams up the advance sometimes. Grinding a little off the length of the spring behind the plunger also helps it to have enough room to move. Even with the adjustment backed all the way out, the spring can be too long by about 1/8" or so.

Also, check the distributor housing for worn bushings that allow the shaft to wobble and change the point dwell and timing. That's another chronic problem.
Jerry

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