Hey Howard,
Unfortunately, you do need to change the springs on the valve. You cannot get to the correct pressures for NAT gas unless you change them. Often times, you just have to buy a new gas valve as furnace manufacturers do not sell many springs, and especially not for older units. There are a couple different gas valves for your unit and we would need the product # of your furnace (next to the model) to find which one you require. Best of luck!
NorthAmericaHVAC.com
Joe