take a look inside may need cleaned and some one that knows how to solder electronics
rework the circuit board
if its lived in an area where humidity is high it may have some bad joints in side
also check the relay 19 and relay 30 articles
this relay supplies power to the whole bike
it's relay 30 on the 660 bikes drawing
relay 30 powers the ignition unit it may seem to run ok
but the tach does not like the power being supplied so it goes nuts
on my rt125 the tach acted up for several days
then bike died
sorted it to bad relay 19
after replacement TACH problem went away
the contacts in the relay went bad
viewtopic.php?f=4&t=2085dave
watch the spade lugs on the wires they get weak and do not grip the wires
they also break off the wire ...wires in side the covering turn to green slime and all thats left is the covering