The in-cylinder flow in the pentroof chamber of a four-valve. single-cylinder, spark-ignition optical engine was quantified by laser Doppler velocimetry(LDV) during compression and correlated with combustion development especially under lean mixture conditions. First, the flow characteristics near the spark plug were quantified under motored conditions at engine speeds of 1000 and 2000rpm and then the cylinder head was modified with sleeves introduced into the two intake ports which generated a stronger tumble motion. The measurements of the flow by LDV confirmed that the modified intake ports with sleeves resulted in 100% higher mean convective velocity (~2Vp, Vp=mean piston speed), and 30% higher turbulence levels (~1Vp) near the spark gap at the time of ignition relative to the unmodified ports. Combustion development was monitored by a pressure transducer and the flame propagation was visualised by a standard and a high framing rate image intensified CCD camera which allowed comparisons to be made of the initial flame images generated by the sleeved and non-sleeved port configurations and their cycle-to-cycle variations under lean burn conditions.