Brain Teased,
I usually replace the orings in my engines once a season, but it depends on the use of the engine. Also if you spend alot of time tweaking the adj. screws it is possible for the o-rings to tear.

If your adj. screws wander from vibration, then I would say you definitly need to change your o-rings. Just be sure to put a little oil on them when you install them so they have a little lube going back in or you might catch the o-ring and tear it.

BK