An　experimental　study　was　conducted　on　a　S.I.　engine　fueled　by　compressed　natural　gas　and　hydrogen　blends　(HCNG),　in　order　to　test　different　engine　operating　parameters　that　affect　lean　combustion　limit　(L.C.L)　of　HCNG　engine.　Firstly,　constant　ignition　timing　and　ignition　timing　under　maximum　L.C.L　(L.L.T)　conditions　were　compared,　then　L.L.T　conditions　were　adopted　in　this　paper.　The　results　indicated　that　for　each　condition,　neither　over-retarded　nor　over-advanced　ignition　timing　are　advised　in　order　to　achieve　leaner　combustion.　L.C.L　increases　with　hydrogen　fraction　in　the　blends,　and　slightly　increases　with　throttle　opening,　while　decreases　when　the　engine　speed　is　raised