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MG MGB Technical - 45D Distributor Vacuum Units

Hi,

I removed my Vacuum Unit today for cleaning and noting of serial numbers etc to ensure correct unit is fitted as I have too much advance with manifold vacuum attached at Idle?

My Distributor is a 45D4 - 41610, 54428147 (correct by Haynes manual) and fitted with electronic ignition.

The 3 numbers denoting start, max and someother vacuum seeting found on the vacuum unit are:

3 - 11 - 12

What is the correct Vacuum unit for a 18V engine with 41610 Distributor as 3 being the start of advance seems low?

Is my unit correct or has someone replaced the original with a non-MGB unit?

Thanks for all your help,

Rees
RH MARSHALL

I have a 45D4-41610 and that has the same vacuum unit.
John
john wright

3"Hg advance starts -
11"Hg advance finishes -
12 degrees advance at distributor Crank advance is double.

This is about right for 75 on cars, but it varies, and I do not have any UK specs. It is identical to 76-on Federal US cars, but I also do not have the actual Lucas part number to compare.

FRM
FR Millmore

Try here http://www.mgb-stuff.org.uk/curvestext.htm#curves
Geoff Ev

Hi All, like many have said this is a great Forum, but can become cloudy as so many of us have convining thoughts that are often erroneous! Me included.

Back to my Distributor problem.

I set advance at 10 deg BTDC at 1000 rpm with manifold blanked, top of vacuum open. Set idle and car runs OK.
Reconnect vacuum and I see my Advance walk downwards to 20 Deg BTDC; surely something is wrong? Quite clearly the diagphram is not punctured but is the spring weak as 20 Deg seems an awful lot hence I wondered if the 3 11 12 unit was correct. No I have not yet measured maximum advance and currently have my Dizzy apart for cleaning.

On subject, what does the 2 mean on the centrifugal flyweight

Please note that I am running on a Weber 45DCOE152 so using vacuum is questionable as Carb does not have any upstream port therefore previous owners have tapped into the Manifold where a vacuum is present at Idle.

Any thoughts

Rees
RH MARSHALL

"with manifold blanked"

There's the clue. With manifold vacuum you get high vacuum at idle and hence high vacuum advance. With carb vacuum, from the original SU carbs anyway, there is zero vacuum at idle and it only comes in as you open the throttle. You should get up to 24 crankshaft degrees of vacuum advance at 11 in. Hg., I'd have expected at least that at idle, so more than 10 degrees additional advance.

You should always use vacuum advance unless the car is used primarily for competition for its improved touring economy, and carb vacuum if you can get it is preferable. Manifold vacuum was just a cheap way of getting the emissions down at idle. When you connected the vacuum and saw the increase in advance you should also have noted an increase in idle speed. That's an increase in idle speed for the same throttle opening, hence you can turn the idle screws down a bit, hence use less fuel and emit less pollution for a given idle speed.
PaulH Solihull

The use of the Weber means the engine is modified. What if any other mods are in play is critical.
In any event, what the engine needs now has little or no relationship to what is "correct", in terms of parts lists and books.
You can attempt to tell us what else has changed, if you actually know with certainty, and if you are lucky somebody with experience on a similar setup can give you a starting point.
The single "right" way to sort it is to get it on a rolling road with a knowledgeable operator.

I agree with Paul, you should have your static setting plus 24deg = 34deg +/- 2deg, with vac connected. If you do not, you either have very poor idle vacuum (less than 11"Hg - measure it), pointing to radical cam or bad cam timing; or, the vac unit is faulty or mismarked, or it is possible that the distributor advance plate has been pinned or blocked to limit max vac advance.

The extreme idle advance this gives is normally suitable only for the lean mixtures/EGR/low compression of emissions engines; however, radical cams can give much of the same characteristics at idle, so it may be near to appropriate.

FRM
FR Millmore

I have a vacuum gauge which reads 15" at idle hence the amount of advance at idel is correct; basic + FULL vacuum.

Should I accept this or return to Mechanical Only? I guess the standard engine with SU carbs needs vacuum and having a Weber without vacuum port is causing probs.

I am tempted to modify the Main Venturi screw to accommodate a vacuum hose-adapter and drill through to allow a "ported" vacuum line to Dizzy. Whether it's the right position well hell, who knows and I can always reurn to a standard Choke post trial!

I read somewhere that another experienced engine-tune chap always sets maximum advance at 4000 rpm and adjusts idel to suit with mixture screws, throttle plate position etc etc.

Thanks

Rees

P.S. ALL my checks are in my garage, can't afford a Dyno run!
RH MARSHALL

15 is about right at idle, on a standard engine it should go up a bit as the revs are gradually increased, mine goes nearly to 20, then start to drop as the throttle is opened further under load. On the overrun it should go higher still towards 25. So try connecting and disconnecting the vacuum at a higher reading and see if the timing goes further. If it does then maybe you have a leak in the capsule diaphragm which is preventing it reaching its maximum advance. Try sucking on the pipe leading to the capsule. You should be able to move the points plate, block the end of the pipe with your tongue and hold the vacuum and hence the points plate movement. If the points plate moves back then there is a leak, which could be in your tongue of course. If you can hold a vacuum but the plate doesn't move maybe the points plate is stiff, or you aren't sucking hard enough.

As far as ditching the vacuum I can't see why, it obviously is working to some extent, so will aid touring economy if that is how you use it. But both the mechanical and vacuum curves are only going to be approximate anyway. They always were, got less relevant with leaded fuel, could be way off original through wear, and with a modified engine are pretty irrelevant anyway. Unless you spend a lot of time on a rolling road, or keep swapping springs and weights anyway, you are not going to be able to do much about it.
PaulH Solihull

My vacuum gauge is calibrated 14-18 "Late on timing"
18-23 "Normal" I expect near 18" generally.

You: "reads 15" at idle hence the amount of advance at idel is correct; basic + FULL vacuum."
Me: "static setting plus 24deg = 34deg +/- 2deg, with vac connected. "

Do not screw with your Weber, unless and until you thoroughly understand the precise location of the vacuum port, which is critical.

Consider that a session with Peter Burgess won't cost much if any more than a tank of fuel, which can be saved easily by getting the advance right. Not to mention more power and saving your engine.

FRM
FR Millmore

This thread was discussed between 18/06/2012 and 20/06/2012

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