2010 Yamaha YZ250F — Owner's Manual
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Using a digital engine tachometer for idle speed adjustment, detect the engine idling speed by bringing the sensing element "c" of the engine tachometer close to the ignition coil "2". This section is intended for those who have basic knowledge and skill concerning the servicing of motorcycles (e.g., dealers, service engineers, etc.) Those who have little knowledge and skill concerning servicing are requested not to undertake inspection, adjustment, disassembly, or reassembly only by reference to this manual. It may lead to servicing trouble and mechanical damage. The valve clearance should be adjusted when the engine is cool to the touch.
The piston must be at Top Dead Center (T.D.C.) on compression stroke to check or adjust the valve clearance. In order to be sure that the piston is at Top Dead Center, the punch mark "c" on the exhaust camshaft and the punch mark "d" on the intake camshaft must align with the cylinder head surface, as shown in the illustration.