Machine will not sew forward without backstitching

Question

this old janome sewing machine( my guess its a early 80's model) has two dials on the front, what apears to be the stich size regulator and a pattern selector dial. It also dosnt apear to have a drop feed buttton anywhere. I tried to select a stitch to stop edges fraying after sewing beautifully and now the machine will only sew forward with a backstitch action ie; three foward two back etc. it dosnt seem to have a problem with any of the other fancy stiches