Excessive back pressure can cause loss of power and overheating.
Following up on the previous post by 'P32'. Back pressure due to corrosion or carbon in the exhaust outlet/mixing elbow is often a culprit. The elbow should be checked as part of annual or bi-annual maintenance. Another cause of excessive back pressure can be the exhaust hose or the exhaust outlet. Assuming that things are properly constucted initally something like an overheating episode can cause the exhaust hose to partailly collapse and raise back pressure. Changing trim of the vessel can bury the exhaust port on certain points of sail and increase back pressure.