vacuum advance unit

[leyther8008]

How much influence does the above have at tickover.
I set my timing using the instructions in the book (vacuum disconnected pipe blocked) got it spot on connected everything up again, took my van out and it wouldnt accelerate at all, I found I had to advance the ignition to get any form of perfomance out of it.
I then found out that the thing isnt working having no effect on the timing by sucking the pipe, so its been like that for a while and presumably why the ignition appeared to be retarded at tickover.
Question is why do you time it without vacuum then presumably the vacuum alters the timing once connected up again, plus what difference does it acctually make to the day to day running of the car as my van seems to do alright without it.

[PSL184]

It just advances the timing at idle/tickover. You don't need it for normal running and alot are probably not working without any owners even knowing! Timing should be set when you are driving the car /van by advancing gradually until the engine starts to "pink" and then just backing it off slightly. Every engine is different and "book" timing figures will not suit all engines.....

[mike.perry]

The higher the engine revs, the more suck, therefore the more vacuum advance at higher engine speeds. The reason that it is disconnected when setting the timing at low revs is so that it does not have any effect on the settings. To set the timing accurately at tickover (1098 6deg at 600rpm) you need a rev counter and a strobe light. Alternatively you can set the static timing when you will not need to disconnect the vacuum pipe.

[Luxobarge]

Not quite. Vacuum disappears with wide throttle openings, so it will retard (relatively) the ignition with your foot on the floor, for maximum power. When you ease off into the cruise, it will advance the ignition and give better economy, but as you aren't asking the engine to do that much work, it won't pink. Also, at tickover it will advance the ignition too, but again, since the engine isn't actually doing much work it won't pink then either - although the vac advance doesn't actually do much of value at tickover, like I said, it's more an economy device for the cruise. That's why you should set the timing with it dosconnected - i.e. in the state it needs to be for max power, although with today's fuels the book figure is only a rough guide, as PSL says connect it up and do the "advance it until it pinks then back off a couple of degrees" method.

The device that advances the ignition in direct relationship to revs is the bob-weight assembly under the points plate - that's why if you are setting the timing dynamically with a strobe, you should do it at prescribed revs, i.e. quite slowly before the weights have come into operation.

I recommend doing the timing using the PSL method, then strobing it at tickover to see what actual setting you have, then if you need to set it again in the future, you can do it with a strobe, which is a lot less hassle than a series of rolling/driving tests. Before you adjust the timing though, check that the points are in good nick, clean and set correctly.

Hope this helps!

[bigginger]

Thank you! Vacuum increasing with revs is a VERY common misconception, but it actually does decrease, entirely counter-intuitively.
a

[MarkyB]

The vacuum advance makes quite a big difference to how the engine operates.
Try disconnecting it and driving the car.
I remember driving my dads A40 and it felt like it had a 600cc engine. pulled over, popped the bonnet and reconnected the vacuum pipe that had come off and viola! it could easily pull the skin of a rice pudding

[mike.perry]

Never too old to learn! Write down a load of rubbish and someone is bound to correct it. Cheers for the explanation.
That probably explains why Ford Pop wipers stopped when overtaking trucks in the rain.

[MarkyB]

What a brilliant invention they were NOT
The faster you go the slower they go
Then, when you stop they try and beat the bonnet to death at top speed!

[bigginger]

a

[bmcecosse]

The vacuum advance does pull the timing up at idle - this allows a small static advance setting for easy starting - then as vacuum develops in the inlet manifold, so the timing is pulled up to give a nice idle. Advice to set timing with vacuum NOT connected is very wrong! Best way is to swing the dizzy back and forth while the engine idles - and settle at the point where the idle is smoothest. However - this may not give the best driving ignition timing - which is best set by advancing till it pinks - then retard slightly. So - there has to be a compromise - do you want smooth idle - or best driving ? The real answer is to modify the advance curve in the dizzy - so you have the best of both worlds. To do this - you need to set it for best idle - then advance it up for best running - noting how much more advance is required (if any). Then - open up the dizzy - and remove some metal from the bob-weight advance stop, so the mechanism will give the extra advance required - then re-assemble and test it all over again. It's trial and error really, and every engine will be different, more so if the engine is non-standard in some way. ie - fitted with a 12G940 head, and/or larger carb and/or better exhaust manifold etc. The '123' dizzy comes complete with a variety of selectable advance curves for easy selection, and has built-in electronic ignition which seems to be very reliable. Costly - but a nice piece of equipment. I wish I could afford one !

[mike.perry]

I have in my collection of distributors some with 13 deg stamped on the bob weights and some with 17 deg. Which is most suited to what engine or is it just a case of trial and error?

[bmcecosse]

The actual advance provided is twice that figure - have to say never seen one with 17, highest I have is 15 - ex Cooper S. I would say the 17 is useful for a modified engine - it will allow a very low static advance and so allow starting in cold weather against a presumed high compression ratio. The 13 could be used on a super(or turbo)charged engine where overall advance must be limited to prevent detonation.

[leyther8008]

I found a really interesing article here
http://www.starchak.ca/tech/pdfs/lucastuning.pdf
but no mention of the vacuum advance just the mechanical side of things.