What allows the automatic transmission to shift in a moped?

I have a 1978 yamaha chappy lb50 with a automatic transmission, It doesn't always shift when it should is there a solenoid or something that allows it to shift, or does it just go by rpms?|||Get a drain pan, drain out old transmission fluid and refill it. The thing is 33 years old, so it's time to GET ER DUN!|||ok