Can you explain the problem more accurately.
If you have criticized other muffler, it is sufficient for regulating the air / gas mixture to adjust a little differently.

Hello,

Yes, it seems that You need bigger nozzles. You should get them at any good Yamaha Dealer.
The original nozzle's size is written on it (very small numbers). Be aware that the rear cylinder's nozzle is slightly bigger than the one for the front cylinder. Talk to Your Yamaha Dealer and describe the problem. He will only be able to make a guess, what size could possibly fit and You have no other choice than giving it a try. It's a bit like digging in the dirt, as long as You don't have someone around who has the equipment for testing and adjusting properly. May be that You have to try different sizes of nozzles until You have a pleasing performance. But this can be done.

Good luck,

Stefan