the old version vas called dimoby, just an automatic clutch, availlable either to start the engine, or working also when the engine runs faster.
the variateur is easy to see, vhen the engine is running.
at low rpm, the engine doesn't move. at high rpm, the engine turns to the rear of the moped...
here is a dimoby:
http://www.mabecanemobylette.com/dimoby.htm
here is a variator:
http://www.solex-motobecane.com/3-embra ... 08-678.htm
the first moped, such as the AV31 had a primary version of the "automatic clutch"...
you can start the engine, as a bicycle, by driving the bike up to 4 mph, then the cluch will start the engine.
at the red trafic light, the engine will still remain running. but if you put the throtle on, nothing will happens. (except the engine going faster)
you must drive yourself the bike up to 4 MPH, then the clutch will operate and the engine will drive you faster...
(sory for my kitchen english, I am not used to explain how a mobylette is (or was) working)
