Okay, problem solved! Thanks to all for your assistance.
The timing is now set with just a barely noticeable clatter between 2000 and 3000 rpms. Is this safe? Most people would never notice the noise so it probably can't do too much damage.
With timing turned back too far, the engine was sluggish, hesitating and backfiring. But no clatter.
There doesn't seem to be much wiggle room in between. The difference between the two extremes is maybe about 4 degrees. I wonder if this indicates a problem with the vacuum advance.
