How do you adjust the fan belt tension on a 1969 karmann ghia?

The pulley on air cooled VW's has shims built into the design of the small pulley mounted to the generator. If you loosen the nut on the generator pulley, you will find that it splits apart, allowing shims to be placed between the pulley halves so as to effectively change the diameter of the pulley by changing the amount of space between the pulley halves. If the belt is too loose, remove a shim, try again. If the pulley is too tight, add extra shims, they are normally "stored" just under the nut and dished washer on the outside of the pulley. Hope that this helps. LL