On these Engines, all the Timing Advance Occurs with next to no opening of the Carbs, the last bit of Throttle Lever movement is where all the GO GO happens. The MerControl of the Era wasn't very Linear and helped increase that Perception.
The Throttle Cable Length is adjusted after the Idle Speed is Set. So whenever the Idle Speed is adjusted, the Cable has to be unanchored, once it the Speed is correct, the Throttle Cable Length is adjust to put a Slight Preload against the Idle Screw Stop. Too much Preload and the Engine becomes hard to shift in-out of Gear, too little or none, and the Idle Speed will be Inconsistent.
Now you can pay attention to one the Arrows in the Pic. The yellow one points to where the Throttle Cable is Anchored, and Adjusted. The Red one is, on later engines to set the Idle Speed. On Yours, it is set up my the Distributor
