As an engine wears the critical tolerances between the rockers and valve tips usually increase and this causes a few problems. They include: hard starting; stiff pull cord; engine oil leaks; lack of power; erratic idle; noisy engine. The gap (lash) between the rocker and valve tip is adjustable and this video covers that process step by step.
Before working on your mower be sure to remove the plug wire to prevent accidental starting, see “Repair Safety Video”.
Valve lash specs are different for all makes and models and on some engines the intake and exhaust lash measurement will differ, check your spec here: