I finally bought a digital timing light.  Followed the Bentley
procedure (more or less):

1. Disconnected the idle stabilizer and plugged the two plugs together
2. Loosened the distributor locknut
3. Started the engine, adjusted timing to 5 BTDC
4. Adjusted the idle screw to get an idle around 900rpm.  The idle
was not rock solid, but reasonably smooth.
5. Checked the timing again -- still at 5BTDC

Then,

5. Stopped the engine, plugged the idle stabilizer back in, started it up.
6. Yikes -- the 5 degrees BTDC timing jumped to about 10-15 BTDC, and
the idle was very fast and surging.
7. Readjusted the distributor to get timing approximately 5BTDC (*)
8. Adjusted the idle screw to around 950rpm.

(*) The idle is rough, and I can see the timing jumping around as the
idle stabilizer advances it.

So this has me worried -- if the timing and idle are OK without the
idle stabilizer in circuit, why should putting it back in cause the
timing to advance 5 or 10 degrees?   Is this the symptom of a bad
idle stabilizer?