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MG MGB Technical - Which vacuum source for Distributor

I haven't been able to confirm which of two possible vacuum sources to use for my distributor.

I have: 72B, SU HS4 carbs, 25D distributor, just rebuilt by AdvancedDistributors.

Recommendation (from AD/Jeff I think) was to use the vacuum port on the rear carb for the distributor.

Caution from another was to NOT use the port on the rear of intake manifold, as that may be too much vacuum for the diaphram on the distributor.

The reason I'm confused is I have two very different vacuum readings; 9 inches at rear carb port, and 15 inches at rear of intake manifold.

Yes, I realize the readings are low, but I cannot afford the luxury of re-building the engine just to get vacuum up as one person suggested. And I know I have slight vaccum leaks in carbs that suggest rebuilding or at the least - new seals on the brass shafts.

So, is 9 inches of vacuum from rear carb adequate or should I move to the 15 inches I get from the rear of the intake manifold?

For now I have it as I think Jeff at AD suggested, distributor to port on rear carb.

Your thoughts.


R.W Anderson

This has been covered before and the conclusion was that there is a diffeence in depression with the throttle closed but less as it is opened and so it is not important which take off port you use. There is a reason for moving the take off point which was emissions related AFAIK.
Stan Best

As Stan says, the only difference is that manifold vacuum is near maximum at idle, carb vacuum is zero at idle. Manifold vacuum allows the carb idle screws to be turned back to achieve a given idle speed compared to carb vacuum, so using less fuel and outputting less emissions. Once the carb butterfly starts moving off its stop, carb vacuum rises very rapidly to meet manifold vacuum, see

The efficiency of the MGB engine is such that the vacuum from both will be higher slightly off idle, than even manifold at idle, then as the throttle is opened further towards full throttle the vacuum gradually reduces to zero. On the overrun manifold vacuum can be as high as 25 in.Hg, but the distributor diaphragm is designed to cope with that. There is the possibility that manifold vacuum makes the engine slightly harder to start, as there could well be some vacuum advance while cranking. There must be some good reason that carb vacuum was used for so long, only changing to meet emissions regs. North America changed in 1971, UK not until September 76. And to dispel another myth, that you need a different distributor if you change between manifold and carb vacuum, even when the UK did change it kept the same distributor for the whole of rubber bumper production.
PaulH Solihull

Oh, in follow-up and for clarification; most emission equipment was previoulsy removed from car. I do still have the valve (PCV/Gulp - whichever it is called) that is mounted next to the valve cover that connects the engine side cover vent hose to the valve that is attached to the intake manifold, and the carbon canister that connects gas tank to valve cover, etc. No pumps though.
R.W Anderson

Use the vacuum source on the rear carb as Jeff suggests. It's more suited to the demands that the engine was designed for. Your engine can't produce enough vacuum to rupture the diaphragm. The maximum amount of vacuum that an engine, in excellent condition, can produce is 29" HG. If you think that you're doing harm to your engine, you'll hear what sounds like a coffee can, full of marbles, being shaken. This is detonation and indicates that your timing needs to be dialed back to a lower setting at idle. RAY
rjm RAY

PCV valve, and there is no benefit in removing that, in fact harmful if you do. The only benefit in removing the canister is to gain a bit of space in the engine compartment, but you then need to take account of that in changes to venting the tank and the crankcase.

The noise rjm describes is pinking (Friday lunchtime in the pub has temporarily driven the American spelling out of my mind), but that usually only happens when accelerating and vacuum (from either source) is greatly reduced or zero. Just retard the distributor fractionally until you don't get any at any combination of throttle opening, revs or load, which of course affects timing across the range, not just at idle.
PaulH Solihull

Paul, Two peoples, divided by a common language. RAY
rjm RAY

Pinging (but I still had to look it up!).
PaulH Solihull

Youth fades, memory fades with it. RAY
rjm RAY

I recently found this on another board. Rather long, but very informative:

This was written by a former GM engineer as a response to a similar question on a Camaro board:

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.


The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don"t work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will 'dither' in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it - they don't understand it, they're on commission, and they want to sell "race car" parts.
Geoff Pollard

Much of what I've been saying for years (especially about non vac 'performance' distributors), except for a couple of points:

"Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation."

Under high or WOT acceleration it is true that vacuum advance is zero, but at part throttle acceleration and cruising you are getting static advance PLUS centrifugal advance PLUS an element of vacuum advance to give total advance, the latter depending on just how far the throttle is opened and the specifications of the can. The point about the vacuum mechanism is that it gives a progressive reduction in its additional advance as the throttle is opened, it's max and min amounts and points being controlled by the spec of the vacuum can. These can start anywhere from 3 in.Hg. to 10 in.Hg., finish anywhere from 8 15 in.Hg., and give from 6 to 24 degrees of additional crankshaft advance - and that's just for the MGB.

"After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle."

I can't speak for American cars but the MGB and its ilk originally had carb/ported vacuum that gives zero advance at idle, and it was because of emissions requirements (71 for North America, not till 76 for UK) that this was moved to the inlet manifold which gives relatively high vacuum advance at idle. More advance at idle makes combustion more efficient which raises the idle speed so the idle screws can be backed off reducing fuel consumption and emissions. For those of you with carb vacuum pull the pipe off the carb and suck on it - hard - and see what happens, and for those of you with manifold vacuum pull the pipe off the manifold and block the manifold port ditto.
PaulH Solihull

Every American car, as well as the imports up until '68, had ported vacuum advance. I've never come across a pre-'68 car that has manifold vacuum advance. They must have been designed this way for a very good reason. Emission controls are a whole different kettle of fish. RAY
rjm RAY

25d was intended to run off the carb, 45's the manifold. The latter was a bit of a design cock up, as the vacuum characterists of manifold and carb are different.
Allan Reeling


Our 1967 Camaro 327 cu. in. w/2 barrel Rochester had full manifold vacuum advance as did our 1965 Chev. 283 cu. in. w/2 barrel Rochester.

However my friend's 1965 Mustang 289 cu. in. w/2 barrel Holley had ported vacuum advance.
Steven 67GT

The 45D4 was introduced for rubber bumper cars, but the UK didn't change to manifold vacuum until September 76 for the 77 model year. The characteristics are only different at idle, just off-idle they are exactly the same throughout the rest of the throttle range, see
PaulH Solihull

When and how much vacuum advance is applied to the base plate is only part of the story. The 45 has a different mechanical advance curve and a higher max advance. One assumes the two are always designed to work in partnership. Hence I wouldn't mix them. Then of course there is my inherent distrust of anything BL did to this engine to improve emissions!
Allan Reeling

BL even went so far as to restricting vacuum advance to 4th gear on later US rubber bumper B's.
Steven 67GT

Steven, My mother purchased a '67 Camaro, new, when they first came out. Unfortunately, she ordered it with the standard 250 CI 6 cylinder. The Rotchester Monojet carburetor, on the six, had ported vacuum. Go figure. RAY
rjm RAY

There were loads of different vacuum and centrifugal advance curves in the shape of individual distributor reference numbers for the MGB over the years, mostly for America. Not sure what is meant by 'not mixing them'.

The restriction to 4th gear was (according to Clausager) to prevent surging in lower gears, which was a side-effect of the American emissions kit. Originally this was achieved by a third switch that operated in the 2/4 plane being wired in series with the overdrive switch operating in the 3/4 plane, to get a 4th gear only signal for the vacuum solenoid while retaining OD in 3rd and 4th. The extra switch was a micro switch, and either they had problems with it or they need to save a few groats, but after only a few months they converted the OD switch to 4th gear only and did away with the additional switch. The irony is that this blocked the effect of manifold vacuum at idle (and gears 1 to 3) and its emissions reducing effects, unless you sat in 4th gear with your foot on the clutch! Not that that would have helped, as presumably you would want to be able to idle in neutral so the idle would have to be turned up enough for that anyway.
PaulH Solihull

"Mixing them"!! Besides the different characteristics of carb and manifold vacuum take off at idle. Vacuum advance and centrifugal advance are expected to work together at part throttle and at different speeds as well as seamlessly move from one to the other. I think a distributor intended to work with a particular vacuum configuration is best used with it, unless the mechanical advance is altered to suit. I've just re-build a chrome dizzy in an attempt to smooth out power delivery. The bob weight springs "looked" wrong. You wouldn't believe the difference putting the proper springs in made.
Allan Reeling

Both centrifugal and vacuum characteristics as selected according to the engine and what the designers want to achieve. For America after about 67 it was more and more to meet emissions targets, which was why they had no less than 16 variations whereas the UK only had two for chrome bumper and one for rubber. All of them work together at part throttle and at different speeds as well as seamlessly move from one to the other regardless of whether the vacuum source is carb or manifold. Vacuum source is irrelevant, as as all throttle openings where you are using the engine they are the same (but I suspect we are never going to agree on this). Changing any aspect of the centrifugal OR vacuum advance characteristics may have a more or less than beneficial effect or performance and economy, it's a reasonable bet that the earlier leaded specs are NOT ideal for today's fuels, and it's also a reasonable bet that most distributors in use today are neither to the original specs through age and wear, or the ideal specs.
PaulH Solihull

Yes Paul, agree to disagree, somewhat! I've only had one R/B B, which "ran on" alarmingly, I wonder now if that was down to the increased idle advance caused by the manifold take off. The static 10/13 deg, plus the 15 or so of vacuum is a lot of advance. I tried retarding to help the situation but that did nothing for performance, so i started adding (then!!) octane booster. It's propensity for this, potentially damaging, action I discovered was fuel (station/brand???)dependent. After experimenting I found an ESSO station which more or less, cured the problem. Variable knock resistence probably being the culprit. (Poor blending????). Did pumps blend then? This must also have been more of a problem in countries where, fuel quality and therefore octane rating, declined before it did here.
Allan Reeling

This thread was discussed between 17/02/2011 and 28/02/2011

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