a reversible motor operatively connected to {TAIL ~ said throttle valve} for operating {HEAD ~ said throttle valve} 
a reversible motor operatively connected to { TAIL ~ said throttle valve } for operating { HEAD ~ said throttle valve }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD