JS60 suddenly runs much higher RPM

While mowing, all of a sudden engine surged to a much higher RPM like a race car and stayed running that way.

Was like cranking up the throttle, except it doesnt have a throttle.

It's 14 years old but well maintained, and running perfectly up till this happened.

I checked air filter, oil, plug - all okay.

Started it again today and same thing.

Any ideas?

Thanks!