You've probably got a set of stuck points at the voltage regulator, the coil behind the "F" wire in the regulator assembly. A wire shorted to ground between the generator "F" terminal and the regulator can also cause the problem you're having. Try disconnecting the "F" wire at the regulator and see if the problem goes away. You could also remove the regulator cover CAREFULLY and tap on the points behind the F terminal gently. If the voltage at fast idle settles down, burnish those points with a piece of clean business card or a slice cut from an index card. You can use 90% rubbing alcohol to clean the points, also.
DO NOT FILE, SAND OR USE OTHER ABRASIVES ON REGULATOR POINTS! If you can't clean 'em up with heavy duty paper, they're probably too pitted to save. Since regulator points vibrate several times a second, and they meet with a slight wiping action, sanding scratches simply increase the risk of sticking.
Changing the spring tension on the VR points regulates the upper voltage limit. More spring tension- - - -higher voltage. Remember that the setting will decrease slightly when the cover is replaced, so adjust the top voltage a tenth of a volt or two higher than you want it to be after the cover is replaced. Max voltage against a FULLY CHARGED battery should be 14.5-14.7 volts, measured at the battery posts.
Jerry
