I didn't know why but the idea is very persistent
Changing the exhaust changes nothing in the carburetor
If you are getting into a higher RPM range and flow is more than
The carb can handle. Because of more fuel loss at overlap or whatever
Then you need a bigger, or more than one carb.
If you enriched the fuel to be correct at the excess RPM
It will be wrong for when you get back to normal
One of the nice things about carbs it's an easy swop
Yamaha Warrior 350 Performance Carburetor 1987,1993-2004 | eBay
Oh yeah I forgot to add congratulations you on a awesome tank