need help with my cbr 600 98

Ok, I Bought my cbr 600 98 like a week ago and the last owner was a pig, the bike was really dirty, but I already took care of this part...
my bike has a 2 brothers exaust and the bike when I give gas the exaust start to "burp", some friends of mine said that it's bad gasoline, some said that the bike needs a tune up, or my just be the exaust that make this noises and it's normal, well before I do anything I'd like to hear from you guys that always help me out, all my friends said that the engine runs stronge and it's really good still, it has 20.000 miles.
Thanks one more time
Rod

Ps. does the manual of my bike show how to tune up the bike
(cbr 600 98), if does I want to buy one cause my bike didn't come with.

2 brothers is a good pipe unless something is wrong with it it should not sound bad. 20,000 is alot of miles, might need a tune up, check your local honda dealer. hope you have been putting 94 or 93 in it, if not try that octane and see if it helps.

Dont worry to much about the Honda owner book, get a Clymer, Haynes or Chiton service repair manual, you can rebuild the bike with one of these. Alot of times you can find them on ebay cheap.