This paper proposes a novel engine starter system composed of a small-power electric motor and a simple mechanical valve train. The system makes it possible to design more efficient starters than conventional systems, and it is especially effective to restart engines equipped with idling stop systems.Recently, several idling stop systems, having intelligent start-up functions and highly-efficient generate capabilities have been proposed for motorcycles. One of challenges of the idling stop systems is the downsizing of electric motors for starting-up. However, there are many limitations to downsize the electric motors in the conventional idling stop systems, since the systems utilize the forward-rotational torque of the electric motors to compress the air-fuel mixture gas in the cylinders.Our studies exceeded the limitations of downsizing the electric motors by mainly using the engine combustion energy instead of the electric energy to go over the first compression top dead center. The starter system described in this paper consists of (A) an electric motor which can rotate an engine crank shaft in normal or reverse directions, (B) a valve train to initiate the intake valve during the exhaust stroke in the reverse rotation, and (C) a control unit to inject and ignite at arbitrary timing.Then, this engine starts-up with the following: (1) the electric motor rotates the crank shaft in the reverse direction, (2) a fuel is injected to the intake port, (3) a generated air-fuel mixture gas is led to the cylinder from the intake port during the exhaust stroke, and (4) the air-fuel mixture gas is ignited in the expansion stroke. Since the ignition generates the combustion energy to push back the piston, the piston is forced to be fallen down and the crank shaft is driven in the normal direction. At last the piston goes over the first compression top dead center by the normal-direction without the torque of the starter motor. After that it begins the normal operation.The mechanism above can decrease the required time and the required torque to start-up the engine, and it leads to the downsizing of the starter motor, as well as a reduction in the battery consumption. When the mechanism is applied to starter-generator systems for motorcycles, the excess capability of the electric generation can be suppressed because it is not necessary to install the large-power starter-generators.