Cylinder head to intake port matching. What you are doing here is just making the air/fuel that is coming out of the intake not have to jump a step or hurdle when heading to your intake valve. Chances are...if you bought a set of after-market carb and manifold set...then the hole on the bottom of the intake is larger that the intake hole on your cylinder head. If you were to look down your intake manifold you would see a lip caused by the head being smaller. This lip will interfere with your intake of fuel and air...it will kind of stumble over it. To fix this, take your intake gasket, place it on the intake port of the cylinder and trace around the inside of it with a scribe. Remove the gasket and you will see how much lager you need to make the beginning of your intake port so that you eliminate that lip. Wen you grind it, make sure to blend the larger part you just made with the smaller part of the intake...like a funnel.
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