[Roadsters] Electrical question

[Gary McCormick]

Not a roadster-specific question, but related to vintage vehicles (my '72 Yamaha): Any tips on tracking down the cause of a battery draining down near flat after a couple of days of sitting? New(ish) wet-cell battery, just topped up the acid this Friday, charged it up. On Saturday the bike started, ran fine (after I cleaned a couple of fouled spark plugs – two-strokes, you gotta love 'em), but went to try it again this evening and it barely lights the neutral light. 

Gary McCormick
'70 Datsun Roadster
SRL311-13291
San Jose, CA