Vacuum advance retard question

[GreaseMonkey]

While I am fiddling with the 280SE and you are all being so helpful, can anyone explain how the vacuum advance/retard should work on my '75 model M110 dizzy?
The static timing is correct by the book, and I run it on 98RON petrol, but I still get knocking under load.
I wondered if the vacuum diaphragm was leaking and the mechanical advance is going too far.
Anyone know how to test this?
I may have a vac leak somewhere, and have not yet got around to checking why the central locking is stuffed, another job for another day, but could that be related?
Also, I thought to swap with the dizzy from my 1980 280E parts car, but noticed that has two vac lines either side of the diaphragm, where the '75 model only has one.
Anyone know the difference, how, why and when they changed?
Once again,
all help appreciated.
Chris M.

 

[John S]

When checking the static setting, did you turn the engine forward then back to activate the points? If you did that the system is not completely accurate due to the slack in the timing chain so you may need to check it again.

Did you also check the timing with a timing light at idle and then rev to see if the advance is smooth?

The 1980 dizzy probably has vacuum retard at idle, as the antipolution changes were made in 1976.

 

[GreaseMonkey]

The timing chain and tensioner have just been replaced and all is mechanically tight, but I will check the advance through the rev range.
Do these distributors have a pair of springs and weights to control mechanical advance like the old standard system?
Maybe there is the problem?
Chris M.