V8i vacuum advance

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.

Reply to
Austin Shackles
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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.

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

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

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.
Reply to
Badger

On or around Wed, 6 Dec 2006 12:50:33 -0000, "Badger" enlightened us thusly:

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

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

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.

Reply to
Austin Shackles

Due no doubt to the gas having a higher octane.

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

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.

Reply to
Badger

On or around Wed, 6 Dec 2006 21:21:05 -0000, "Badger" enlightened us thusly:

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.

Reply to
Austin Shackles

On or around Thu, 07 Dec 2006 07:48:38 +0000, Austin Shackles enlightened us thusly:

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.

Reply to
Austin Shackles

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.

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

Reply to
Badger

On or around Fri, 8 Dec 2006 01:43:55 -0000, "Badger" enlightened us thusly:

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.

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

smartarse :-)

Reply to
Austin Shackles

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