Disco 1 V8 Vacuum advance unit

hi_arshard · Nov 22, 2024

Hello,
I have replaced the Vacuum advance unit, now I want to test it if it is working correctly.
When the engine is in start and when I give a hard suck to the hose connected to the VAU, I see a slight advance in engine rev. But when I cover the hose connector with my finger (the connector on the butterfly ) I don’t feel a suction. Tried rev up to 3000rpm did not feel a suction.

May I know how does the vac advance works the effect to the engine performance?

Thanks

Rangieman123 · Nov 22, 2024

You won't feel much vacuum from the vac advance connection on the manifold as it's located slightly upstream of the throttle butterfly. If you open the throttle to give it a rev you will feel some vacuum but it will never be high.
Take off the distributor cap and have a good suck on the vac line. You should be able to see the baseplate move slightly.
If you try a Mightyvac or something similar you can see the baseplate move properly.
Once you have no leak you'll be fine. Have you checked that your bobweights are moving correctly? They do seize and an rob you of a noticeable amount of power.

JohnKeane · Nov 22, 2024

The vac advance kicks in when cruising on the motorway, at a light throttle the petrol air mix is at its weakest and so the timimg is advanced so the weak petrol mix starts to burn slightly earlier as the mix takes longer to burn than when it is at full strength. With a partially closed throttle the suck from the engine is high which provides the advance. It is really a fuel economy thing and nothing to do with acceleration performance, as soon as you put your foot down vacuum reduces and you lose the advance, at the same time the fuel mix strenthens and burns faster so needs less advance.

Rangieman123 · Nov 22, 2024

Yes JohnKeane is on the money, I should have mentioned that. The vac advance is for economy and not much else.
The operation of the centrifugal advance has a big effect on power and the weights mechanism does need a few drops of oil at service time..

hi_arshard · Nov 22, 2024

Rangieman123 said:
You won't feel much vacuum from the vac advance connection on the manifold as it's located slightly upstream of the throttle butterfly. If you open the throttle to give it a rev you will feel some vacuum but it will never be high.
Take off the distributor cap and have a good suck on the vac line. You should be able to see the baseplate move slightly.
If you try a Mightyvac or something similar you can see the baseplate move properly.
Once you have no leak you'll be fine. Have you checked that your bobweights are moving correctly? They do seize and a rob you of a noticeable amount of power.

Thank you for the reply. Sorry, What is a bobweight how do I check that ?

My issue is I don’t feel the V8 power. and what else should I be checking for ?

Thanks

Rangieman123 · Nov 22, 2024

Firstly I wouldn't describe the standard 3.9 Rover V8 in your Disco 1 as a powerful. The cylinder heads fitted are more suited to a nice spread of torque from low down as they are a heavy vehicle. The standard V8 is not really a revvy engine.
The bobweights in the distributor use a mechanical method of advancing the ignition timing as the revs rise. They do seize up due to lack of "exercise" but can be freed up with some light oiling.
The easiest way of finding out how your Disco goes is either for you to drive another one, or get another enthusiast to drive yours.
Put out an ask on this Forum, and I'm sure you will find some assistance.

Stanleysteamer · Nov 22, 2024

hi_arshard said:
Thank you for the reply. Sorry, What is a bobweight how do I check that ?

My issue is I don’t feel the V8 power. and what else should I be checking for ?

Thanks

The bobweights are to be found under the plate in the distributor, they are sprung so that they stay close to the centre of the distributor under low revs but once the engine revs higher the bobweights move outwards under centrifugal force and move the top part of the spindle so that the spark advances. Thereby firing the mixture at the optimum point for the revs.

https://www.v8register.net/FilesV8WN/180710-V8-ignition-timing-NH5-V8NOTE543.pdf

AllanWorms · Nov 23, 2024

The Disco 1 V8 uses ported vacuum, not manifold vacuum. That's why it's not advanced at idle, but advances at part throttle, such as when cruising, and just off idle.

Disco 1 V8 Vacuum advance unit

hi_arshard

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Rangieman123

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JohnKeane

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Rangieman123

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hi_arshard

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Rangieman123

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Stanleysteamer

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AllanWorms

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