First thing to check is the end of the speedo cable where it fits into the
grease cap on the left front wheel.  The square hole that the cable fits into
often wears out, letting the cable spin.  Or, the clip comes off and the
cable falls in.  If the hole is rounded out, take it off and use a peen or a
punch to squash the rounded part back into square.
       if that's not it, check the back of the speedo, where the crappy
plastic clip is.  Good luck.

Bill Marshall
85 GL Tiico
PHX AZ