Ray, new spring helped, slowed the timing curve but did not completely get rid of the rattle. Phreakish's comment "An engine can "run great" while slowly, or quickly, destroying itself. Simply jumping at the timing may not be the solution to the issue. Ignoring other possibilities is a great way to wind up looking at a rebuild instead of a rejet or new distributor springs" got me thinking that my previous state of tune was possibly right on the edge of detonation, and that a weak distributor spring or bad tank of gas just pushed it over the edge. So I am going back through all systems and double-checking.
