The governor system on a Briggs & Stratton engine is designed to keep the engine that it is attached to from operating at too high of a speed. This prevents excess fuel usage, unnecessary engine wear and can also help to prevent accidents and injuries that can result from a lawn mower or other component using a Briggs & Stratton engine accelerating at too high of a speed.
There are two types of governors used for Briggs & Stratton engines, a pneumatic "Air Vane" governor that monitors air speed from the flywheel fan and the mechanical governor which uses gears inside the crankcase to monitor the engine speed. Both governor system types are able to adjust engine RPMs as needed to control engine speed. When the governor system is in use, RPMs are held at a steady level to maintain a constant engine speed even if the engine itself is under a heavy load.
A faulty governor system on a Briggs & Stratton engine can result in the engine running either too fast or too slow. An engine that runs too slow may have trouble starting or may stall when it begins to idle, while an engine that runs too fast may suffer from excess heat and a shortened lifespan. Both pneumatic and mechanical governor systems can be adjusted by mechanics or at home to bring the RPMs back in line with engine specifications.