Unstable combustion and high cyclic variations of the in-cylinder pressure associated with low engine running smoothness and high emissions are mainly caused by cyclic variations of the fresh charge composition, the variability of the ignition and the fuel mass. These parameters affect the inflammation, the burn rate and thus the whole combustion process. In this paper, the effects of fluctuating fuel mass on the combustion behavior are shown. Small two-stroke engines require special measuring and testing equipment, especially for measuring the fuel consumption at very low fuel flow rates as well as very low fuel supply pressures. To realize a cycle-resolved measurement of the injected fuel mass, fuel consumption measurement with high resolution and high dynamic response is not enough for this application. It is also very important that this measurement technique can cope with fluctuating fuel flows and does not influence the combustion and hence the engine performance - due to the pressure drop over the flow meter. To analyze the effects, the cycle-resolved measurement of the fuel mass was simultaneously adapted with ion current measuring techniques on a small two-stroke SI engine and a correlation between cycle-based variations of the fuel mass and the combustion behavior was performed.