Cerbin,
Those Webers on your engine are 32 years old. Typically, the clearance in the throttle shafts wear to a point they allow air to be drawn in and create a lean idle/part throttle mixture.
With the engine idling, press each throttle shaft (in turn) off axis and see if the idle changes; or squirt some carb cleaner (propane works too [unlit]) at the throttle shafts and see if it affects the idle speed. If so, you have leaking throttle bores and they will need to be rebuilt by someone like Eurometrix. BTW, don't discount the other suggestions provided for you.
Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com