The butterfly assy is the lower intake. It changes the intake path at around 3000rpms from the long small runners for low speed to the shorter fatter runners for higher rpms - an intake tuning trick to make the power band wider.

THe butterflies tend to break off the shaft and eat up the engine when the shaft bushings get loose and let them vibrate, eating up the screw heads. If your shafts are tight, you should be ok.

Not responsible for advice not taken...