As far as I'm concerned, when your OEM exhaust is so quiet that you could almost forget it is equipped with an internal combustion engine, that's a problem. Not to mention, I don't know about you, but I like to rev match my downshifts and a more free flowing exhaust can increase throttle response which does what? yep, you got it, makes rev matching an easier affair, the OEM exhaust creates far too much back pressure for my liking.
So why is jetting required? is the engine that lean? from factory?
Not trying to put you down or blow you off, Discotrash, but . . . instead of "saving lives," loud mufflers close trails. IMHO.
"Free flowing exhaust?"
"Increase throttle response?"
"Too much back pressure for my linking?"
Discotrash, your meaning of these phrases would be necessary for a meaningful discussion of the relationship between the exhaust system and carburetor jetting.
Please disregard my previous post on this thread.
Haven't looked, but bet Googling "carburetor jetting" reveals an article or two on the subject. "Care and feeding of the CVK40" explains how the jets work in your carburetor.
You don't "need" a muffler at all, if you just want to make noise. I believe you'll offend more greatly from noise pollution than you are offended by your KLR250's quietness. Yet, your call, do what you must.