Motorcycle Repair: Honda elite 150, honda elite, valve clearance

Question
We bent the intake valve on our scooter and don't know why.
On start up it started clicking and then we discovered that air was being pushed out of the carburator. I pulled the head and found the bent valve. I checked the chain and timing. everything was okay. We bought a used valve and now it runs like a champ. How could we have bent the valve and more important how do we keep it from happening again.  Thanks,   Greg

Answer
Greg,

-possible causes for bent valve are
over-revving the engine which does not
give the valve spring time to close the
valve before the piston comes up.

-a weak valve spring

-wrong valve clearance.

-driving through water. Water
does not compress if it gets into
the engine. This is more likely to bend
the rod though.

-excessive carbon build up on the piston and/or
valve.

-dropping dirt or other uncompressible residue in the
spark plug hole.

-very loose timing chain or broken chain guides.

-dropping foreign objects into the carb mouth.

that's about all I can think of right now.