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MG Midget and Sprite Technical - weber/1500 ignition advance

good morning malc...new thread as requested
Iwas given a nearly new genuine weber off a 1500 but no manifold. I subsequently located one of these and it had no hole for the vacuum adv connection. I wish I could remember who I spoke to but it was someone in the know as opposed to myself. The advice was deff go with the non vac dissy. The dist came from the man on ebay who does everything for breakerless ignition.
Put it on, I cant be sure of the ig advance but it was lots. I will have to look it up but the car really goes well on what it has
rgds tony
a boyle

Hey Tony (and others).

Here are my thoughts on vacuum advance and DCOEs. See what you think. Sorry if it seems a little basic at times and asks a lot of rhetorical questions! Also, I have yet to properly test the below in real world driving, but I am confident in the theory. Also, sorry for the lengthy post :-)

At its most simplest, at any given driving condition your engine needs fuel and air in the correct ratio and a spark at the correct time to produce “optimum” power (optimum is a bit of a loose term I know but you know what I mean, right? I use the word a fair bit throughout as I can’t think of a better word).

Breaking things down into individual components the job of each component is:

- The carb: to mix fuel with air in the correct proportion.
- Mechanical advance: Advance the ignition timing as RPM increases (performance)
- Vacuum advance: To advance the ignition timing under part throttle conditions (cruising and economy) vacuum advance does not operate under full throttle conditions.

So, for a given driving condition, all the carb is doing is metering the correct amount of fuel required for the engine. Everything else being equal, it doesn’t matter whether it is an SU, DCOE or a trained monkey (maybe Prop can work on this…?!) doing the measuring, the amount of fuel required for “optimum” power is the same.

So, beyond the carb, whatever type it might be, the engine and ignition system do not care how the fuel/air mixture was measured, just that the spark plug is fired at the right time for “optimum” power. So this raises the question: why use vacuum advance when using SUs and not with a Weber?

I have not found a convincing answer to this because I don’t think there is one. The myth that you don’t use vac advance with a Weber probably comes from racers, where you are always full throttle and a vac advance is therefore not operating so not needed. But for road use the only argument against hooking up the vac advance is “just because you don’t” which isn’t convincing.

I have read/seen arguments saying that a Weber has the “wrong kind of vacuum” for operating the vac advance. The SU used a “ported” vacuum source, if you are using a Weber it is likely that you will be hooked up to “manifold vacuum”.

For a ported vacuum the hole is only just on the air filter side of the throttle plate, therefore when the throttle is completely closed, there is no vacuum advance. For a manifold vacuum source, the hole is on the engine side of the throttle plate and therefore sees full vacuum when the throttle is closed (I have pics of both of these, I will try to find them). I do not see a problem in hooking up to manifold vacuum (and actually think it is better) because:

- Beyond very small openings the ported hole is uncovered and sees the same vacuum as the manifold vacuum port on a Weber (see link 1).
- Assuming an SU carb and ported vacuum, why (for the sake of argument) at 5% throttle do you get full vacuum advance and at 4.99% throttle zero advance? Or at 1300 RPM full vac advance and at 1299 RPM and lower, no vac advance? In this regard, I think manifold vacuum is better as it is more progressive and doesn’t suddenly cut on/off at some arbitrary point (discussed more in link 2). Removing this step change in ignition timing should therefore mean smoother power delivery on small throttle openings.
- Your engine should pull a steady ~20 inHg vacuum at idle, more than enough to fully activate the vac can so there will be no timing flutter. (You should pull a vacuum source from each runner and combine the signals to stop the signal pulsing that would occur using a signal from a single runner).
- Too much advance whilst starting shouldn’t be an issue as the vacuum signal whilst cranking should be too low to add significant advance.

I am sure you could compensate a fair amount for the lack of vacuum advance with a well curved mechanical advance only distributor, but it isn’t quite the same. An example, cruising on the motorway you might be doing 3200 RPM and pulling ~12 inHg of vacuum. On my dizzy this would give me full vacuum advance (12 degrees) and full mechanical advance (16 degrees) plus static (10 degrees) which is 38 degrees (this figure holds true for either SU ported or Weber manifold vacuum).

If you were to tune for this amount of maximum mechanical advance then you would need 22 degrees static advance (maybe tricky to start?). You would have 32 degrees at 2000 RPM and 38 degrees at full throttle. I am no P Burgess, but I think this would cause so serious detonation issues! I am sure you could re-curve the mechanical aspect but it will always be compromised and you will miss out on the driveability and economy benefits that vac advance gives.

(by the way, I am guessing a little but I think the 16 degrees mechanical advance is conservative from a performance perspective, maybe you could go up to 20 degrees mechanical, therefore 30 degrees max advance, plus 12 degrees vacuum = 42 degrees at cruise).

Some background reading and where I got a lot of my thoughts and info from:

Link 1 (look for and press on the “chart” link): http://www.gofastforless.com/ignition/advance.htm
Link 2: http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

The only situation I haven’t fully thought about is what happens when you go from full throttle to no throttle at high RPM (extreme overrun) would this be a situation where additional advance would be undesirable? … Hmmmm…

I will find some pictures of how I have set things up (there is a vac take off on the DCOE runners, you just need to buy some hose tails) and I welcome your thoughts, cheers,
Malcolm
M Le Chevalier

Is there any reason you cant simply drill and tap a hole in the manifold for a dissy vacume advance

Prop
Prop and the Blackhole Midget

You haven't said what sort of Weber it is and some are designed with a facility to provide vacuum for a distributor. A photo would help identify it

Vacuum advance is designed to improve (part throttle/small throttle opening?) fuel consumption. If you are interested in fuel consumption then why fit a Weber (especially of the type without vacuum advance). Alternatively, probably a good, rather than poor or average, inlet manifold for the Weber will make a bigger difference to the fuel economy than vacuum advance will.
Daniel Stapleton

Sorry, this is a follow on from another thread, it's a Weber DCOE 40.

I believe all DCOEs have a plug next to the idle mix screw and progression hole cover that leads to manifold vacuum. A couple of M5 thread hose tails, a tee and some vac pipe from ebay and you are done.

I have read there are some rare DCOEs with ported vacuum in the style of SU carbs.

Cheers,
Malcolm.


M Le Chevalier

As an aside, I wonder if that mounting stud is fouling on the carb... something to check tomorrow! :-)

Malc.
M Le Chevalier

Tony, Malc is correct; fit a vacuum advance.
Nick Nakorn

hi nick
just for the record re the previous thread mine is a single 45dcoe..As I do not do many miles, the mpg is not an issue. Plus the fact my old vacuum distributor went to another needy midget owner!!!
However I was interested in the vacuum pick up off the carb. I will look see if there is one on the 45
rgds tony
a boyle

I have a 40 DHLA with a vacuum take-off, see image. When the throttle is fully closed there is a small amount of the drilling visible on the manifold side, in idle position I don't known whether this would be ported or manifold vacuum. Alfa Romeo seemed to think a vacuum advance was worthwhile.


David Billington

malc
I looked at my carb re your drawings, it does not have the take of point you point out. Perhaps the 45dcoe is different.
It is deff a genuine weber
the only problem is if I let the electric fuel pump
run before starting If there is too much fuel about the engine is slow to turn over but this clears quickly with the pump off and full throttle.

tony
a boyle

Nick, neither you or Malc are necessarily correct and the fact that the DCOE isn't generally fitted with an appropriate vacuum outlet speaks for itself.

Tony, if you really do want to fit vacuum advance from the DCOE then you need to establish whether the vacuum will draw from one of the carb inlets or both. If one of them then the regular Sprite/Midget vacuum unit won't be suitable as it draws from the manifold and thus a pair of carb inlets.

Dave - I don't agree that Alfa thought vacuum advance was a good idea because they fitted DCOEs to a lot of cars!
Daniel Stapleton

Daniel,

Maybe it was a fuel economy thing as that is one of the latest of the 3 pairs of DHLAs I have off Alfa Romeos, the earlier ones don't have that feature but do have a point like Malcolm mentioned.
David Billington

Daniel, heaps of cars use manifold vacuum (ie, not from a carb vacuum port) to ensure that there's plenty of available advance at low revs; for town driving and pootling about it gets rid of flat spots. Clearly, as revs increase at wider throttle openings, the vacuum tails off. The fact that the DCOE has no vacuum port doesn't change or effect the need for a vacuum advance at low revs. As Malc says, the carburation doesn't alter the general timing requirements at low revs. Vacuum advance at low revs is not essential, but it's better.
Nick Nakorn

Glad someone understands what I am saying Nik. The method of fuel delivery has no bearing on the need for a correctly timed spark which varies with both RPM and load, for the Midget we use mechanical and vac advance. Modern cars with EFI use maps and open loop calibration against RPM (crank sensor) and vacuum (the manifold pressure or mass flow sensor). Or you could invent any other alternative, another trained monkey perhaps!?!

It's unsubstantiated statements like "it doesn't have a vac outlet so doesn't need one" that perpetuate the thought that if you use a DCOE your engine suddenly no longer needs and will no longer benefit from a vacuum advance.

Hey Tony, getting back to your question, do you have any pictures of the top of your carb? I am interested. What sort of manifold did you end up with?

Although, if you are happy with the way your car goes then good work and maybe we shouldn't mess with it! I also love the sentiment that "MPG is not an issue". Good man, I hate those conversations that people have "ooo, my car does 52 MPG, oooo, mine does 59, blah blah" I have no idea what my modern does, I just drive it however I like, have fun and why the needle says 0, put some more in :-D

Cheers,
Malcolm.
M Le Chevalier

I dug out my AR Alfetta manual earlier and the screws Malcolm pointed out earlier are referred to as "vacuum port blanking screws" and definitely lead to the manifold side on all my DHLA.
David Billington

This thread was discussed between 30/03/2015 and 31/03/2015

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