4 stroke 50cc scooter makes loud buzzing noise at 55 KPH and won't go over 60KPH?

I have a fairly new 4 stroke 50cc Honda scooter. I recently put in a new CDI hoping to increase performance. It helped a little bit, but when I go over 55 KPH I start to hear a high pitched buzzing sound and the bike won't go past 60. Even on a flat surface.

I'm not sure what the problem is. Does my bike have some kind of limiter? Is it the pulley or muffler? Or is 60 kph the fastest my bike will go?

Thanks!

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