Reed valves are the most common method used to control the intake of fresh air and fuel into the crankcase of a high performance two-stroke engine. While they can be quite simple in terms of mechanical design, their operation is highly dynamic and can be influenced by many other components.Previous publications from The Queen's University of Belfast have shown the derivation of mathematical models and their verification by measurements from a firing engine at relatively high engine speeds, up to 9,500 rev/min.In this present paper, measured and predicted data for delivery ratio and reed tip lift are presented for a 125 cm3, single cylinder engine over a range of speeds up to 12,290 rev/min. Steady flow discharge coefficients are measured and used in the mathematical simulation. Four variations of reed valve material/thickness are investigated in the firing engine. Correlation between measured and predicted delivery ratio is good over the speed range and various reed specifications. Correlation for reed tip lift behaviour is reasonably good, considering the highly dynamic operating conditions. Overall, the enhanced simulation model can be used to determine an optimised reed specification with a high level of confidence.