PRD Fireball Shop Manual/Carburetor
Not unlike any other carburetor, eventually the seals will become corroded, the pathways will become gummed up, and the carburetor will require maintenance to restore it to proper condition. Using higher quality fuels will prolong the service time you can receive before a rebuild is required.
All legal carburetors in the USA are HL type Tillotson carburetors.
Maintenance and Adjustment Procedure 
Tillotson have provided a comprehensive service manual for the HL series carburetor. This is the best reference for maintenance, and troubleshooting. You should also utilize the PRD Fireball Owner's Manual to reference tuning instructions specific to the Fireball.
Tillotson have produced an instructional video which walks you through the exact process of working on one of their carburetors; most are very similar.
RLV have provided an instructional video on partial blueprint of a HL-166.
A myriad information can be found on Tillotson's Service/Downloads section of their website.