The crankshaft drives an intermediate multifunctional shaft on the side, which in turn drives the coolant pump and the two camshafts via the cam chain, while also acting as a counter balancer. A hydraulic chain tensioner ensures reliable chain tension to achieve highly precise valve timings.
Specifically developed for the 250 EXC-F, the 6-speed Enduro gearbox has relatively short gears from 1st to 5thand an overdrive 6th gear. The transmission is distinguished by easy and precise shifting.
The 250 EXC-F engine can be started with a kick starter as well as an electric starter. Well protected behind the cylinder, the electric starter is connected to the starter freewheel on the ignition rotor via the starter drive in the ignition cover.
Engine Management System (EMS)
The Engine Management System by Keihin with electronic fuel injection and a 42mm throttle body offers a spontaneous response and maximum engine performance. An oil-cooled 196 W AC alternator provides sufficient energy for the fuel injection, lighting and other electrical loads. The efficient ECU offers fast and absolutely reliable data processing. With an optional map-select switch from the Power Parts program, the EMS can allow the rider to select different engine characteristics. In addition, the optional User-Setting-Tool can offer the facility to readout engine data with a laptop and change the ignition and injection mappings with a few mouse clicks.
The 250 EXC-F near enough can guarantee top positions in the highly competitive Enduro series, as it has already been proven with an array of World Championship titles. The construction of the engine shares many base components with the successful 350 EXC-F, and boasts numerous advantages in terms of torque and power. The 250 EXC-F leads the class in performance, thanks to its dynamic yet agile power delivery and an advanced fuel injection system.
The single piece basket of the DDS clutch (damped diaphragm steel) is machined from solid high-tensile steel in one piece together with the primary drive gear. This process guarantees unbeatable reliability and, together with the thin steel linings, contributes to the narrow engine width due to its compact rivet-less design. The diaphragm spring makes it possible to implement an additional rubber damping system inside the clutch hub, which guarantees reliability for the transmission and good traction. Finally, the hydraulic clutch mechanism by Brembo in conjunction with the diaphragm spring offers a very light operation and good controllable modulation of the clutch.
The centrepiece of the DOHC engine is its cylinder head with two overhead camshafts and DLC coated followers. The large, flow optimised ports in combination with large intake valves ensure a considerable flow rate, which in turn provides significant performance. The camshafts have been designed to achieve maximum power and torque figures while maintaining an Enduro specific power delivery.
Cylinder & piston
The short stroke and large bore allow the use of large valves. The lightweight, forged bridged box-type piston guarantees optimum performance and a dynamic power delivery, while the low oscillating mass reduces vibrations.
The 250 EXC-F’s crankshaft has a flywheel that is designed to achieve optimum traction. The crankshaft employs a plain big end bearing. Two force-fitted bearing shells run directly on the crank pin. The necessary oil feed for this bearing is ensured by the engine’s pressure lubrication. This design increases the engine’s long-term durability and lengthens crankshaft service intervals, which is a customer benefit in terms of time, effort and cost.
The pressure the cast crankcase has been designed with significantly reduced wall thicknesses compared to cases produced by gravity die cast. The aluminium alloy utilised has great tensile strength while ensuring high resilience against stone-chipping and other damage.