to add on, this so called valve is the "power" valve for 2
stroke engines, located after the exhaust port. not to be confused
with air-intake valves like boyessen reeds.
4 stroke engines have an exhaust valve that closes completely to
allow compression of the air-fuel mix as it enters the engine. 2
stroke engines have no exhaust valve to do this, only an exhaust
port, that is open and closed as the piston goes about its cycle.
it's a little hard to visualize lar...
2 stroke engines compress the air-fuel mix by use of back-pressure
from the combustion wave. at low revs, the power valve is "closed",
but in fact it is only half-closed. this helps charging of the new
air-fuel mix for the next engine revolution. it is at this time
when carbon deposits (due to incomplete combustion and 2T)
accumulate on the valve. given that the rider doesn't "open" the
valve every now and then, the carbon gets caked up, and jams the
valve. aggressive throttle action does help prevent this jam, but
doesn't prevent carbon altogether.
race bikes can afford have open valves at all rpms because race
riders maintain a high rpm most of the time. an open valve provides
the maximum power (among other things working together), so they
get the best out of their machine at all times. for us, it's not
practical because we have traffic lights. 
honda's RC valve (the one on SP, dunno about other models) works by
rotation, so it's not so easy to break lar, as compared to
aprilia's power valve.
hope this helps...
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rock hard... ride free...
