I have a 7518 installed in my table and have spun bits from 1/8"OD to 3-1/2"OD (Marc Sommerfeld's big panel bit). My observation is similar to yours...
When I turn on the router, it fires up (relatively) slowly, goes a bit above the "set" speed and then slows to the right speed. When the bit is small it can go over the set speed quite a bit but when it has something resembling a load (i.e. a big bit to accelerate) it just goes over a little bit (10% or so). So, with it set to 10,000 rpm it might go to 11,000 rpms before slowing, but no where near 21,000 rpms.
BJ's right though. If it doubt, take it to the PC shop. A large piece of sharpened steel, spinning at 21,000 rpms, would be nothing to mess with.
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