When your riding lawn mower is revving high it's often a problem with the governor system that can be a result of a faulty governor spring.
A clogged carburetor, jet, dirty air filter or even a broken linkage within your governor mechanism or a worn out governor component inside the engine could also cause the riding lower mower to rev high.
Another cause of a riding lawn mower revving high is a leaky or bad carburetor intake gasket allowing too much air into the system which throws the air and fuel mixture off.
A carburetor that is not set correctly for the fuel to air mixture ration can also cause a riding lawn mower or other small engine to rev high.
If you've replaced the carburetor with a new aftermarket carburetor the fuel to air mixture adjustment may be off.
I had this happen with one of my riding lawn mowers and couldn't figure why it would always rev high.
Finally I remembered about the fuel to air mixture screw and adjusted it back and it stopped the revving high of the engine.
The fuel mixture screw on a new carburetor is often underneath a seal that you can carefully drill out or use a small punch to knock it out.
Then you can use a screw driver to adjust the screw until it slows down to normal rpms while it's running.
Carburetor linkage that is bent or sticking can also cause the riding lawn mower to rev high as well.
A Governed engine that is revving up and down is called Hunting, and it's always caused by the engine running lean. The carburetor needs cleaning/servicing, or the intake tube is leaking.
Of course, the Spark Plug and Air Filter should also be replaced at the same time to make the engine reliable.