1990 CBR1000, no top end power

Hey guys,
Was wondering if anybody could help with my bike. I just bought it and it has no top end power, seems to do good to about 4500 to 5000 rpm then the power drops off, it still gains rpm but slower and there is just no power, The guy before me said he put carbs off of a different cbr1000, I dont know why other than he said it was easyer than cleaning the stock ones. he also resealed the tank and thought the breather might be clogged, I blew the breather out with air, it did have some resistance but air did travel through it. the air cleaner is new K&N. any help would be great thanks.