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MG Midget and Sprite Technical - Vacuum Advance & Weber DCOE

Hi folks,

I've recently fitted a single Weber 40DCOE to my 1500 midget instead of the SUs. On removing the SUs i noticed i have the vacuum take-off for the distributor which now has nowhere to go. My question(s) to you are as follows:

1) Is there a place for vacuum take-offs on the Weber. I've been told it's number 48 on the attached pic but i have no idea how i'll attach a vacuum tube to that.

2) Do i take a tap off the brake servo vacuum pipe?

3) I've been considering this anyway but should i get a new distributor without vacuum advance? If so, which one?

thanks!!


Jamie Watt

For road use, you'll want a vac adv system... it's a economy device that helps the engine to be more efficient at less than wide-open throttles (which is almost all of the time) incl cruising.

Tapping off the brake servo vac pipe is reasonable.

A
Anthony Cutler

Thanks for the quick reply, Anthony. My next question is, if i do go ahead and upgrade my ignition system from the standard, what is the best one to get? I'd like to get a new coil and distributor i think.
Jamie Watt

I have the same setup on my Midget 1500.

Unlike SU and Zenith carbs, WEBER DCOE's are not designed to be used with vaccum advance distributors... instead I'd recommend fitting a mech (centr.) advance distributor set to run up to around 30 deg max advance. Improve spark with a higher-output coil.

Simply tapping into the manifold with a vac take-off will not work properly on a DCOE. I've tried it, and fumbled with it... will waste fuel and not produce the extra power which is reason you chose a DCOE to begin with. There are numerous threads on here about setup and tuning Webers. Check 'em out.

It made a world of difference in the performance of my car.

Ron Koenig

>>WEBER DCOE's are not designed to be used with vaccum advance distributors...

Have to disagree. When you have a part-throttle, the cylinder isn't being filled and so the effective compression is low. This means the swirl (that carries the flame front) is slow, so that timing needs to be advanced to compensate. Vac advance does this, whatever system is used on the inlet side i.e. 1 or more carbs, fuel injection.

[As a corollary, forced injection needs less advance with boost pressure, as ao arnold and Bob will know.]

The myth that webers don't needs vac adv stems from the racers... given that they are at WOT or nothing, then the vac system would never come into play to assist economy.

HTH

A
Anthony Cutler

I am afraid who ever told you that Ron has given you the biggest load of codswallup I have ever heard. As Ant has suggested the vac advance is not a "power" performance tool but is an economy and slow running tool. Without it the Weber will drink fuel on normal operation and is probably why the other myth about webers exist which is they are not economical. Without vac advance they will definately not be economical. My MGA's economy went up by 10% when I tested the vacuum effect!

Why you experienced the effects you have described with your weber mate is god only knows but from my experience suggests exactly the opposite and logic would back that up.

James as Ant says plumb it into the intake manifold either in the servo take off or another handy tapping.
Bob Turbo Midget England

Some Webers did have a vacuum port but it wasn't the part you indicated. That screw might lead to a vacuum source though, I just can't say for sure. The acutal vacuum ported Weber looks like the one in the photo. One problem with using vacuum advance with a Weber is that most of the intakes are indivitual runner designs which get a lot of pulsation in the vacuum signal and can drive the dissy nuts. I'd suggest some type of small vacuum resevoir to help smooth out those pulsations.


B Young

Thanks for all the replies. My research seems to point in the direction of hooking the vacuum line up to the inlet manifold as well although now im thinking whether or not it's worth replacing the dizzy with 123 ignition or something similar? Essentially im totally lost as to what i should do! How about the lumention kits, i take it they don't interfere with the basic principals of vacuum advancing?
Jamie Watt

Best thing to do is to establish how much vacuum you have at most driven speed (engine rev in 4th)by hooking up a vacuum gauge and drive around for a period, say two weeks.

Then you will know the conditions to which you can select a vacuum canister.

I had trouble with my vacuum, coming in to soon, way too much vacuum for the original canister causing the engine to pinking. Solved it with a heavier canister.
Alex G Matla

Alex
Explain..
do you mean the size of the canister?

Would it help if I tap in to both of the runners of my 45 Dellorto ;) manifold.
Then have a chamber to level things out and then go to the vac connection on the dissy?
Onno Könemann

My understanding was (and I can't remember where I got this from or why it is true..) was that on the overrun Webers generated a mental amount of vacuum that made the standard vacuum advance go bananas (a technical term :) )

I have a weber with a 123/tune and no vacuum advance. Maybe it's not economical when I drive, but then, neither am I :D
OrangeSpyderMan

Onno, the Lucas vacuum advance diaphragm cannisters are rated at different levels of vacuum to acutate. The number in inches of Hg I beleve is stamped on the housing. I beleive that Alex is saying that changing to a higher rated cannister eliminated a lot of timing flutter as it didn't react to the lower inputs as easily. I can't remember right now what the options are, but as I remember there are three different vacuum levels in common usage. Jeff at Advanced Distributors can certaily tell you what's available. http://www.advanceddistributors.com/
B Young

Spiderman? How can the weber generate mental amounts of vac on over run??? They are no different to any other carburettor in so much as they are a throttle disk that closes when off the throttle the engine determines the vac created not the carb!!

Bill, Alex is saying nothing of the sort he is suggesting that his vacuum advance canister was advancing the timing too much and was therefore causing the ignition to be too far advanced and pinking at throttle openings less than max!

Whilst that could have been the case, to cure that he could have reduced the overall advance, he could have purchased a dissy with less mech advance etc etc. Without knowing the full facts it would be difficult to assess if changing the vac unit was the best solution although it suited Alex so that is fine for him. That said it will not be a fine result for everyone.

Jamie do yourself a big favour.

Plumb the vac advance into the manifold and set you timing in the following manner.

Disconnect the vac advance and rev the engine to 4500RPM and at that point set the timing to 32 degrees BTDC and then try the car. If it pinks then retard he timing to 30 degrees. I would be very surprised to hear if that did not work fine.

If you do not understand how to do this then please ask further
Robert (Bob) Midget Turbo

I am most definitely an amateur mechanic and the thought of vacuum gauges and twin port inputs genuinely terrifies me. Would it not be better to scrap the vacuum advance dizzy in favour of 123 ignition or maybe even just don't connect the vacuum pipe and try to get the timing set up accordingly?
Jamie Watt

Jamie it is not difficult and even with the 123 you would still need vacuum I think!

You can forget about the vacuum advance if you are not concerned about smooth slow speed running and fuel economy. If these are not on your list then simply forget about it but for the benefit of future posters do not then start posting comments like my car is heavy on fuel and I have a DCOE fitted. LOL
Robert (Bob) Midget Turbo

Jamie
The 123 comes with or without vac advance and again the vac will give you better economy.
Leaving it off does not do any harm other than higher fuel consumption.
But connecting it is no rocket sience ;)
Onno Könemann

I think I'll go for the lumenition magnetronix and leave this job to the professionals!
Jamie Watt

I changed to a stiffer (needing "more" vacuum) cannister, taking the max advance for what I could get. The old canister was just to weak, with twin HS4 carbs the butterflies hardly open at cruising speed so very high vacuum.

Can't exactly remember values, but can look them up if someone really wants to know.
Alex G Matla

"Spiderman? How can the weber generate mental amounts of vac on over run??? They are no different to any other carburettor in so much as they are a throttle disk that closes when off the throttle the engine determines the vac created not the carb!!"

Like I said, I've no idea. Now you've actually encouraged me to think about it, I can't see why it would.

Oh well, that's another urban myth I can cross off :)
OrangeSpyderMan

Jamie, leaving it to the professionals means you must tell them what kind of carburetion and so on you have.
An of the shelf "performance" ignition, ie without vacuum advance just doesn't cut it.
IMHO.
Alex G Matla

Robert, I guess I mis-understood what Alex was saying. For information this chart has the various vacuum advance and retard assemblys used on Lucas distributors listed by application and wheither it used ported or manifold vacuum as the source. As I understand from other things I've read the numbers in the Vacuum spec column indicate in order,
1. the vacuum level in inches of Mercury when the unit starts to advance, 2. the level when it's totally advanced, and 3. the total advance in degrees. As you can see, if you want your vacuum advance to come in sooner or later or give more or less total advance you can select a unti that best meets your engines needs.
http://www.teglerizer.com/mgstuff/advance_curves.htm
B Young

Exactly Bill!
Alex G Matla

Ah so it is the unit on the dissy.
Now I understand what we are talking about ;)
Onno Könemann

Please note that this is a 1500 NOT a 1275

It's up to you - you won't get economy on a weber anyway so you choose!

Most electronic ignitions are good, but ultimately you get what you pay for, and do get a coil to match at the same time as many problems arise when they don't and they will just blame the other one!
rachmacb

Rach don't talk about things you know nothing about ;P

Webers can give economy though a SU will always be slightley better.

The principle of vacuem apply to both the 1500 and 1275.
Every N/A piston engine for that mather
Onno Könemann

If you are going to use a Vac take-off from the manifold then you will need to take it from each runner using a balance pipe, and even this will not be ideal.

As has been said, Webers and their ilk produce large pulses in the inlet runners which MAY upset your dizzy (hence the balance pipe). This is the major reason for me running a TPS sensor as opposed to a MAP sensor on my Climax race motors.
Deborah Evans

Onno go and play with your little engine!!

It wasn't that I meant but that the rev range is different for setting up the advance curve.

No-one had answered his other point despite him asking several times.

And properly set up carbs are more economical!
rachmacb

Deb, that's why I thought about a vacuum resevoir to help balance out the pulses as well. I'm going to face a similar problem with my inline 6 running on three Weber/Mikuinis as far as vacuum pulsations to the dizzy, that's why I'm so interested in this thread. Appreciate your input.


B Young

Bill is that a 6 in an A?!
Onno Könemann

I have run a couple of webers without a balance pipe between the inlet tracts. Never had a problem with that but I can see the benefit of having the balance pipe, even if it is just to avoid undue wear on the vac mechanism, otherwise I think perhaps the pulses are so fast that the vac mechanism itself dampens out the pulses. I think it must be remembered that if the enine is revining at 3000 rpm that is 3000 vac pulses in a tract every minute or 50 per second, not sure the vac unit could detect such fast pulses?


With vacuum advance I have easily been able to achieve as good fuel economy as with any other carburation and in fact with my MGA I get better fuel economy thn with the original SUs. To me a weber is the same as any other carb it is simply a device to match fuel ratio to air consumption. That means it must be the same as an SU unless it is running rich! Most webers I have seen tend to run rich and are poor regulators ue to the choke being sized too large.
Robert (Bob) Midget Turbo

Bill is building a hillbilly healey ... MGA with a 6 cylinder jeep engine. ;)
Trevor Jessie

Lol I was thinking that for a B ....

I'm sure that Mr Vizard will bow down to the superior knowledge of others regarding webers ..... Perhaps they should write a book :)
rachmacb

At least it will look better than a healey!
Onno Könemann

Jamie

Or.....

Rather than a dizzy set up there are various programmable ignition systems.....

I use the Megajolt setup and am very impressed, means making changes to the engine in the future (if they effect the timing characteristics) are easier- plug into laptop and adjust figures.

Although it might not be your thing if as you say "I am most definitely an amateur mechanic" :-)

Just another suggestion for you..
M Slater

Mr young is of course correct .....
and also consider the amount of timing retard per depresion of the throttle needed to prevent detonation. Generally a motor will loose a bit of fuel economy by elimiating the vac advance and also the advance curve and total might be affected. And a DCOE is on the rich side and not an economy carb as was said above.

Inspecting the carburetor port that was originally used with the stock carb you can tell what is needed ....port or manifold vacuum. More than likely it was ported. Either change the advance curve in the dist or change the vacuum diafram on the dist( and possbly new linkage to the advance plate) which will accept manifold vacuum.....

Plan B of course is to just run any old thing or combinations ...."whats causing a big stumble off idle on my car"...is then asked.

J. Blow

How does the megajolt stuff work? Im now thinking i need to have consistency through from the coil to the dizzy and electronic ignition just so i know that everything works. The lumention magnetronix looks decent to me and they sell it as a complete kit so it should all work together. My big decision now is whether or not i keep the original vacuum advance dizzy...
Jamie Watt

Megajolt replaces the dizzy and coil with a crank sensor and coil pack plus a control box. It is a fully mappable electronic distributor-less ignition system.
Deborah Evans

You can still leave the distributor in location, just in case - however, I notice on the other thread you didn't want to pay too much, and, well, that would pretty much rule it out as it's not the cheapest option!
rachmacb

Im a true Aberdonian, Rach, parting with money is not something i enjoy doing! ;)
Jamie Watt

Thank you Deb...

"If you are going to use a Vac take-off from the manifold then you will need to take it from each runner using a balance pipe, and even this will not be ideal."

I listen to everything Deb says and treat it as gold. I have not failed in doing so.

And to everyone else, THIS is why I'm reluctant to post comments on these threads, I'm always scorned and ridiculed. I noticed too that one comment will throw a whole thread off track. We've got someone who has little experience with these carbs asking a simple question and we end up bickering. :/

Let's enjoy driving our LBC's (or working on them as is usually the case).




Ron Koenig

Lol Jamie I noticed that :p!!!

That's kinda why I said it now before you went down that track and spent money you don't want to - yes, it's a good system, but for everyday driving and a limited budget, it's neither worth it or lots of gain.

If you go down the electronic direction, then the best advice is to source both distributor and coil together - it saves time and money down the road.
rachmacb

Thanks Rach - the car is going into the garage tomorrow to take care of some other bits and pieces that im no good at so i'll take their advice on the best ignition system for the car. Glad you stopped me going down the Megajolt route before my wallet was opened!

Ron - the tinterweb was invented for people to bicker harmlessly and divert threads off-track, that's why i love it so much!! :D
Jamie Watt

My (limited) experience with Webers, albeit on a 1275 not the 1500 is that they need more advance at idle than the SU's, so whatever ignition system you choose, the advance curve needs changing from stock, because if you dial in that advance at low revs you'll end up with too much at high revs unless the curve is changed.
OrangeSpyderMan

A little tip that may help...
You can even out the vac pulses from a Weber by putting a small inline fuel filter in the vacuum line.
Bob T

This thread was discussed between 01/08/2011 and 08/08/2011

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