Quoting from the B31 owners manual...."the lever on the outside of the rocker box, should have appreciable free movement before it starts to open the valve.........Adjustment is normally carried out by means of the cable adjuster on the rocker box cover, but the actuating arm can be removed and reset to any position on its serrated shaft".
Now if my memory serves me correct it is a fine spline of about 5/16 inch dia. and should prove enough adjustment. Your cable adjuster screws into the rocker cover and is the anchor end for your cable outer. The lever will come off by careful levering after removing the nut. If the problem cannot be solved by adjustment, I think the cable may be incorrect. Remember the free movement quoted during your adjustments.