Reeds perform the same function as the intake valves in a four-stroke but are a lot simpler and don't have an actuating mechanism. They are simply flat pieces of thin metal or fiber material that allow air into the cylinder but not back out. They are located in the intake tract between the carb and the cylinder. If you remove the carb and look down the boot towards the cylinder, you can see them. I'm not sure of the symtoms of bad reeds, but if they haven't been replaced recently, I would do it. I noticed a difference when I replaced the aftermarket reeds in mine with new ones of the same type. They will cost about $25 and are easy to replace.
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'85 ATC250R
.030 Over Wiseco
FMF Pipe/Answer silencer
36PJ Keihin
K&N
Boyesen reeds
Magura hydraulic clutch
Roost Boost
MSR bars
OMF swingarm skid
ITP Beadlocks
Three Wheelin' Since '84