i have a derbi senda 50cc, on starting it revs straight up to 900 rpm. ?

generic answer as not familiar with the bike. Presuming this has a carb, check the butterly valve inside incase the spring / linkage internally has become damaged/ detached leaving the throttle in an open position. Choke on? if it has one. Check for the carb drawing in air around the gasket area.

mark
November 2012