Woah. This needs sorting quick.
The engine to exhaust seal should be airtight. If not the bike will go massively lean. Hense the 3500rpm.
Using the choke is correcting the mixture after a fashion. Seal the pipe up then check your air filter. That may be partially blocked but you will lose a little go as running lean gives more power. Problem being the cyl runs too hot and pistons melt, (yes litterally). So a slight trade off is needed.
If you keep running with the loose exhaust you WILL BLOW THE MOTOR. Costing at least a rebore.