Stihl MS 251 Instruction Manual - Page 19
Engine, Fit new piston rings - 18
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3.7 Engine Always check and, if necessary, repair the following parts before looking for faults on the engine: - Air filter - Fuel system - Carburetor - Ignition system Condition Cause Engine does not start easily, stalls Oil seals in engine damaged at idle speed, but operates normally at full throttle Engine pan leaking or damaged (cracks) Intake manifold damaged / bore blocked Remedy Replace the oil seals Inspect engine pan, re-seal or replace if necessary. Clean bore or install new manifold. Engine does not deliver full power Piston rings worn or broken or runs erratically Muffler / spark arresting screen carbonized Air filter dirty Fuel hose kinked or torn Intake manifold damaged / bore blocked Fit new piston rings Clean the muffler (inlet and exhaust), replace spark arresting screen, replace muffler if necessary Clean or replace air filter Fit new hose or position it free from kinks Clean bore or install new manifold. Engine overheating Insufficient cylinder cooling. Air inlets in fan housing blocked or cooling fins on cylinder very dirty Thoroughly clean all cooling air openings and the cylinder fins Air inlet in fan housing dirty Clean air inlet on fan housing 18 MS 231, MS 231 C, MS 251, MS 251 C