I installed my 366 spring last night. This was my first modification to the truck. The pump was bone stock. This was a great thing to do. The end result is well worth it. I wanted to drive the truck first before I did anything else to see the difference with just the spring. I did not touch the fuel screw or the timing. I think I am going to bump the timing tonight maybe 1/8" to see what it does. I don't want my fuel mileage to drop any, so I am not going to fool with the fuel screw.
I have to go back under the hood anyhow to try to lower the idle some, I think I got it set wrong when I put the screw back in. It is idling to high. I also need to re-set my TPS because it is not shifting into OD the way it used to. It got late and I ran out of time.
Next mod. will be a pot. for the dash to control the overdrive. After that comes the pin fix or KBB or whatever it's called. Then I should be good to go for a while.

 

Yea you better get on that kdp.

 

Just turning the fuel screw in will not by itself reduce your mileage. It will, however, let you put your foot into it more, which might effect your mileage, and let you generate nice big clouds of black smoke!

It all comes down to the right foot and how hard it's pressing on the loud pedal. At cruise, it won't be using any more fuel, but the throttle will be more responsive, since you can get the fuel up quicker.

 

Exactly, the fuel screw alone won't reduce mileage, it's how you drive it. In fact, turning the fuel screw can allow the fuel to atomize the fuel better, and can actually increase mileage, if you can keep your foor out of it.