Do you have a budget in mind ?
And would you buy off e-bay, as theres allways some @ on there.
RD04 is one to go for if your handy with some spanners - better built for offroad, but due to age of them, will probably need some maintenance before your trip - otherwise any RD07 would be a good buy.
Things to watch out for - corroding rims on the early bikes, rotting exhausts, fuels pumps, regulators/rectifiers.
all bits that can be easily replaced - but watch out for the RIMS as some are made of cheese

.
If your doing 2 up, then maybee an aftermarket or re-built rear shock - thats the last thing you want to break if youre in the middle of Africa.
Just my 2p's worth - you'll find plenty of great info on the site, and i'm sure others will follow with more/better advice

Bookmarks