Vacuum Advance is needed to adjust the distributor timing in relation to the load/no load put on it in increasing or decreasing acceleration of rpms of the engine. A vacuum mechanism is usually attached to the distributor in a way that it moves the breaker plate, usually with a vacuum line from the carburetor.It will advance the timing on acceleration and retard it on deceleration such that the plugs will fire at the correct time during compression/power stroke resulting in the best performance/economy of the engine at all speeds.Of course all said, it's not a perfect world so timing adjustments have to be made or at least checked periodically. And if you add perfromance options, well that's even more issues.That's why many,many people are switching to newer electronic ignitions making all this easier and simpler-NO POINTS !
Regarding the "large knob" on 235's: It should be filled with chassis grease(lithium is ok) and turned in periodically.In my case whenever I think of it.I don't know a time frame for it.If you remove it you'll see it contains a small stiff tension spring with a bakelite plug on the end that rubs against the distributor shaft.It all acts as an upper lubricant device for the distributor shaft and really doesn't need alot of maintenace.Just keep it filled.Sorry this took so long to explain.Hope it helps.
