Brick cleaning acid works wonders on dirty exhaust headers but an alternative that I have recently found is to use a wire wheel on a bench grinder.
Works really well and then just needs finishing off with some relatively fine wet & dry paper and Solvol Autosol.
Brian
PS Please excuse the messy bench - I got too many jobs "in progress"
Exeter, Devon, UKhttp://www.ttr250.com - The one and only dedicated TTR250 FAQ!
TIP: For easy viewing bookmark the "Recent Posts" view - http://ttr250.activeboard.com/p/recent/
Teesside, UK
96 TT250R Raid
87 Kawasaki KX500