Your problam sounds curiously similar to what I had going on with my ''69 144s.

I kept lookig for some sort of battery drain, etc. and just could not find any.

One evening while cranking the engine, I noticed a small whif of smoke of the front sarb (SU- H6).

My front choke cable was heating up and smoking what was probably some thin film of oil up there!

I traced the problem to the short where the twin choke cables pass through a small guide hold on the steering column bracket. Over the years the bracket has chaffed off the cable's plastic jacket. All the battery cranking power was in my hands, literally!