The ignition is 6 volt. The 8 volts was for extra cranking power out of a 6 volt starter. If a 6 volt battery will turn the engine, it will run on 6 volts. Something I have done in the past to get old engines running is hook a 12 volt battery to the starter to make it spin fast. I then disconnect the wire going from the ignition switch to the coil and hook a 6 volt battery to the coil and ground. That way I get the 12 volt cranking power and a full 6 volts to the ignition. This is of course only to get it running because there will be nothing to charge either of the batteries.