This is caused by the engine being overloaded – being asked to produce more power than it is capable of – because:
1. The propeller has been fouled by a rope or other debris, fishing nets etc.
2. The propeller is too large for the engine.
3. The exhaust is restricted, perhaps flattened by equipment as it passes through a stern locker, for example.
4. The injection timing is incorrect. Check that the bolts holding the high pressure pump to the engine are tight.
5. The engine is starved of air. Check that the engine compartment vents are not obstructed and the inlet filter on the engine is clean.
