One of the more common reasons for slow engine turnover using an original six volt system is under size battery cables. Most of today’s auto parts stores only stock the smaller diameter 12 volt design. Unknowing owners mistakenly replace their original worn cables with shinny new ones that are as much as half the diameter as needed. In Addition the owner does not know he needs two. Ground cables: One from the battery to the frame and one from a starter mounting bolt to the frame.
Six volt starters require twice the electrical flow to operate properly. Don’t blame your six volt system for slow unacceptable starter motor speed! Many restorers go to the expense of changing their 6 volt system to 12 volts. They feel their original system was inferior and believe what they did was necessary. The problem could have been corrected with just adding the three proper cables.
Remember: Millions of cars and trucks were made with 6 volt electrical systems each year. If they had not operated correctly they would have come with 12 volt systems!