Vacuum advance gear inside distributer

[mojobaby]

Its been some years since I installed a new vacuum advance unit to my distributer and I've been wondering about the little gear inside the distributer. It looks like a spur with teeth.

Can anyone tell me what its purpose is and how it works. When I put the new vacuum advance unit in, I connected it to the gear in the same position as the old one, which I think was 4 teeth from the bottom.

 

[JdeW]

Basically that thingy is meant to fine tune the vacuum advance ignition timing by x degrees, by heard I thought the delta is 10 degrees.
The vacuum advance was invented to get good ignition with a lean gas mixture. If you suddenly release the accelerator pedal at full throttle at high revs or driving idle, you get the highest vacuum. Then only very little petrol comes in compared to the air, the mixture becomes lean so the ignition must then start earlier because a leaner mixture burns worse.

 

[JdeW]

Addition information GTL ignition: vacuum advance at the following underpressure (in mbar) and in distributor degrees:

100-2
200-3
300-5
400-6
500-7
600-7

So above 500 mbar no advance takes place means that in crankshaft degrees the extra 7 degrees vacuum advance stops at 14 degrees.
Vacuum advance is only there to improve efficiency and for nothing else.

 

[mojobaby]

Thanks Joop, before I installed my new vacuum advance unit, if I released the accelerator on a downhill, I would get very uneven and jerky movement and the occasional backfire (which sounded quite cool) Thats when I realised that my advance unit was broken. The new unit has solved that issue.

In the photo above you can see a little bell shaped thing below the cog. The little bell has a notch on it that slots into the teeth of the cog. In its current position, if I suck on the pipe that goes to the carb, the points advance.
If I pull the bell outwards, the cog can be rotated. If I rotate it clockwise to the 12 o'clock position and try to suck, the points don't move at all.

Where the bell is positioned now (in the 5th space) and if the clog s rotated further anticlockwise so the bell notch is in the 1st, 2nd, 3rd or 4th space the points advance much more.
So 12 o'clock position, no advance and 6 o'clock position a lot of advance.

 

[petak]

Very interesting discussion.
Did not realise it has any matter where you "lock" vacuum .

If i understand correctly, fifth space is maximum you can advance vacuum.



Mine is also at fifth space

Thank you Joop for another detail

 

[mojobaby]

The Ducellier distributer also has a centrifugal advance system, with little springs and weights.
I think that it works only at high speed as opposed to the vacuum advance which relies on manifold pressure as per Joop's info.

 

[JdeW]

The Ducellier distributer also has a centrifugal advance system, with little springs and weights.
I think that it works only at high speed as opposed to the vacuum advance which relies on manifold pressure as per Joop's info.

The centrifugal advance only works during acceleration, but when we are driving at a stationary "high" speed, the vacuum advance kicks in for economical and fuel-efficient driving.

 

[angel]

The vacuum advance unit has a spring at the atmosphere side of the diaphragm, so that advance build up is gradual with increasing vacuum. Total amount of advance is relative to travel of the actuating rod of the unit. What the cam does is to add or remove preload to this spring. Less preload (1 on petak's picture) = steeper vacuum advance curve (line in fact), more preload, opposite effect.
What's interesting is that some carburettors have the vacuum take-off downstream of the throttle, therefore more engine load produces less ignition advance, while others have it upstream of the butterfly, adding advance as the engine load increases.