There actually is a way to make a pretty accurate SWAG at ignition timing using a vacuum gauge, but the procedure doesn't even remotely resemble the urban legend that's promoted here. Disconnect and plug the vacuum advance line at the distributor, set the engine speed to 2,000 RPM or a little higher, and attach a vacuum gauge to the port in the intake below the carburetor (full manifold vacuum). Advance the timing until the vacuum peaks and then begins to drop, then turn the distributor housing back and forth until the highest reading is achieved. That method is MUCH more effective than trying to do the adjustment at idle. Not correct by any means, but better than some of the other methods.
Jerry
