sewing backwards in forward?

I have an old Minerva machine with external motor recently had a few shocks from metal so husband replaced the motor with one I had from another machine which was used very little....No more shocks but sews backwards ...any idea why...its a basic machine with slider for stitches 1 to 4 and up for backwards ...now does backwards and forward only when in the up or backwards position so I have no control over length of stitch....