On account of increasingly strict needs on engine efficiency and emission control, the fuel to air ratio (FAR) control arrangements for internal combustion engines (ICEs) are still incessantly developed. The effects of time delay limit the FAR controller gains and degrade the system performance. This paper addresses a control system design for FAR in ICEs that suffer from unknown time-delay induced due to several reasons such as unknown evaporation time, unknown fuel quantity injected into the combustion process, and an imperfection in the model used to describe FAR and its relation to the surrounding factors that affect the combustion process. A new theorem is proposed in this context depending on Routh-Hurwitz technique and manipulating poles cancellation control method to achieve the required control action. A mathematical proof is introduced for the proposed theorem and a numerical simulation is performed to illustrate the validity of the control scheme.