I put in the AFC spring hit yesterday and the kit came with two springs. Should I have used the lighter spring or the heavier spring. I put in the heavy spring but think that was wrong, the lighter spring should allow more low end fuel. correct?

 

Which spring to use depends on how you use your truck. If you tow and haul, use the heavier spring with the timing set for towing. If you run empty most of the time and have your timing set up, use the light spring.

The spring does not really allow more fuel, what it does is regulate the AFC manifold pressure pressure setting at which the AFC releases the fuel stop to enable the pump to operate slightly richer fueling at lower engine loads. At higher engine loads and speeds nothing is done by the AFC kit and your inherent pump and injector settings determine the fueling rate.