not being a carb guy i will probably get my butt chewed on here but here goes. i will try to say what i know. the needle and seat control gas level in fuel bowls. stock carbs work fine with moderate inclines. when you do much trailriding-hillclimbing the stock n-seats either flood or starve the engine for fuel. spring loaded ones help or solve the problem. holley and edelbrock have both come out with "offroad carbs" that have that option. the metering rod and jet kit is for tuning for performance and altitude adjustments. i currently have a edelbrock 750cfm electric choke carb on my 454 #1411. i put the springb loaded n-seats and tweaked the jet-rods(4500ft altitude) runs nice! /forums/images/graemlins/peace.gif