Vacuum advance questions

Engine Oily Bits, Ignition, Fuelling, Cooling, Exhaust, etc.
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Clifford Pope
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Vacuum advance questions

#1 Post by Clifford Pope » Wed Mar 22, 2017 11:24 am

I noticed recently that the braided earth lead in the distributor was looking very wispy, as if it might break at any moment, and managed to find an ad on eBay selling earth leads for all 45 distributors. It's cheap and the right length, with an eye terminal on each end, but it's not braided. At least I don't think so, but it's plastic covered and looks and bends exactly like an ordinary bit of wire.

Anyway, I fitted it, giving it a loop that can coil/uncoil as the olate advances, which I checked by sucking. Then I noticed that the main input wire from the coil to the points terminal isn't braided either. Obviously it can't be, because it's a continuous piece of wire from the terminal, through the rubber grommet in the distributor, and on to the coil. So exactly the same conditions apply as for the earth.
Why the fuss about needing a braided wire on one but not the other ?

The car runs perfectly and idles nicely, and it's now got a good earth wire and an old spare just in case.
I tested the advance by pulling the pipe off the carburetor while idling, and it made no difference. I checked the pin-hole and it felt clear when I poked a paperclip down it, but on sucking the pipe found it was blocked, about an inch down the hole. I dug out a packed wadge of hard soot, and finally got an air passage, but on connecting up found that the engine ran much better as it had been before. The pipe has been doing absolutely nothing for several years.

Is there any point in now re-tuning to try to make it run smoothly again, or should I just block up the vacuum connection and keep it as it was? (It's a 2500S engine with HS6 carbs)

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Re: Vacuum advance questions

#2 Post by Vox » Wed Mar 22, 2017 1:06 pm

Can't speak for the 45D but on the 25D I think part of the reason the earth is a braid may because of its position as the moving plate moves over the top of it, or at least near enough to be a worry, and a wire would be too thick but the braid is flat. The plate end of the braid is also virtualy spot welded (or spot soldered i suppose) to keep it as flat as possible and is very hard to replace.

The other wire to the terminal post in a 25D also needs to be flexible (although I don't imagine any more so than on the 45D) because of the plate movement and I had an Ebay special that was a wire rather than a braid (and a fairly stiff wire at that) and that eventually fatigue fractured - or rather I caught it just as it was about to fracture so wasn't stranded - and I replaced that with a NOS Lucas soft braided one and it has been alright since.

With the 45D built in points lead it is possible that it is deliberately made from super flexible wire (like the silicone ones you get for some multimeters) to account for it but I don't have one to check.

Curious as to which hole you dug the crud out of on the vacuum advance - do you mean the one at the carb end, the diaphragm itself or the pipe? I am not convinced my vacuum advance does much either!

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Re: Vacuum advance questions

#3 Post by Clifford Pope » Thu Mar 23, 2017 7:15 am

Carburetor end.

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Re: Vacuum advance questions

#4 Post by Alec » Thu Mar 23, 2017 3:08 pm

Hello Cliff,

the vacuum advance's operation is designed for mid range (cruising) and I don't think it operates very much at idle. It should make it run better at design range as the engine fuelling is set weak there, (or should be).

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Re: Vacuum advance questions

#5 Post by pete lewis » Sat Mar 25, 2017 1:40 pm

yes its vacuum is developed from around the throttle plate position and nothing to do with manifold vacuum
as alec says its a economy device that advances the spark mainly when cruising
has no real effect on performance , but gains about 4mpg on a run.

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Re: Vacuum advance questions

#6 Post by Clifford Pope » Wed Mar 29, 2017 11:34 am

Yes, I've got that now. I always forget that vacuum in this context means vacuum inside the carburetor not manifold vacuum.

I can see how that would only apply in mid range. At idling there's lots of manifold vacuum but it's blanked off by the butterfly. At open throttle there's not much vacuum anywhere, so in between is in between.
I just had a momentary thought that mine might be tapped in the wrong place, but you are right, it's just upstream of the butterfly spindle. That would explain why it might be prone to carboning up - a small stream of mixture round the edge of the butterfly, so an ideal accumulation point for the usual throttle body crud.

I'll see what happens to fuel consumption now it's working for the first time in years.

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