Bruno_c

I suppose you have checked your choke system, so I think it looks like a weak ignition.

It is a MZ (from east Germany) bike modified with a home made twin cylinder engine.

How did it run before modification?

I suggest you should first determine if the malfunction affects one cylinder or both. Pull one plug cap then the other and see how it works.

I would first check ignition timing with a strobe lamp.

If the battery is almost discharged, even a small draw will cause a big voltage drop. So in my opinion, the first thing to do is to have...