For an engine to run well, all it needs is good compression, the right supply of fuel, proper intake and exhaust flow of fuel, and a spark to start the combustion process. Over time, these ingredients get compromised which ultimately leads to power loss. This process is explained by Jason Fenkse in his video below:
According to him, exhaust system restrictions, for example, clogging of catalytic converter may be caused by a rich fuel mixture and may lead to increased pumping losses. Also mentioned by Fenske is a stretched throttle cable, that could be a potential cause of an engine’s loss of grunt.
On the other hand, clogged fuel injectors that get worn out over time and are not able to send enough fuel at high speed could easily be a reason of engine’s short life.
An engine can also lose compression due to poor valve sealing. Carbon deposits compromising the seal between the valve and valve seat can be a reason for this loss of compression too. Last but not the least, degraded or fouled spark plugs and deposition of carbon on pistons causing hot spots may also lead to loss of power of the engine.