Thread: What does it do?

AFAIK, the vacuum advance, does, or should do exactly what it says,,
as revs increase the "vacuum advance" pulls on the distributer, increasing the sparks angle of advance.

basically more revs, more advance on the distributer,

more advance on the distributer means the plugs fire earlier in the cycle,,

thats where the "6 degrees Before Top Dead Centre" at idle can change up to what? 32?

it can get very complicated,, flame paths is an interesting subject though,,

Close Pedro, but not quite right. As the vacuum is increased, the ignition advance is increased. It is not related to engine revs.

the main thing you need to know........is.........

if the vacuum advance unit is faulty.......you will use more fuel than you need to.......about 4-5 litres more per 100kms to give you an idea........

if your cruise control isnt working......it may have a vacuum leak......this wont help either.......




and.....vacuum leaks shouldnt be ignored......they can lean out individual cylinders enough to blow a head gasket or worse.......due to the higher temps from running too lean.......

so,, what increases vacuum?

There is max vac in the plenumn chamber ( where the vac advance connects) when the throttle body butterfly valve is closed. This is when you want the timing to give you best economy.

When you plant the foot ( wide open throttle), vacuum at that point is close to zero, and the timing is adjusted to give you best power.

A faulty ( leaky ) vac advance will also stuff up your engine idle as there is an uncontrolled vac leak.

Plenty of places do reco units for around $65 - Carb and Gas in Wooloongabba are one

The 63 Falcon I used to own had vac operated wipers. The speed of them could be controlled by your right foot on the throttle . Put your foot down and the wipers slowed almost to a stop

Bruce.

So how do you know it's faulty other than using more fuel?

Pull the distributor cap off and look down in the guts of the dizzy. Apply vacuum to the unit (hook up a hose and suck hard). You should see the base plate move.

Engine performance will tell you.
If your VA diaphragm is split the timing can not advance to place the spark at the right time for high revs, your engine will not want to perform at revs, or at worst, will not want to build revs up at all.

Sorry big jon, I think you got it back to front.

Engine revs and throttle setting combine to "sort of" control the vaccuum induced in the intake system. That vaccuum is then utilised to control spark timing placement, rotationally, so that the flame has time for combustion.

(Yes, vaccuum will decrees in time at high revs, but it does not do much in the way of timing change because that has been over and done with by about 2,500 to 3,000 RPM.)

Static engine timing is held by spring pressure on the advance retard plate. As throttle is opened engine revs rise and vaccuum rises, (think of an engine as an air pump, pump faster, make bigger vaccuum) the advance retard plate is rotated against the springs advancing the firing point. This is in response to the time the flame needs to complete the fuel burn.
( Flame propagation and flame paths are indeed very interesting Pedro. Ever had a look through a "colour tuner" window? Ooh, pretty.)
The faster the revs the less time for fire, so the spark is initiated sooner to give the time needed for complete combustion before the exhaust cycle starts.

(32 degrees before top dead centre? What the hell revs are you pulling to need that much advance? And can I drive it?)

To prove it to yourself, remove the dissy cap, remove the manifold end of the little black rubber hose that is the vaccuum advance retard hose. AND SUCK. And you really have to suck hard.
The vaccuum you induce will move the points, or what ever, plate, only slightly, but you will see movement. Or, you will hear yourself sucking through the split diaphram.
Pretty simple to replace, they are a sealed unit that is held to the side of the dissy by one only little screw. Inside the dissy there is a link held to the plate by a "c" clip. Remove the hose on the unit, remove the screw, carefully remove the "c" clip. ( it is tiny and can fall into the bottom of the distributor, you don't want that.) Throw the junk unit over your favourite shoulder, and reverse the process. Job done.
Or, you could vary that for fun, and remove the screw last, then reverse the process for installation of the new unit. Screw, clip, hose, replace dissy cap, start the engine and rev her up.

Of course this is the ancient pre computerised way of trouble shooting.
Now a days the car has a whole computer to tell you what is wrong and these backyard boys think it is just a matter of replacing a lead or two. Now, where have I heard that in a bloody holden add before.