PW80 stumbles at high rpms

I have a 2000 PW80 and I have installed a new carb on it and it starts and idles great but when you are riding and get on the gas it stumbles as the rpms come up. I have tried new plugs and have cleaned the air filter. What would the factory settings be for the carb? Do I need a new size jet or change the needle setting? Any help will sure be appreciated.
Thanks

It runs ok if you just let it take its time to get up to speed on the street but tahats not where its normally ridden. I also did install a lawn mower muffler on it at the end of the stock pipe just so it wouldn't be so loud and the lawn mower piece is very open and does not appear to be very resrictive.
Thanks