An apparatus for detecting unstable combustion in an internal combustion engine

A saddle riding type vehicle driven by a straddling rider includes an engine having a combustion chamber, an inlet pipe for introducing a fuel-air mixture into the combustion chamber, an air throttle valve attached to the inlet pipe for adjusting quantity of the fuel-air mixture supplied to the combustion chamber, an inlet pipe pressure detector for detecting, as inlet pipe pressure, a manifold air pressure in the inlet pipe downstream of the air throttle valve, and a determiner for determining that combustion in the engine has become unstable based on variations in the inlet pipe pressure.