Adding Hydrogen–Oxygen (HHO) into traditional fuels enhances combustion efficiency and mitigates CO2 emissions. However, due to the high flammability of hydrogen, designing a specialized fuel supply system for engines is imperative to ensure safety of fuel supplying. One of the problems with high-speed motorcycle engines’ HHO supply systems is backfiring. It is crucial to make sure that the amount of hydrogen left in the intake manifold does not beyond the lower flammability limit in order to reduce the chance of a backfire. The HHO/air ratio and injection pressure both affect the highest concentration of H2 that remains in the intake manifold, or ϕH_port reaches 0.074 and 0.172 at 0.3 bar injection pressure and 7500 rpm engine speed, respectively, meaning that the HHO/air ratios are 3 and 5%. The HHO/air ratio injected into the cylinder must be limited to values of 5.75%, 6.3%, and 6.9% for HHO injection pressures of 0.1 bar, 0.3 bar, and 0.5 bar, respectively, in order to keep the H2/air ratio in the intake manifold below 4%.