1941 Packard 120 Sedan turning over slow....?

My buddy and i have been working on his Packard for some time now. It ran fine back in november, but now it wont start. 6V system, positive ground. The starter was rebuilt last year, and it is fine. Solenoid is new, battery is new and charged. Timing is correct, plug wires are correct, firing order is correct. All the grounds are good, and the conections are good. It has spark and fuel, but it just cranks over so slow, and it has us both puzzled. We know the battery is good because we can throw it in his 53 Plymouth and it fires right up. Having restored old cars, I know they crank over slow, but this one is really sluggish. I am out of ideas, any suggestions before we convert it to a 12V system?