The advantages of air assisted direct fuel injection systems to achieve high atomization degree into the combustion chamber of SI engines are well-known. The solutions up to now proposed appear anyway poorly tailored to be suitable for small engine applications. In fact scaling down such existing systems for automotive applications, they present mainly two drawbacks: the costs and a difficult tuning of the very low quantity of fuel required per cycle. Moreover the amount of electric energy required makes the engines not self-sufficient. To overcome the above mentioned problems, Piaggio has developed a completely mechanical low cost fuel injection system, named FAST (Fully Atomized Stratified Turbulence), which does consent a very atomized and stratified mixture lean combustion process, i.e. a dramatic improvement of emissions and fuel consumption. After general considerations, the application of such system to a small capacity 2T engine is analyzed. Particularly the lean combustion capabilities, that remain no critical over a wide range of global A/F ratio and for various engine load requirements, are evidenced. The practical results for a 50cc scooter in real field conditions fully confirm the validity of the proposed solution that appears a good candidate for ULEV small SI engine applications to comply with the future requirements in terms of emissions and fuel consumption but still maintaining high output characteristics and low costs.