What To Watch For
Problems associated with the GS?
Yes, there are...
Most of these problems stem from owner neglect. Since the GS keeps on running no matter what you do to it (or don't do to it), people tend to forget routine maintenance. If you're out to buy one, look for signs of neglect. These include: black engine oil, low oil level, seepage through head gasket (manageable) or barrel gasket (expensive and time-consuming!), a noisy crankshaft (terminal!) a noisy gearbox, a noisy camchain... you should have got the picture by now. This may or may not be a reason not to buy it; much will depend on your mechanical skills and your willingness to work on the bike yourself. Most GS's are quite easy to work on.
And then there's the problems with the electrics. Most of these can usually be explained by the fact that these are usually old bikes, which have been tampered with over the years.
There is one serious problem that probably affects all GS models: the charging system. This problem, usually ascribed to the stator, is in fact caused by the regulator/rectifier. You can read more about this in "The Stator Papers".
This problem is so serious and so devastating that it contributes greatly to the GS having a very bad name as far as electrics is concerned -- as someone said "with an electrical system so bad that it makes a Yugo look good".
In general, this statement overdoes the case. The electrics I've seen on non-tampered, garaged bikes are quite good, and in particular on the GS1100E that I bought when it was 14 years old, I've seen some wiring and fittings which have withstood the test of time quite well. Mind you, if you buy any classic GS other than the shafties, which has been carried through until 1986, it's likely to be at least 13 years old even as we write this, and on any bike of that age wiring and connectors are prone to suspicion!
Other than the electrics, there's nothing specific to watch for that you shouldn't be aware of when buying any secondhand.
One other area to watch for is the fuel tank. Many times the bike may have been sitting for a long time with the tank partially filled or even empty. Moisture attacks the unpainted steel in the tank causing it to rust on the inside - particularly at the seam between the sides and the bottom of the tank. So take a look in the tank paying close attention to the bottom seam (it looks like a crimped area). Take the gas cap off and look inside the tank for any possible evidence of rust. Hopefully, after all this you will have a tank with no major rust issues.