A new concept of mathematical model of gas flow through reed valve fitted to a small two stroke petrol engine with inlet port controled by piston was performed. Load and movement of reed petals, distribution of pressure and filling the crankcase by assumption of unsteady turbulent gas flow m an inlet pipe were determined in detail. The movement of a reed petal is solved by a more complete system of partial differential equations in forced vibration of plates. The results achieved from the simulation were verified by the experiments conducted on a real power-driven two stroke engine. The pressure measurement in an inlet duct and a crankcase helped to compare the computer simulation and tests results. The paper covers a theoretical model with mathematical equations system and their solution. It gives some diagrams of pressure, valve lift, velocity and delivery ratio obtained from calculations. Then the experimental work on a 125 cc one cylinder, two stroke engine, research stand and results are described. The comparison of the results from simulations and tests for the different geometrical dimensions of the system confirm the correctness of the mathematical model. A good correlation between the theory and experiment gives the possibility of the application of a more complete simulation model of the crankcase delivery through the reed intake valve in all types of two stroke engines. This work is a continuation of the same studies which were conducted earlier but presents some aspects of a gas flow through a reed valve, load and motion of the petals and pressure distribution, not mentioned in the previous papers. It referes to all types of two stroke engines with a fitted reed valve, in particular to engines with a piston-controlled inlet port used in motorcycles, agricultural and marine equipment, small vehicles and others.