Hi Chaps,
Debris caught behind the reed valve will only cause excessive oil blowout through the oil breather, thus very bad running if you've got the breather threaded straight through to the airbox. It is a pretty reliable little thing, but very easy to replace if anything does go wrong. I've pretty much replaced once a season but have one happily lasting just over 2 years.
hope this helps,
Cheers,
Simon