Electric Mystery

[njlevett@juno.com (Nicholas J Levett)]

For a 1980 MGB.  If the battery is not fully charged, the starter turns
over fine but it will not start.  The spark seems weak (yellow vs blue)
and even with starter fluid, will not fire.  When I jump started it, it
finally started.  One clue is the ignition light "glows" while driving
at night.  Why would the battery be strong enough to turn the starter but
weak enough to produce a weak spark, and not start?

Any ideas?

Thanks.
Nick