The biggest difference in the model J and the Cobra convertor is the primary pressure valve. The model J had a primary seat that was a blade with a square rubber piece glued to the blade. The Cobra convertor uses a plunger with a rubber plug on the tip of the plunger. It was very typical to experience primary side failures, resulting in high fuel pressure, in the 1,000-2,000 hour range. The Cobra convertors primary fuel pressure valve is much more durable. As for the blue spring vs the orange spring, the blue spring is a 1.5 inches water column to open the secondary pressure and the orange spring opens at 1/2 inch water column.