lotta different Ferris machines out there - pls post back with machine model, engine make, engine model and engine type/code.
M/W, do the simple things - completely drain fuel, replace with fresh - remove engine shroud and clean fins (look for mice chewing on plug wires), etc.
Certain Briggs engines need valve adjustments, but we need at least yer engine model/type/code info to give any decent advice.

whups - missed the Kawi 21 hp on yer post title, but still need rest of info..