Vacuum Advance Unit - any reccomendations please?

[Gresley]

I have noticed that my Stag has been feeling a bit 'off' over the last few weeks, not pulling as she should and very wooly on the choke, and I have also noticed popping while changing gear. I checked the carb diaphragms which are fine, checked the mixtures etc which were also fine. Tonight I have just been out and tested the vacuum advance unit and I find that it is definately perforated - damn! This was a new part four years ago...

So, could anyone reccomend where to buy a new one from please? Or I wonder if they are all the same and don't last long?

Cheers

Brian.

[V Mad]

Hi Brian
How do you know its perforated? The usual red herring is that the vacuum pipe is leaking and leads to a false diagnosis.

If you have elmimnated the pipe, and are sure its faulty try Lee at H&H Ignition. You will need to state which dissy type you have (41525, 41336 etc) or quote the number on the vacuum advance.

[wilf]

I know this is radical but.............you don't actually need vacuum advance. It hs no effect upon anything other than the very lightest of throttle openings, and is only there for light throttle emissions/economy. It certainly won't affect large throttle opening performance. Unless of course you have such a massive vacuum leak through the torn diaphragm that is throwing off the mixture? Try blocking the vac pipe off and see what happens.

I think your symptoms may be rooted elsewhere.

[Gresley]

Hi guys,

Thanks for the replies. I'm fairly sure that it's the vacuum advance unit because Ihave a hand held vacuum pump and I can't get it to hold a vacuum when connected directly to the unit.

Wilf - it does run absolutely fine on a wide throttle opening so I will blank it off and see how it is.

Chris - I will give him a ring tomorrow if I can and see what he has, I've seen their adverts now that you mention it.

Thanks again,
Brian.

[Arbman Al]

Hi Brian

I was running around for about a year before it was realised that the vacuum advance unit wasn't working. Took it to Russ for a tune up and he spotted it

I get about 3-4 mpg more now it is working properly, so I wouldn't advise running with it disconnected for too long.

Cheers....Al

[Motorcycle mad]

Surely the vacuum unit is to help the engine run smoothly when changing gears etc, the vacuum unit advances the timing when accelerating, if it were not there you would possible have a flat spot in your accelerating.

Ken

[jbuckl]

distributor doctor?

http://www.distributordoctor.com/contact.php

no connection etc

[V Mad]

Hi Ken - the vacuum advance is fed by a port on the carburettor just above the throttle valve. You get maximum vacuum/hence max advance on small throttle openings. When you accelerate, the vacuum drops and so retards the timing (decreases the advance). This aims to time the ignition at the ideal point for best efficiency. If you disable the vacuum advance you might notice a slight lack of pick up from the engine, and you would get a few mpg less as well.

[GDPR]

So lets completely ignore the true function as taught to me by the author of the Motor Vehicle apprentices bible, Victor Hillier and Fundamentals of Motor Vehicle Technology (the are some on here that should get a copy and read it before posting) and my lecturer during my years at Croydon Technical College - lean mixture on cruise takes longer to burn, so the vac advance is there to provide an earlier spark to ensure complete combustion of the leaner charge - better cruise power, better emissions, better MPG.....

Modern petrol engines still have it, just modified digitally from MAP sensor data...

[wilf]

That's what I said to begin with!


Having dabbled with many a tuned engine though, I know it is possible to up the static advance until you get issues with hot starting, then back it off a bit and limit the mechanical advance to the normal "all in" figure at the correct revs. That way you can eliminate vac advance and still reap the benefits of advance at light throttle openings. And if you have a carb like a Holley or Edelbrock where you can adjust "cruise" mixture separately from "power" you can lean the thing out a lot in the cruise before the engine will complain. That's how the V8 in the Cobra is set up, not that I particularly care about economy in that thing anyway.

Race engines run with locked or non adjustable advance/retard frequently - of course racers don't worry too much about "driveability" anywhere other than wide open throttle!

[Gresley]

Thanks for the info... I did not really understand how the vacuum advance works but just as has been said she runs fine on a wide throttle opening but has lost her sparkle on a light opening, so I am now convinced that the vac unit is at fault. H&H and Distributor Doctor both have them so one will be in the post to me shortly. Help and advice much appreciated...

Brian

[wilf]

Please let us know what difference it makes Brian, as I am very interested to understand the characteristics of the TV8 with regard to advance/retard. Engines vary so much in that respect - all down to combustion chamber design IMHO.