I'm adapting another exhaust to my 650l, its a stock muffler from another bike so it's not extremely open, but more free flowing than the goofy looking stock thing. I'm not changing the intake for now...is it going to be likely that I'm going to need to re-jet? if so which direction will I be going, slightly larger main? I haven't done it yet, I just want to be prepared when I do, I don't want to bolt it on then have it runnin funny while I order jets..

Best I can say is when I put a CRF250X Muffler on my XL600 I had to go up in the mains a couple. Mines a twin carb so I don't know whats normal in your single carber.In the 600 I went from I think about 118's to 122's in both carbs,, It's a little rich but I can live with it. Thats all I changed..still starts and runs fine with the pilot jet and settings on the needle at stock. Mississippis about sea level isn't it so I'd say you'd be in the same boat as me in a Tsunami..

If you follow the uncorking procedures posted here (search "Dave's mod" and you'll find a ton of info) and other thumper boards, you will end up opening up both your intake and exhaust. Opening up the exhaust without opening up the intake is counterintuitive... gotta do the intake as it's mostly FREE! :bonk:
At any rate, Stock XRs are jetted waaaaay lean from the factory, so you will definatelly need to go larger on the main and pilot jets. :ride:

thanks, I'll look into doin those Daves Mods this week.

what's common as far as simple intake mods? just removing the snorkle and using a more free flowing filter like K&N or UNI?

yep thats all you need for opening intake, drill the 2 rivets out. and the UNI is good cuz its a great filter, way better than stocker. if you open up intake i would say daves mods jettin will work well, 55 158