Brian S

Brian S

Well-Known Member
I have an M reg 3,9 efi Discovery

a few months ago I changed the Vacuum advance unit on the distributor.

I thought all was well, but recently when I reset the timing I did it with the vacuum advance pipe off as instructed, but after I fitted the vacuum pipe it made no difference to the timing. Is this right? I would have expected a little change.
 
It will only work when the throttle is opened, and then only a small amount unless you really bang it wide open. It will then return back to where it was, as the centrifugal weights take over as the engine rpm increases.

It works on changing vacuum, and on steady-state running, doesn't do anything much for you.

Think of it as an accelerator pump for the ignition.

Peter
 
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You can test the vac advance unit by sucking on the pipe going to the adavnce bellows. You should see the base plate rotate slightly.

Or using a timing strobe gun, sucking on the pipe should result in the ignition advancing.
 
So when idling there is not enough vacuum from the normal point to make a difference, but if I suck on the tube it should change the timing a little.

Cheers guys I'll give it a try.
 
Brian S said:
So when idling there is not enough vacuum from the normal point to make a difference, but if I suck on the tube it should change the timing a little.
Click to expand...

That's about it, the device works on 'changing' vacuum rather than 'steady state' vacuum.

Peter
 
On a similar point - trying to test the vac unit and on my D1 3.9 i can draw air into my mouth when sucking on the vac advance / retard unit's pipe, its a very long time since i had a car with a distributor and i carnt remember if this is correct or if it indicates a punctured diafram ?? ANy clarification appreciated! Ta
 
What confused me was:

"the device works on 'changing' vacuum rather than 'steady state' vacuum."

Which lead me to think there might be a "bleed" of air to allow the unit to reset to nil when a steady state vacuum was present????
 
Can anyone clarify for me about this sucking air issue on the vac advance??

If a steady state vacuum does not activate the advance - only an increase in vacuum then is there an air bleed to the "vacuum" side of the advance unit to allow compensation??? and if so is this the air i can suck into my mouth or is the unit knackered - if not how DOES it work only on change in vacuum state??
 
theres no air bleed just diaphragm moves to amount of vacuum created by engine the greater the speed the greater the vacuum,if you can keep sucking air its broken
 
Brill - thats what i thought, but obviously i did not want to replace a unit that was supposed to allow a a little air to bleed through and therefore ok! Thanks!
 
No, there's no bleed, the vacuum advance works at speeds not that much above tickover, you'll see if you look carefully at the inlet manifold that the takeoff point for the vac. advance is just downstream of the throttle butterfly (closer to the air intake), at tickover it has no vacuum applied to it, just above tickover and you will get full advance, much above tickover and the throttle plate is too far open to pull any vacuum on the vac advance port. It is designed to help low engine revs fuel economy.
 
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RobCavendish has hit the nail right on the head.

Its only purpose is to advance the ignition when cruising at part throttle, for economy.

The difference it makes is noticable in a road car, but will cause more problems than it solves in a motor that is used mostly off-road, in which case the baseplate inside the body of the distributor is best welded to the body with a few tacks, and will stop any unwanted fluctuations in the ign timing.
 
funnily i allways believed advance was there to counteract the costant time fuel burn takes with increasing revs ie burn has to start earlier the faster the engine goes if its going to be complete
 
You're also absolutely correct.

This is taken care of by the mechanical advance (bobweights) further down inside the dizzy. These rotate the triggerwheel a few degrees around the main shaft that its mounted on, as rpm increases, so as to pass the pick-up sooner.

The vacuum capsule moves the plate that the pick-up is mounted on, and is only there to improve economy in the cruise.
 
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