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Engine changing speed.

Engine speed will automatically increase for a short time.

Air in the fuel system. Leaking diaphragm in feed pump. Fuel pump leak. Dirty filters.

Engine runs irregularly.

Air in fuel system. With larger engines (OM363, DAF575 for example) the fuel has a separate crankcase where engine oil has to be put in, if this is not there the pump and regulator will work more stiffly and wear out faster.

Engine runs wild.

Crankcase breather mounted on an air filter should be regularly checked for contamination. With a heavily soiled air filter, oil can be sucked out of the crankcase when the crankcase breather ends in the filter.