I have a 12V battery with a 6V starter and no issues. You just can't run the starter too long as it will burn out quickly. I think spacing is the issue here.
Do you have a idle mixture adjustment? What 'gas valve' do you refer to? If this is the one under the tank then my guess is that you have real carb trouble! I would suggest removing the carb for a full clean and inspection. What make is the carb on your tractor?