Can any of you explain the Lucas dizzy centrifugal advance mechanism to me?

I've checked my centrifugal advance mechanism. The rotor arm can be easily and smoothly rotated counter-clockwise what appears to me to be about 10-15 degrees. I didn't break out my protractor. It snaps back smartly to its normal position when released.

What I don't understand is why it's movement is counterclockwise. I must admit that I'm assuming the dizzy spins counter-clockwise in the XJ-S. I haven't turned it over without the cap on. The vacuum advance rotates the pickup clockwise around the inductor rotor to advance its position relative to the rotor so I assume that I'm correct on dizzy rotation.

So why is the centrifugal action to rotate the rotor arm counter-clockwise? Logically the action at speed will be to rotate counter-clockwise further with increasing rpm, retarding the timing. ???

Im sure someone will straighten me out but I'm surprised I haven't found this detailed in all the literature on the subject. I know the way to true understanding lies in taking one apart and reassembling it.

Bonus round! Video question: How many cylinders are misfiring?

 

The dizzy turns anticlockwise. When the advance turns the rotor in the same direction (anticlockwise) the spark (ie the rotor in the dizzy that distributes the spark to each cylinder) arrives at a given point (in this case the post in the dizzy cap for, say, cylinder A1) EARLIER than it would if the advance had stayed in the same place. A bit like advancing your watch hands gets you to opening time faster!


Advancing the ignition point means that the spark ignites the petrol in cylinder A1 at a rotational point earlier before TDC than it otherwise would on the compression stroke. Say 22 degrees BTDC instead of 15 degrees BTDC. This is because the flame-travel-time to burn the petrol effectively does not alter as the revs increase, so an earlier (ie advanced) spark means there is adequate time for the petrol to burn to give more optimum power and efficiency at those higher revs.
Greg
I would say from several thousand miles away that none of them on that bank are!

 

Thanks Greg! I think I see where I was getting confused now. Centrifugal advance pushes the rotor forwards to deliver spark to wire sooner, while vacuum advance is pulling the pickup clockwise to be triggered sooner.

Al makes sense.

Bonus round follow up:
Really? You don't think that idle is missing a cyl or two? Other side is nice and steady with no poofs.

 

Nearly, the Vacuum capsule on the Lucas dizzy also advances the rotor on the V12, in fact I am not sure there are any advance weights in the Lucas dizzy! The vac capsule is just lever-powering mechanism; it is just a matter of which side of the fulcrum the vac capsule is hooked up to.
Greg


Bonus round: I posted that NO cylinders were firing, not all of them; but there is some misfire/irregular fire that is for sure! new plugs, HT leads, rotor ?

 

Here is some info on the subject:
https://www.jaguarforums.com/forum/x...timing-160787/

 

Bonus round final answer!

One. Piston B4 is not firing in the video.

I am pleased to have solved this puzzle!

 

HT leads is correct. One lead completely failed and couldn't carry a spark. This amazes me. How can it NOT conduct? My finger literally conducts better than it can.

The HT leads routing is really poor. I found the updated routing and Its better but there's just a ridiculous amount of conflict and way too much opportunity for leaking.

I'm going with some custom cut Taylor wires in a larger diameter with straight distributor boots. Going out the top of the dizzy where there's plenty of room to keep them neatly separated and be able to choose a routing rather than be forced into it.

Now, back to the advance mechanisms! I just don't understand what I've quoted here. I've pulled the cap, applied vacuum to the vac adv and watched it rotate the pickup clockwise around the stationary timing rotor ( 12 pt star ); advancing the timing. I understand that other models reverse this action and retard the timing. I've also pushed the rotor counter-clockwise against the springs which I now understand advances the timing.

I don't follow how you're suggesting that the vacuum is somehow involved in moving the rotor. I havent seen them yet, but there have to be weights down in there somewhere that pull against those springs when you spin em. Please help me understand what you mean. Sorry if I'm being dense. Maybe another cup of coffee will help.

 

I am not familiar with this distributor. But, all are somewhat related.


1. Determine if yours uses both vacuum and centrifugal force to control timing. Many variations have been used in other critters over the decades.


2. Find an old tech GM or similar point and condenser distributor. A teaching/learning aid so to speak.


3. Rotors are firmly fixed to the distributor shaft. It in turn is driven by the engine, usually off the cam in some, but not in most OHC engines.


4. The plate with the points or conducter pickup do rotate move just a bit. The vacuum can pulls it. Advance in some, retard in others.


5. A bit fuzzy here. Need that teaching/learning unit. Centrifugal force throws the plate in slots, controlled by springs. Spark come earlier.


I'll stop here.


Carl
and

 

My '86 Lucas dizzy has both vac advance and centrifugal advance.


I've pulled many distributors apart. I'm familiar with the centrifugal advance mechanisms used in US and several foreign makes. I was just trying to follow Greg's post and it didn't click for me.