Rick,
I would say it would be rare for the breaker cam to be worn enough to cause your problem.For a good anylisis of the cam contour clean it up good and compare the cam profile adjacent to the contact area of the rubbing block.
Rotating the distributor in it's clamp will change the ignition timing all through it's range of automatic advance/retard.I think it would be good for you to move it slightly to test the result.To turn the body in the direction of shaft rotation will deliver the spark later (retard) and vice versa.
A retarded spark from optimum will make the engine seem lazy to respond to throttle opening, possibly backfire through the carburetor, and lessening of power.
An spark advanced much from optimum will give a sharp resposive engine off idle but reduced power as the automatic advance comes into play, limited usable RPM, bucking and jerking when under load and possibly coming off load, pinging or knocking under load, and irregular idle and light load.
Many people setting the timing"by ear" set too advanced because they mistake the quicker response off idle as an indication of a more favorable setting not realizing what happens when the automatic advance comes into play.
Is this jerking you describe something you can repeat or just a random thing that happens under varying conditions?
Under what load(throttle opening) and speeds does this happen?
When driving in top gear, level road, at 35-40 MPH if you step down on it some for 10 seconds then let up slightly what happens? smooth transition back to lower throttle opening?
