Yamaha TZR50 Engine howl up to 14'000 rpm, 50ccm

I am howl up the engine for this Video. I make a stop after driving 60 kilometers.
The engine was on the normal temprature. I let the engine after makeing the video on that the waterpump still keep cooling the engine.

I dont know why the horn is beggin to shout when you are going more than 13'500 rpm.