The valves are double seated. they need to be fully open or fully closed to seat properly, or you may leak past the valve shaft packing.
I have been using 5-7# AC to 15-17# Ox for years and adjusting the torch as needed
A few things discussed above and a few not: Always back off the regulators, always transit with the protective caps on, cinder block doesn't stop a CO2 tank with the valve busted off (but the water heater does), Never transit in a closed trunk, never oil a regulator, always know where your hose is, know what is on the other side of what you are cutting/welding, keep the fire extinguisher handy.
