I modified the "cabin heater splitter box" closing plate attachment arm. The factory bend the control cable around a screw on the closing plate arm, therefore no adjustment available. After talking to BDH, I modified the attachment point which will allow adjustment of the closing plate position. I have drilled a hole through a AN3 bolt which will allow me to move the position of the cable on the arm, therefore position of the closing plate. The cable will be a little offset but shoud be OK, not a lot travel required.
