20 thou is generally the accepted gap for mag and points ign, going up to nearer 30 thou for CDI ignition.
If you set the gap to 20 thou you should get a nice fat blue spark when just spinning the mag over by hand. If the spark is feint, pale or non existent at that speed either the leads/pickups/carbon brushes are shot, points gap is wrong, the internal timing is wrong, or it is time for a mag rewind. 1001 starting problems are fixed with a rewound mag!
Cheers,
Rich
