Todor Skrimov finished as a top scorer with 18. Valentin Bratoev added 17 (15 spikes).
With the win co-hosts leapfrogged Iran in the Pool G standings moving to second place.
Next for Bulgaria – the clash on Saturday with the unbeaten USA and Iran faces Canada.
The 2018 championship is the 19th staging of the FIVB Volleyball Men's World Championship, contested by the senior men's national teams of the members of the Fédération Internationale de Volleyball (FIVB), the sport's global governing body.
The final tournament is being held in Italy and Bulgaria from September 9 to 30. For the first time the tournament is jointly-hosted by more than one country.
The final six will be hosted by Italy at the Pala Alpitour in Turin.