The aim of this paper is analysing of performance parameters and emission values of an SI engine fuelled by acetylene-hydrogen and acetylene-methane blends. The experiments were carried out at a fixed bmep (brake mean effective pressure) of 2.095 bar, a load of 30 Nm and an engine speed of 1500 rpm under lean mixture conditions (lambda=1.3-2.8). Moreover, the flow rates of the blends were changed as volumetrically. The parameters of brake thermal efficiency, specific fuel consumption, cylinder pressure, heat release rate and emissions were analysed for each fuel blends. The experimental results showed that the values of specific fuel consumption are declined between 18.5% and 20.1% by hydrogen addition and raised about 64% with methane addition in the blend. The values of brake thermal efficiency are declined between 6.2% and 3.3% with the addition of hydrogen and are decreased about 10% by the addition of methane in the blend. The curves of cylinder pressure and heat release rate are advanced to top dead centre by the adding of hydrogen to acetylene. But the curves are retarded with methane addition. The adding of hydrogen in acetylene leads to a decrease in CO and HC emissions and an increase in NOx values for fixed lambda. But the increasing of methane fraction causes a rise in CO and HC emissions and a drop in NOx values for fixed lambda.