Indicator flasher not properly working

My Vespa special from 1976 has indicators on the end of the handlebars. The old flasher unit is a bi-metal type relay working of the 6V generator, no battery. The old unit is broken and I am trying to replace it with a new one. I have tried two models WRE03 and WRE01, but both are switching the indicator bulb on, but not off for long enough. It is almost constantly on. The new flasher have two pins, so they fit into the wiring. Any ideas what I might need to modify so it works. With the old flasher it was quite easy to change the switching rate as there was a tension screw which was tightening the wire across the bi-metal. The new flasher dont have this. Any ideas would be welcome.