The amount of delay in horizontal scrolling is controlled by bits 7-0 in
BPLCON1. You set the delay separately for each playfield; bits 3-0 for
playfield 1 (bitplanes 1, 3, and 5) and bits 7-4 for playfield 2
(bitplanes 2, 4, and 6).

   Warning:
   --------
   Always set all six bits, even if you have only one playfield.  Set
   3-0 and 7-4 to the same value if you are using only one playfield.

The following example sets the horizontal scroll delay to 7 for both
playfields.

        MOVE.W  #$77,BPLCON1+CUSTOM