Weak Spark Problem

[JimTravis]

What would be the most probable cause for a week spark? With the plug out and grounded to engine, the Spark in a dark shop appears to be extremely weak. I have put this motor together from various parts I have had laying around so nothing is "known to be good" . I did eliminate the coil as a problem by putting a new one on. Again- I am getting a spark but it is very faint and the engine will not fire. Thanks