Throttle Balance
On the technical forum, the top post refers to a Maintenance Manual. There are a couple of different versions to download, in PDF format so you'll need Acrobat reader, but very quick to download. The one I used came from the ibmw website. It isn't an official BMW document - it's far more useful than that. It tlaks you through three people's methods of doing it. Read them all before you start. Pretty simple, though.
I used it for exactly wht you want to use it for - with a bit of effort, I managed to balance my throttle bodies far better than they came away from the 600 mile service. And the BMW techie has zillions of pounds of computers to help him!!! You'll need a vacuum gauge of some sort, either mercury or dials.
Patience, and knowing you're going to have to live with the results, can have a very positive impact on fiddly jobs like this.
(I have an R80RT that can be balanced just about perfectly by ear!)
Good luck,
Robin.