Generally the problem with most engines occurs at the far end of the needle valve.

Bad fuel, cheap fuel, not enough oil in the fuel, dirt in fuel, moisture in fuel, bad tank assembly, low tank installation, hole in fuel lines, and even filters in fuel line can cause untold problems.

Too much prop, props that don't track, crappy spinners, unbalanced props.

Sucking up dirt, running without cleaning after a muffed landing, not running engine dry at the end of the day.

Varnish, running lean, loose backplates and heads.

Did I mention that most problems happen at the needle valve? Quit running your engines so lean. If you need more power, find a better prop or put a bigger one in the plane.