Lean the engine until it starts to die..then richen up again to about half way between the two..ie from when the engine starts to die to fully rich..
remember leaning is better for the engine than running fully rich all the time,saves avgas as well..
if in doubt always air on the rich side...prolonged over leaning can damage the engine..holed pistons etc
all this is if your aircraft doesn't have a EGT probe..PM if you need any more help..
