Many things affect the tune a carb needs, dom touched on a few. One he mentioned that I think often gets overlooked or is misunderstood is elevation, and as far as a carb is concerned it can be a BIG one. The higher up you go in elevation the lower the barometric pressure/air density becomes. For instance Vegas a great barometer is 28.00,(actual readings not the corrected ones used by your local weather man) in the midwest 29.5 is a typical good day. The lower barometer reading will require less jetting to achieve the same AFR's or tune. Not to mention the myriad of internal engine varibale that can effect the engines actual pull on the booster itself, the harder it pulls on the booster the less jet it will need in theory and vice versa.