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Engine does not deliver full power.

Fuel filters clogged.

Replace filters and bleed the engine.

Tank vent clogged.

Pierce / clean the vent.

Air in the fuel system.

This can be a porous fuel hose, but often a filter / bleed nipple is no longer properly sealed. In the event of an air leak, you can use the lever on the fuel feed pump to pump up fuel, if this results in faster starting, then you have an air leak. When bleeding the fuel system, it is also necessary to bleed the high pressure pump and in some cases the injectors.

Air supply to the engine not OK.

Clogged air filter or insufficient air supply to the engine compartment.

Injector worn out.

With older engines, the spray pattern may no longer be good, so that the fuel is not atomized correctly and does not ignite completely. Worn injectors have gray (blue) smoke, this indicates that the injector tips are in the first phase of wear.

Compression too low.

Valve clearance is not good. Worn piston rings / cylinder wall. Burned-in valves, a cause of valve clearance that is not set correctly. Leaking head gasket.

Cooling.

Too little cooling can also reduce engine power. Air leak in the seal of the strainer cover. Water pump defective (impeller is missing 1 or more vanes and or cover is worn too far) or an air leak at a hose clamp. Thermostat defective. Heat exchanger is dirty.

Exhaust pipe.

In case of a narrowing in the exhaust, the engine will not be able to release its exhaust gases and this will result in a decrease in power. Constrictions can occur because the inner lining of a rubber exhaust hose comes loose. This is usually because the hose has been too hot or is old.

Alternator.

With small engines, the alternator can be overloaded with a defective voltage regulator, usually accompanied by black smoke. A too heavy alternator also causes problems with a small engine.

Dirt in the propeller.

The motor cannot lose its power because there is, for example, a piece of rope or a fishing net in the propeller. Hitting backwards can help.