I'm about to fit a pair of stainless headers (possibly Motad?) and a free-flowing Rino Racing alloy tailpipe from a Honda CRF (off ebay) to my UK spec gs and am looking for any advice on where to start with rejetting. I plan on keeping the stock air filter unless it isn't possible to get the mixture right without junking it for something else. Does anyone have any suggestions on where to start? I've searched previous threads but most everything involves a K+N or aftermarket filter of some sort. I am doing this mainly to get rid of The Flat Spot at about 5-6k rpm but also to loose several kilos of rust-prone stock mild-steel sh**! Thanks in advance.

PS I believe my stock mains are 115 (way lean?) and the needles are stock at 3rd groove from top.

Message Buddah, he seems to be the jet master around here.

What year is your bike?.....later bikes have pilot, mid main, and main jets.....earlier bikes up to 2001 or so, have only pilots and mains.     Just opening up the exhaust side shouldn't affect your mixture too much, but you probably want to jet to 125's to keep it a little rich.....check the Wiki....

I never understood how they bikes in some countries ran 115 mains.
Anyway, I'd jet it to 125/40 or or 127.5 or 132.5/20 depending on what your bike year is. Yes sorry its pretty confusing. However stock filter+pipe off another bike = standard operating procedure so no problem. That 115 however is a puzzle.
Oh, 115 means its 89-00, so 125/40 ... want a jet pack, 9 or of 10 people from other countries buy from the buddha.  ... naaa not a problem.
Cool.
Buddha.

My 97 Euro spec bike ran 115 stock jets, latter replaced with 125 as suggested here 

Haynes says that in UK( and Europe I guess ) 96 and 97 models came with 115 jets 

Thanks folks. My bike is a UK '99 with 115 mains and 40 pilot as stock. Do you think I should just change the mains for 125s and stick with stock pilots as a starting point? Should I just leave the needles alone? Also what is your jet pack Buddha? Would it suit a UK spec bike? Thanks again for the advice! 

They had the same types of jets, just different size.
BTW didn't UK bikes have 40 pilots.
My pack has washers, float bowl bolts, mains, and pilots.
If you got 40 pilots there is no reason to have just the mains sent all the way from US. Just buy the 125's somewhere over there. Large round slotted mikuni's.
In some places the modding of bikes are illegal, so evidently they dont sell them.
Cool.
Buddha.

I'll swap the 115s for 125s and take it from there. Thanks again for the help.