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> Distributor vacuum advance questions
webmonster
post 6 Sep 2014, 05:34
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I've just finished having a checkover of the Guzzlers carbs and timing (yes yes, I've distracted myself from doing the splines on the Oxford's diff...)

Carb balance at idle is good, mixture seems OK.
Using my timing light I can see that the mechanical advance is working and I had the dizzy checked over last year and it was within the correct specs. He worked out the max advance at 4000 engine rpm is 24 degrees, so perhaps a fraction beneath the factory specs.

I'm not convinced about the vacuum advance though.
It was measured as 7 degrees (presumably at the specified settings of 5-12-8), so it is 1 degree off. Not sure if that affects things a lot.

I tested the vacuum line - it works. Not 100% sure about the fittings at each end.
Hooked up another vac line to the dizzy and sucked. I could hold it against my tongue, so there is no leak. It took quite a strong suck to make it move.

I held the other end of the car's vac line (connected to the carb) and revved the engine but I couldn't feel much vacuum at all. Should I be able to?
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wolseley16/60aut...
post 6 Sep 2014, 11:42
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I used to use a plasticy thing called a 'Mityvac' vacuum tester years ago to check for vacuum leaks etc.....
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Llansadwrn
post 6 Sep 2014, 13:33
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When you rebuilt the carb did you blow out all the orifices with air pressure? If so then you'll have plenty vacuum, it's very difficult to test it objectively without using a vacuum gauge. The problem is pretty much always at the distributor end, not at the carb/manifold end. Vacuum timing a degree out is not significant IMHO.
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enigmas
post 6 Sep 2014, 22:13
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The vacuum advance is designed to improve fuel economy on a moderately cammed engine (infer standard factory spec) during cruise conditions. Turbulence at these speeds is not optimal so to improve combustion conditions additional advance is added to the timing at this rpm range. Vacuum advance is taken from the port in front of the throttle blade (air filter side) not behind it. Unfortunately the current fuels available today don't have the same properties or burn rates, so the original timing specs are more a starting point than a hard and fast rule.

This post has been edited by enigmas: 6 Sep 2014, 22:19
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Llansadwrn
post 7 Sep 2014, 01:40
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Get this, throw the old distributor away and (I believe) plug the vacuum port:

http://www.123ignition.com.au/

and more info:

http://www.123ignitionusa.com/

and the mother site in Holland:

www.123ignition.nl

This post has been edited by Llansadwrn: 7 Sep 2014, 01:59
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webmonster
post 8 Sep 2014, 09:37
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QUOTE (Llansadwrn @ 7 Sep 2014, 13:40 ) *
Get this, throw the old distributor away and (I believe) plug the vacuum port:

http://www.123ignition.com.au/

Interesting, but I can't see how it would 'know' the difference between part-throttle and full-throttle conditions.
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webmonster
post 8 Sep 2014, 09:42
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QUOTE (enigmas @ 7 Sep 2014, 10:13 ) *
The vacuum advance is designed to improve fuel economy on a moderately cammed engine (infer standard factory spec) during cruise conditions. Turbulence at these speeds is not optimal so to improve combustion conditions additional advance is added to the timing at this rpm range. Vacuum advance is taken from the port in front of the throttle blade (air filter side) not behind it.

Rats - my cam is a little hotter than factory spec, but not too wild. I have had ideas of replacing it for a factory standard one at some point... (IMG:style_emoticons/default/dry.gif)
Vac advance taken from in front of throttle blade - makes sense, otherwise you'd get massive advance on the over-run.
QUOTE
Unfortunately the current fuels available today don't have the same properties or burn rates, so the original timing specs are more a starting point than a hard and fast rule.

Difficult to change vac advance without getting hold of a new unit, I imagine?
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