V8i vacuum advance

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Austin Shackles - 06 Dec 2006 12:40 GMT

3.5i hotwire. The vacuum advance pipe to the dizzy has a gadget in it
which, on my one, serves merely to prevent the vacuum from getting to the
distributor. I've turned it round so that the "dis" side is attached to the
"carb" end of the pipe, which now lets the vaccum through...

what does it do and can I either leave it the wrong way round, or remove it,
or should I actually replace it with one that works correctly?

Observed symptoms were absence of any detectable vacuum in the pipe with
engine at about 2K revs, and absence of any vacuum advance. Having reversed
it, I now get vacuum and advance.

and the motor runs better :-)

also advanced it 3 degrees ahead of "book" which makes it better on LPG in
the absence of any cunning ignition processors.

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Austin Shackles. www.ddol-las.net my opinions are just that
"There is plenty of time to win this game, and to thrash the Spaniards
too" Sir Francis Drake (1540? - 1596) Attr. saying when the Armarda was
sighted, 20th July 1588

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Badger - 06 Dec 2006 12:50 GMT

> 3.5i hotwire. The vacuum advance pipe to the dizzy has a gadget in it
> which, on my one, serves merely to prevent the vacuum from getting to the
[quoted text clipped - 3 lines]
> what does it do and can I either leave it the wrong way round, or remove it,
> or should I actually replace it with one that works correctly?

Apply a vacuum and hold it there on the carb side, it should allow the
vacuum through after so many seconds. It is a delay valve, fitted to prevent
spurious pinking when coming off and on the throttle quickly. With it fitted
the wrong way, it'll actually be allowing the vacuum to be held in the dizzy
side (under control of the delay unit) after you've pressed the throttle
pedal again and increase the risk of inaudible high-speed pinking which will
wreck your pistons!
I've never seen one fail. In all honesty, you could probably just remove it
(My 4.7 doesn't have one at all) completely but check carefully for pinking.

> Observed symptoms were absence of any detectable vacuum in the pipe with
> engine at about 2K revs, and absence of any vacuum advance. Having reversed
> it, I now get vacuum and advance.

How long did you keep it at 2000rpm for? Needs to be at least 30 seconds,
from memory.

> and the motor runs better :-)

Not for long if you get high-speed pinking.......

> also advanced it 3 degrees ahead of "book" which makes it better on LPG in
> the absence of any cunning ignition processors.

Check carefully the accuracy of the TDC marks, then try running 9 degrees at
idle, no vac. I had a 3.9 in about 18 months ago that wouldn't run smoothly
unless it had at least 12 degrees at idle, turned out the factory marks were
12 degrees out! And yes, it was the correct original pulley and
pointer...... shocking, that's the worst one I've ever seen though.
Badger.

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Austin Shackles - 06 Dec 2006 18:34 GMT

>> 3.5i hotwire. The vacuum advance pipe to the dizzy has a gadget in it
>> which, on my one, serves merely to prevent the vacuum from getting to the
[quoted text clipped - 13 lines]
>pedal again and increase the risk of inaudible high-speed pinking which will
>wreck your pistons!

>I've never seen one fail. In all honesty, you could probably just remove it
>(My 4.7 doesn't have one at all) completely but check carefully for pinking.

it does a bit of pinking (audible kind, under load) on petrol, but not on
gas.

>> Observed symptoms were absence of any detectable vacuum in the pipe with
>> engine at about 2K revs, and absence of any vacuum advance. Having
[quoted text clipped - 3 lines]
>How long did you keep it at 2000rpm for? Needs to be at least 30 seconds,
>from memory.

seems like way too long. I'm inclined to remove it altogether if I can find
a suitable length of pipe.

>> also advanced it 3 degrees ahead of "book" which makes it better on LPG in
>> the absence of any cunning ignition processors.
[quoted text clipped - 4 lines]
>12 degrees out! And yes, it was the correct original pulley and
>pointer...... shocking, that's the worst one I've ever seen though.

hmmm. It was set at TDC, as per the book, and not pinking; at 3 degrees
advance it's pinking slightly, although that could be related to the vacuum
delay thing; I'll try removing it. 30s delay for the vacuum to be applied
to the dizzy seems to me to be far too long; if it needs the vacuum advance
to work properly that means it's running too retarded a lot of the time,
especially in variable-load conditions. Also, it's run most of the time on
gas, so more advance won't hurt it. I've had some of the gas-only engines
running quite a lot of static advance, but this one needs to be able to run
on petrol as well, so the timing will have to be a compromise.

I thought the "avoid pinking when jumping on the throttle" aspect was done
by the fact that there's not much vacuum there; at lower revs with the
throttle open it don't suck much anyway.

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Austin Shackles. www.ddol-las.net my opinions are just that
"Remember that to change your mind and follow him who sets you right
is to be none the less free than you were before."
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Badger - 06 Dec 2006 21:21 GMT

> >> 3.5i hotwire. The vacuum advance pipe to the dizzy has a gadget in it
> >> which, on my one, serves merely to prevent the vacuum from getting to the
[quoted text clipped - 19 lines]
> it does a bit of pinking (audible kind, under load) on petrol, but not on
> gas.

Due no doubt to the gas having a higher octane.

> >> Observed symptoms were absence of any detectable vacuum in the pipe with
> >> engine at about 2K revs, and absence of any vacuum advance. Having
[quoted text clipped - 6 lines]
> seems like way too long. I'm inclined to remove it altogether if I can find
> a suitable length of pipe.

From memory, it's about a 20-odd second delay. Good idea to remove it
altogether though.

> >> also advanced it 3 degrees ahead of "book" which makes it better on LPG in
> >> the absence of any cunning ignition processors.
[quoted text clipped - 18 lines]
> by the fact that there's not much vacuum there; at lower revs with the
> throttle open it don't suck much anyway.

But if it's connected back to front, the vacuum as you close the throttle
will be applied instantly and will stay there when you re-open the throttle
causing the timing to remain advanced when it shouldn't be, the exact
opposite of what is really meant to happen.
Badger.

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Austin Shackles - 07 Dec 2006 07:48 GMT

>But if it's connected back to front, the vacuum as you close the throttle
>will be applied instantly and will stay there when you re-open the throttle
>causing the timing to remain advanced when it shouldn't be, the exact
>opposite of what is really meant to happen.

yeah, I see that. I'll get rid of it :-)

I've never noticed a need for such a thing before.

The motor was running very "flat" on gas and petrol, hence the
investigations. Not pulling convincingly and lacking power at the top end,
and swapping the delay thing round made a noticeable difference. I'm not
sure it stays advanced though. more anon, when I've looked again.

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Austin Shackles. www.ddol-las.net my opinions are just that
"Chuck didn't reply, so George swung round in his saddle. He could just
see Chuck's face, a white oval turned toward the sky.
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Austin Shackles - 07 Dec 2006 18:54 GMT

>>But if it's connected back to front, the vacuum as you close the throttle
>>will be applied instantly and will stay there when you re-open the throttle
>>causing the timing to remain advanced when it shouldn't be, the exact
>>opposite of what is really meant to happen.
>
>yeah, I see that. I'll get rid of it :-)

Pulled it off and substituted a plain bit of pipe; it now doesn't pink on
petrol, and I've tweaked the timing to 6°BTDC at idle.

Having studied the unit, and tested it under pressure on the mig gas bottle,
putting pressure on the "dis" side (not a huge amount, might've been about
10 psi) it allows gas through *very* slowly, all the time. I guess that's
how it make a delay; even when there's suction in the pipe it takes some
time to suck through to the dizzy. either that or it's broke.

The stub on the inlet already only has a small hole through it (probably
about 1mm, maybe less), which will in any case slow down the vacuum getting
to the dizzy.

In normal driving, I suspect that this one was not getting any vacuum
advance at all, since you don't normally have static conditions where the
vacuum is constant, unless at steady speed on the motorway.

anyhow, no audible pinking on petrol despite being provoked, at 6 degrees
advance; it does feel slightly hesitant off the mark now though (only on
petrol, not on gas), so I don't think I'll advance it any more. Might be
able to stand a few more degrees if we put super-UL in, but that costs more.
Running on gas it's way better.

What I'm wondering is how those ignition processor things you can get work.
I can see that it'd be tolerably easy to make the spark more retarded than
the dizzy switching, but more advanced? Only way I can see is that it
actually triggers off the pulse before the one it's generating a spark for
and then delays it by almost 1/8 of a turn of the distributor shaft at the
given speed.

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Austin Shackles. www.ddol-las.net my opinions are just that
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Badger - 08 Dec 2006 01:43 GMT

> >>But if it's connected back to front, the vacuum as you close the throttle
> >>will be applied instantly and will stay there when you re-open the throttle
[quoted text clipped - 25 lines]
> able to stand a few more degrees if we put super-UL in, but that costs more.
> Running on gas it's way better.

Most rover V8's will take a bit more than 6 degrees without pinking on pump
unleaded, and be more responsive because of. Try 8 or 9.

> What I'm wondering is how those ignition processor things you can get work.
> I can see that it'd be tolerably easy to make the spark more retarded than
> the dizzy switching, but more advanced? Only way I can see is that it
> actually triggers off the pulse before the one it's generating a spark for
> and then delays it by almost 1/8 of a turn of the distributor shaft at the
> given speed.

You advance the timing by the designed ammount, the device retards when on
petrol.
Badger.

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Austin Shackles - 08 Dec 2006 10:14 GMT

>> anyhow, no audible pinking on petrol despite being provoked, at 6 degrees
>> advance; it does feel slightly hesitant off the mark now though (only on
[quoted text clipped - 5 lines]
>Most rover V8's will take a bit more than 6 degrees without pinking on pump
>unleaded, and be more responsive because of. Try 8 or 9.

while it's not pinking, it seemed a bit hesitant off the mark on 95 octane;
mind you, I've not checked the tuning on petrol, so that could be a fuelling
issue.

>> What I'm wondering is how those ignition processor things you can get
>work.

>You advance the timing by the designed ammount, the device retards when on
>petrol.

hmmm. well, you *could* do it like that, aye :-)

smartarse :-)

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Austin Shackles. www.ddol-las.net my opinions are just that
"The breezy call of incense-breathing Morn, The swallow twittering
from the strawbuilt shed, The cock's shrill clarion, or the echoing
horn, No more shall rouse them from their lowly bed."
Thomas Gray, Elegy Written in a Country Churchyard.

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