Bruce, it's fine to jump start it with 12V with the 6V system all properly connected, then pull the jumper cables once started - BUT if 6V won't turn it over with the plugs out, you have problems that need to be found, you should be able to turn it over by hand with the plugs out, just by holding a tension on the fan belt and turning the fan .... battery defective? cables too small? ground connection bad or not directly on engine? starter motor bad or binding?
you need to figure out why it won't crank before thinking about firing it up, if it's 'none of the above' it may be incorrect assembly of the engine, don't just say 'it's tight cuz it's new', a new engine should still turn by hand
Bill
