[Rendoz]

When I bought the bike, 08 250R, the previous owner told me that he installed a jet kit on the bike but never jetted it. I didnt fully understood what he meant by it. He also mentioned that it could be running rich, which I think it does because it backfires like crazy . So what do I do now? Do I need to get it tuned or something? Also, the bike came with a Yoshi slip on.
I have zero experience with motorcycles, this is my first bike

 

[Rendoz]

It backfires all the time, on easy decel its sounds like its the exhaust is bubbling and especially loud on hard decel. I think he just did it himself in his garage. Its a Dynojet kit and he didnt give me any stock parts