Vacuum advance on SU HIF44. | MG Midget and Sprite Technical

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MG Midget and Sprite Technical - Vacuum advance on SU HIF44.

Whilst making experiments to measure the degree of vaccum at the engine breather spout on my SU HIF44 -- ( you might remember the tail end of last year, when we were discussing engine oil sucking ) --, I noticed that the distributor vacuum advance take off, on my HIF44, is on the MANIFOLD side of the butterfly, not the air filter side. Hence the distributor is exposed to full manifold vacuum when the throttle is closed, advancing the timing too much at the wrong time. When I fitted my HIF44 years back, I retarded my ignition a tad to compensate for poor running (without realising that it was manifold instead of ported vaccum), and forgot about it.

Then as part of the oil sucking experiments, when I put my twin HS2 carbs back on, it was too far retarded, and wouldn't tick over properly, because the vacuum take off on the HS2 is carb side, and not manifold side. However, having set the timing back to where it should be for HS2's with the correct vacuum take-off, I noticed it was more responsive to the throttle. Previously I just figured you lost a bit of acceleration with a single HIF44, as opposed to twin HS2's, but now I wonder if you should.

On the 25d4 with HS2's, as you open the throttle, the distibutor gradually sees increasing vacuum, and advances, until the vacuum drops off at fully open throttle. -- As I understand it. But with the HIF44 fitted, mine has been operating more or less in reverse it seems to me. So I've disconnected the vacuum and it generally runs smoother.

I might re-site the vacuum take off on the HIF44, as per HS2's, by drilling an other port. But before I actually do, 3 questions.

1). How much less fuel efficient(environmentally unfriendly) is it to run without vacuum advance on a 25d4? Or to put that another way, am I wasting much expensive petrol?

2). How much of a performance hit is there, if you disconnect vacuum advance?

3). My HIF came from an MG metro. How does the vacuum advance mechanism work on those didtributors?

Lawrence Slater

The other option rather than drilling etc is to fit a 23d distributor which doesn't have any vacuum advance (fitted to early spridget 1275's and mini cooper s) or remove/disconnect the vac advance from a 24d and change the advance springs to the 23d type.

Bob Beaumont

I have an MG Metro HIF44 and an MG Metro dizzy.

Acceleration is spot on, the carb is very responsive and gave good figures when Peter Burgess fettled her on his rolling road. (the old one, must book a new visit soon)

The vaccuum take off just in front of the dashpot seems to work just fine for me.

Maybe it's the MG Dizzy that does it.

Bill1

Yup, that's why I asked Bill. The vac advance on the MG Metro distributor, must work differently to the 25d4 that I have. Does anyone have a description of how the MG Metro distributor vacuum advance works?

Yup I could do that bob, but I've already got 3 good 25d4's. Cheaper to drill a hole in the HIF44. :).

What of my other questions? How much petrol am I wasting, and how much(if any) performance is lost without vacuum advance?

Lawrence Slater

The HIF44 has full vacuum at idle/throttle closed as an emissions measure. As soon as you open the throttle to do any kind of acceleration/steady movement, the vacuum is identical to any other set up. You simply set timing as usual (vacuum line disconected!) and then adjust idle speed on the carb to compensate for the high idle you get from bags of advance at idle.
If you have to re-time (more than a couple of degrees anyway for optimum running) with a carb swap, you're just not timing it right in the 1st place. Or the problem lies with something else.

http://www.mgb-stuff.org.uk/ignitiontext.htm#vac

It's about MGB stuff, but the information applies to any car that uses a dizzy and a vacuum module basically.

Roadwarrior

Yes, I guessed it's an emissions device, I was asking how much petrol I might be wasting, and performance I might be losing by not using it. Timing is set right!

Very interesting that MGB link, but according to this, --- http://www.minimania.com/SU_HIF44___HIF6_Vac_pipe_take_off_ --- , manifold vacuum doesn't suit A-series engines, and gives a rough idle, which is just what I found.

I'll drill another take off as he describes.

Lawrence Slater

Lawrence

Interesting stuff. I too have HIF44, no idea which car it originates from, 123 ignition - no idea of distributor but it is not original - and vac take off just behind flange to inlet manifold.

Appreciate your concerns, but frankly my timing is simple trial and error using a useful nearby long hill. Any problems are sorted by rotating distributor CW/CCW accordingly. For me, there are simply too many unknowns to time by the book as it were.

It now runs more or less spot on, and compared to the original properly set up SUs, definitely more economical and an improved response.

Mark O

Hi Mark.
Not really concerns as such, more just wondering. And the thing is, I only changed to the HIF on a whim really, because it was cheap to do it, fashionable at the time (lol), and wear in the spindles of my HS2's made balance at idle more "fiddly". Above idle though, it wasn't a problem. I like the HIF for it's comparative "simplicity", but other than that, I wouldn't say I gained anything at all by swapping, and had to retard the ignition from "normal" to get a decent idle.

Then as an experiment with oil sucking, when I swapped back to the twin SU's I noticed that actually, the engine seems more responsive with HS2's, and apart from the slight fiddle to get balance at idle, I wondered why I bothered to swap to an HIF in the first place. Then noticing that the HIF take off was manifold and not ported, I disconnected the vacuum, and got a much better idle, with the timing set as per book.

So I wondered if there was a definitive answer about manifold vacuum advance vs ported, vs no vacuum advance at all. And the answer as usual to these type of questions, is less than certain.

The MGB experience piece says it doesn't matter if your ported or manifold, and Kieth Calver says it should be ported. The only certain answer seems to be when racing, that vacuum advance does matter because you spend most of the time with your pedal on the floor. But I don't race.

It'll cost me nothing to drill a hole and convert the HIF to ported vacuum advance. Then I'll have the definitive answer to whether or not it makes any difference on my engine.

Lawrence Slater

Lawrence.

I'd be most interested to hear how you get on with this. Keep us all posted please.

Gavin Rowlesx

Hi Lawrence,

When I got my '74 US spec midget it came with a 25D4, and the vacuum was taken at the manifold (twin HS2s, no vacuum port provided in them). Sort of like your experience, when I rebuilt the car (the first time) I put in a new vacuum unit (maybe from Moss, or maybe from VB, I don't recall) and the advance under acceleration was way off. Once I had identified that the vacuum was not at all what it should have been I looked into options like buying another vacuum unit or what have you (and I didn't know then about the differences between manifold or ported vacuum sources for timing advance), so I plugged the ports (on the distributor and on the manifold) and drove it like that for 15 years.

My MPG the whole time was 24 (US gallons), or about 29 (Imperial gallons).

I suspect that if I installed a proper vacuum unit, and drilled the carb body for the ported source that I'd maybe / maybe not see any difference in my mileage. Also, maybe there would be a reduction of emissions in some circumstances.
Maybe this information is helpful to answering your question.

YMMV,
Norm

Norm Kerr

ooh! here it is:

http://www.460ford.com/forum/showthread.php?t=117504

Excerpted key point which directly addresses your question (and reminds me why I never felt the need to drill my carb body):

"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."

Norm "manifold vacuum for me, then" Kerr

Norm Kerr

My HIF44 has the vacuum take-off (just) on the manifold side of the throttle butterfly as well. It seems to work well enough. As far as I can recall, it responds as well as the previous twins, and certainly gives better fuel economy at 40 to 42 mpg. - But then I don't live in London ;-) !

Manifold vacuum is greatest, not at tick-over, but on the over-run, when the engine is still turning over fast, but throttle is shut, largely closing off the available air supply. But the vacuum also is relatively high as the engine is accelerating and trying to suck in lots of air. It is at its lowest on a constant part throttle opening, as when cruising at a steady speed on the M-way.

I suspect that the vacuum advance unit actually responds to a fairly modest reduction in pressure which is why it works with the normal twin SU take off. The greater suction provided by a manifold connection is just surplus to requirements so works just as well. What may be of greater significance is when that suction occurs, rather than how strong it is.

Guy W

Thanks for that Norm. I've read the quoted bit, and I'll read the whole lot later tonight.

Guy, you suspect right. The standard vacuum unit on a 1275 HS2 equipped Spridget (UK spec anyway) is, type number 54418274, and 5-8-3. Vacuum advance starts at 5ins, finishes at 8ins, and pulls 3 degrees of advance. So the effect manifold vacuum has on it, must mean that it's not often pulling less than 3 degrees of advance.

Lawrence Slater

PS. I should add that it's 3 degrres in the distributor, not on the crankshaft. I think that explains why I had to retard my timing when I put the HIF on. Manifold vacuum simply causes too much advance on idle, and makes it run badly.

Lawrence Slater

here's more reading on this subject, this time from an MGB forum

http://www.mgb-stuff.org.uk/ignitiontext.htm#vacmod

excerpt:
"the only difference between (manifold and ported (carb) vacuum) is at idle and just off it. Manifold vacuum is high at idle reducing to almost zero as the throttle is moved towards fully open. Carb vacuum is zero at idle as the butterfly plate covers the port and the port is effectively on the piston or low-vacuum side of the butterfly. As the throttle starts to open the port is uncovered and is effectively moved to the same side of the butterfly as the manifold port i.e. the high-vacuum side. Therefore the vacuum rises very rapidly, and when the throttle is only slightly open it becomes the same as manifold vacuum, thereafter it reduces gradually to almost zero as the throttle is moved towards fully open, exactly as manifold vacuum does."

Norm

Norm Kerr

Hi Norm. That's the link posted earlier, but now having read more on the subject, I don't agree that it doesn't matter if you use ported or manifold vacuum.

That's a B series engine, and I've now read several texts that say the A series engine is better on ported vacuum advance. The A+ as I understand it, was speced to use manifold vacuum advance, and presumably had a different advance curve requirement. Otherwise you could just swap it to ported vacuum, just for the hell of it, since it wouldn't matter.

I think it depends on how the orignal was spec'd. My engine was spec'd with ported vacuum advance. I've found that using manifoild vacuum, throws the timing out at idle, and give me a bad idle. I have to retrad the timning to compensate, and that affects the engine above idle. It now seems logical to me to simply revert to ported vacuum, and restore the base timing to the spec in the book. So that's what I'm going to do. :).

Lawrence Slater

Without getting into the technical aspects of manifold v carb vacuum, the higher advance at idle with manifold vacuum should give a much smoother idle, not a rougher one - I'd be looking into that!

Chris at Octarine Services

Well the fact is that it doesn't Chris. Using manifold vacuum advances the timing at idle by 6 degrees(at the crank). That's on top of the static. How can that be correct, and not make any difference, or improve the idle.?

Even with my twin carbs on, if I swap to manifold vacuum for the advance, the idle isn't right. I'm not that well versed in advance curves, cam overlap etc, and whatever else is going on, but the engine setup was designed and runs well at idle, on ported vacuum. Swapping to manifold vacuum, it runs less well. It surely follows that I'm simply better off by swapping back to ported on the HIF, exactly as keith Calver describes.

I've also found an answer to my question about petrol consumption. It seem that without any vac advance at all, I'm likely to loose up to 5mpg. So I won't simply leave it off, as was one of my considerations.

Lawrence Slater

Well if it is correctly matched to a 45D4 dissy and correctly set up it will do.

The manifold vacuum and 45D work as a vacuum retard system, so the advance responds to meet the same engine demands as the carb / 25D vac advance system but runs a higher advance at idle allowing a leaner mixture hence saving fuel and lowering emissions.

Chris at Octarine Services

Lawrence,
please don't misunderstand my excerpts from those links, my intention was not to say that there was no difference, but that there was a big difference at idle

as a result, the distributor must be made to match which kind of vacuum source is used - mixing them up can cause trouble

Norm

Norm Kerr

Just to add to this, when I took the little bue car to the RR (I had previously discarded the 59D4 vac system) they found engine wasn't getting enough advance early on. A couple of runs showed where the advance curve needed to go, and the bob weights and springs were duly replaced with early Cooper.This instantly inproved the idle and gave midrange another 8 bhp for the same overall advance.Probably improved the fuel consumption too - but by how much I don't know.

Fergus

This topic has got me digging out my notes to refresh my memory. Granted, all of the links I've provided are for other engines (mostly the GM V8 seems to attract people to write in depth explanations of how timing works).
I am sure the guys over at Advanced Distributors, or many experienced engine tuners could teach us a lot, but since that is their business I wouldn't want to ask them to give their valuable knowhow away. So, I read as much as I can and look for common themes that would apply to all engines, and then look for evidence of how to apply that to an A series engine. In any case, a modified engine needs to be carefully set up to optimize its performance and these guides should be taken as merely guides to help grasp how to logically approach it.

For an unmodified engine I'd follow the shop manual for setting the timing. But the interesting thing is how much BL changed the timing each year in the US, starting in '68, in repeated efforts to meet the tightening emissions requirements, while the UK Midgets kept the '67 spec, so studying this stuff has helped me to better understand what the heck they were trying to do when they made those changes.

Here is another link explaining the difference between manifold and ported vacuum, and the reason why ported vacuum was adopted for emissions control:
http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf

Brief excerpt: "When emissions became a priority to vehicle manufacturers, a method had to be found to reduce emissions at idle. The amount of Hydrocarbons emitted out of the tailpipe can be drastically altered by changing the timing: Retarding the timing reduces Hydrocarbon emissions. But retarded timing adversely affects gas mileage at cruise. So a method was needed to retard timing at idle, yet maintain it at normal levels for cruise. The solution was seen to ìturn off the vacuum advance at idle, yet have it operate normally under all other operating conditions." By using ported vacuum, they could use a vacuum advance which provided good characteristics while driving, and shut it off at idle. If manifold vacuum was used, a vacuum unit that retarded the timing enough at idle to meet the emissions target would have caused terrible idle and cold start performance.

Norm

Norm Kerr

One thing that I find really fascinating about this subject is that every time I run across a number that is used as a rule of thumb for, say V8 engine timing decision making, like for example these two limits are referred to by many authors, writing about different kinds of engines:

"max total mechanical advance should be checked that it does not go above 32~34 deg"

"max total cruise advance (mech + vac) must never be allowed to go above 52~54 deg"

Then, when I then go and study the numbers in the Bentley manual, I see those very same limits showing up there as well.
And, when I had trouble burning exhaust valves in a freshly rebuilt, stock 1275, after confirming everything else, it turned out the root cause was an improper vacuum advance unit that was taking the cruise advance way above 60 deg.

It may be only a coincidence, since the burn characteristics of each manufacturer's head must be really different. But anyway, I thought it was an interesting observation.

Norm

Norm Kerr

Here is perhaps the best link showing the difference between manifold and "carb" vacuum sources and what they do to running (this from an MGB site):

http://www.mgb-stuff.org.uk/ignitiontext.htm#vacmod

scroll down to "Vacuum Advance - Carb vs Manifold", where his experiments comparing the parts, and measurements, showed, for an MGB:

"The important thing is that both carb and manifold vacuum give the same results in most normal driving conditions. The only reason for the change is that manifold vacuum results in a higher idle speed than carb vacuum. This allows the idle screws to be backed off slightly to achieve to same idle speed, which reduces fuel consumption and hence pollutants. The final thing to remember is that UK cars didn't get manifold vacuum until September 76, but had the same engine and distributor from the start of rubber bumper production in 1974 to the end of production in 1980. Which itself is surely proof that the two are interchangeable."

What this makes me think is that perhaps RoadWarrior was right in his initial post near the top of this thread, perhaps the issue with your engine is not where the vacuum is being taken from, but some other issue with your timing?
As I had found with my engine, an incorrect part used in the system caused me problems that took systematic searching to track down and remedy.

Interesting, too that this MGB information indicates that perhaps BL used advance vacuum differently than the Americans did (note my earlier (US engine) links all referred to manifold vacuum being the usual way and ported vacuum adopted to reduce emissions, but the MGB story in this link went from ported to manifold). Such a difference highlights how important it is to separate the "universal truths" from "specific to a particular engine" types of information.

Here is one more link, this one written for the A series engine (finally!). I should have started with this one, but I found it last:
http://mgaguru.com/mgtech/ignition/pdf/tuning_lucas_dist.pdf

He describes at length the differences between parts available for tuning Lucas Distributors, and compares some aftermarket / alternative OEM distributors in depth.
Also, he does not try to state what is "best", instead he shows what is available and what can be done, but then also states:

"What is clear is that ignition advance curves are simple to change and assuming the correct advance curve is known, it is easily obtained. It should also be clear that an existing curve should not be trusted to either be correct for the application at hand, or that the distributor is delivering it accurately. Each distributor should be checked through its full range of operation so that a single-point calibration setting (like setting the timing at idle), when it is in the engine, will deliver the intended results."

Norm

Norm Kerr

Chris. Exactly. As I said, I have a 25D4 distributor, and you've now answered the question from my first post, and confirmed my suspicion. MG Metros have a distributor that works in reverse as far as vacuum advance goes, -- when compared to the 25D4.

So yes, I could get 45D, or simply drill a hole in the HIF.

All in all, if I do say so myself, I think this has been a good thread.

When converting from HS2's to an HIF, you also need to consider where the vacuum advance is sourced (if it bothers you). And if you get the chance, grab the 45D from the car you might be taking the HIF from. Maybe a pity I didn't as it was sitting right under my nose, but easily solved by drilling a hole.

Norm. Thanks for all those links, I'll read and digest them today. God knows why(I love saying that, even though I'm an infidel), but this subject seems to have got me unduly interested. :-.

Lawrence Slater

Not so easy to swap to the Metro dissy, as the mounting is different, and possibly the drive, also.

Bill will know, as he has done it.

Dave O'Neill2

Yes indeed.

The mount block in the engine has to be butchered and the drive shaft has to be swapped from the Metro too.

And sadly you have to use the 90 degree dizzy cap as no alternative ones can be found fer luvvermunney.

But you get MG Metro performance and economy.

If anyone wants the info I can pop it in here.

You do need a lathe to cut down the mounting block thickness though.

Bill1

I did have some side-entry 45D dissy caps. I may have a couple left, if anyone needs one.

Dave O'Neill2

I think the issue was finding a side-entry cap that fitted with screws rather than clips for the Metro dizzy. Brain fade, so I am not now sure, but I know I modified a dizzy cap using resin to build up the area prior to drilling a couple of holes for screw fitting. The problem with straight entry caps is that the leads then rub on the steering column and eventually start shorting.

Guy W

Does the Metro use screws rather than clips?

My racer had a 45D with clips, but the caps have screws. I used to file a groove in the cap boss for the clip to locate.

Dave O'Neill2

Mine uses screws Dave, but of course I meant to say I could only get a straight sticky outy cap to fit it

Obviously didnt try hard enough :(

Could be the story of my...

Bill1

Aha the Metro dizzy is a Lucas 65DM4 and is different to and smaller than the 45Ds.

Pictured below is the 65DM4 (this one suits some Minis it seems.

Bill1

Aha, the plot thickens!

Dave O'Neill2

Lawrence

Two points

Myself and many others seem to recommend fitting the HIF 44 - so I guess you are 95% there by having one fitted.

Fine tuning thereof does indeed appear - from this very thread - to be highly subjective. By all means I would try a tapped hole - you can always plug it if it does not work, but again I would state that there are simply far too many unknowns to state with any degree of certainty just what the result would be. Still think it would involve a CW/CCW tweak of the distributer until you are happy - ish.....

Mark O

I changed to HIFF and Aldon distributor at about the same time.

HIF was from a 1275 Ital and has the vaccuum take off manifold side of the throttle in the carb body. The Aldon has a vacuum advance and they seem to work very well together- though I haven't had it on a RR.

For the distributor cap, the 45D based Aldon came with a top entry with clips. Like Dave...I bought a screwed side entry and spent about 30secs with a dremel to put some grooves in so it would hold with clips. Aldon wanted something like £25 for a side entry cap....the screwed one was about £6 off ebay.

My notes suggest it was for a 2.0L Rover SD1 part DDB194.

(like this one for £7.95 delivered... http://www.ebay.co.uk/itm/Ford-crossflow-Lucas-side-entry-Distributor-cap-DDB194-/170647998650)

Dean Smith ('73 RWA)

Hi Mark,
When I fitted my HIF (10 years + ago ) I fairly soon worked out that in order to make a decent idle, I had to retard the ignition, but I didn't know why, and didn't look into why. I just accepted it, and assumed the slightly reduced response on the HIF, compared to the HS2s, was characteristic of the HIF.

Then at the tail end of last year, when I first realised that the vacuum take off was on the manifold side, I realised why I was having to retard the ignition with the HIF fitted. So when I put the HIF back on again -- after experimenting with oil sucking, whilst running with HS2's ---, I left the ignition timing at the correct static setting, and disconnected the vacuum connection on the HIF instead. Consequently I got a decent idle at the correct ignition timing. But I didn't really fully understand the implications, and put it on the back burner until I started this thread.

Now though it all seems blindingly obvious to me.

Static timing on the 1275 Spridget is 7deg btdc.
Vacuum advance is an additional 6deg at the crank, at 8Hg vacuum.
My manifold vacuum at idle is 20Hg.

Thus with the HIF fitted and the vacuum advance connected, at idle, my ignition timing is never less than 13deg btdc. 6 degrees above what it's supposed to be.

In order to rectify that, I was retarding my ignition by some amount that I didn't measure, but that gave me an idle. That was great at idle, but meant that I was running retarded throughout the rest of the rev range. Hence I now realise why my HIF performance seemed "flat".

Now I've got the vacuum disconnected from the distributor, I've got a decent tick over, and the timing is correct, static and centrifugal. But I'm not getting any vacuum advance when it's needed, which apart from affecting economy, I've read also affects performance to some small degree. How much, I don't know.

But the obvious and simplest solution, is to drill a hole in the HIF, on the air filter side of the butterfly, and connect the vacuum advance to where it was designed to be when using a 25D4 Lucas distributor.

Lawrence Slater

Hi Lawrence,
I have been following this thread with some interest and would be keen to establish further detail on your intention to drill and fit a new vac take off on the HIF44, location, hole size, take off insert etc. I have a similar set up to yours but would rather run with the vac connected.
Ross

R Livingstone

OK, but I still do not appreciate the relationship between extra vacuum and retardation. I have always set timing at dynamic - with vac disconnected - and taken it from there. Personally a bit of slightly uneven idle now and again is just a trait of the HIF 44 and a trade off against the improved overall performance. You may be chasing shadows, but I would be interested to see if drilling achives anything.

Mark O

Lawrence,

If you drill your HIF44 for ported vacuum, you will want to make sure that the hole ends up so that it will be on the air filter side of the throttle plate only when the throttle is fully closed.

It is a pretty small target.

If you put it further away (towards the air filter) than that, then you will have some part throttle situations where no vacuum is provided to advance your distributor, and the result will be the same, "flat" response that you had noticed, under those conditions.

Themes:

vacuum advance is used to provide more burn time for lean mixtures

Part throttle cruise is lean (the smaller the throttle opening, the leaner)

Idle is (usually) lean, too (and that's why manifold vacuum is used on engines with a lean idle, because it improves the burn time and helps improve the idle and start-ability of those engines). Apparently BMC engines with SU carbs had a pretty rich idle and didn't need it, until the '70s when the US spec cars did, and they switched to manifold vac to help with that.

Norm Kerr

One more observation Lawrence:

static timing 7 deg

When you check the timing at idle, with the vac disconnected, the mechanical advance should be 6 deg at 800RPM, so the combined advance at idle is 13deg (this is the usual stroboscopic timing target shown in the manuals for the UK spec 1275).

The total mechanical advance should be 22deg + 7 at 3600RPM (there is a small typo in the Bentley manual, on page 37, it shows "at 5600RPM", but then that is corrected later in the supplement pages).

A good check to do, whenever troubleshooting an engine running issue, besides checking the valve clearances, and the points & condenser, plugs and wires, is to measure the total advance at high RPM. If it is 13deg at idle, and 29 at 3600rpm (and doesn't go above that), then your mechanical advance system is basically OK (the Bentley includes the advance at several RPMs for you to check the springs are also OK too).

The vac advance should be checked that it starts to move at 5"Hg, is "all in" at 8"Hg. You can't really measure the advance in degrees without running the engine. So, if you idle the engine (and if you are using manifold vacuum, and if your cam and head breathing set up are actually pulling more than 8"Hg at idle - check it to be sure because hotter cams tend to pull less vacuum), then you should measure 7 + 6 + 6 at 800rpm, or 19 deg.

If you are worried about that amount seeming like it might be too high or something, you can confirm it by driving and listening for audible pinking under various light throttle loading conditions. For best economy, power and drive ability, you want your vac to be providing the most advanced timing that your engine can tolerate without pinking (find the limit then back off 2 deg).

OEMs tend to spec the vac on the safe side to avoid damage from build tolerances, poor maintenance or fuel octane. Carefully setting up the vac curve to your particular engine should release some hidden power (and fuel economy) in the mid range.

Note that you always want your timing advanced as far as your engine will allow, without pinking, so you can even measure the total mech advance (vac disconnected to avoid working at cross purposes) and increase it until you find the limit, then back off 2 deg.

Norm "studying up on ignition design these days" Kerr

Norm Kerr

Ross, Mark, Norm, et al.

From the link I posted earlier. Kevin Calver says ---

"The solution is easy - remove the vac take-off from the manifold and fit it back to just in front of the butterfly where it should be. If you need to drill a port to accept the vac pipe take-off it needs to be sited 9/16" back from the carb to manifold mounting face, offset slightly to one side. I.e. NOT at the dead centre, or 'peak' of the butterfly bore. And usually set off away from the crankcase breather port. Usually the vac pipe take-offs fit a 1/8" diameter hole - but measure whatever you're going to use before drilling. You're looking for an interference/air tight fit."

Mark said. "I have always set timing at dynamic - with vac disconnected -- " .
Which is, on a perfect distributor with no worn parts, the same as setting the engine to the static spec. With dynamic, the vac is disconnected, and the figure given for btdc takes into account how far the plate has been moved by the centrifugal weights.

As Norm has just posted, vacuum advance is used under the conditions he describes. Not using it, results in a less efficient burn, which not only means less economy, but a loss of power. I don't know how much, but if the fuel isn't being burned properly, there must be a loss of power, as well as a waste of fuel.

My logic on this simple. I accept that BMC designed 1275 Spridget engines coupled with a 25D4 distributor, to function best using ported vacuum. I've changed to a carb(HIF) that only has a manifold vacuum take-off, which advances the timing too far at idle. So I'm going to restore the ported vacuum take-off(little or no vacuum advance at idle), thereby restoring the correct timing at idle, whilst allowing for the timing to be advanced by vacuum at the appropriate times according to load and rpm.

Lawrence Slater

Norm.

My manifold vacuum at idle, is 20Hg, as posted this morning.

Hence at idle, with the HIF connected manifold vacuum advance, I'm way over the 8Hg needed to pull an unwanted additional 6 degrees of advanced ignition timing, at idle, which is the wrong time.

Lawrence Slater

I can't wait to get my B back together. It left SU with a single HIF6 carb, with manifold vacuum IIRC.

It always seemed to work well despite its low compression North American spec engine.

It won't be running that engine, though I will be retaining the carb.

Dave O'Neill2

How long's it been off the road then?

Lawrence Slater

Only 14 years.

Dave O'Neill2

Such much?

It's probably grown into a "C" by now. ;)

Lawrence Slater

I had a problem similar years ago on a late mgb that was converted to earlier carbs. The vacuum advance on the distributor was a mismatch of parts causing a stumble and generally not behaving correctly.

I found my answer here:

http://www.teglerizer.com/home.html

Click on "mg automobiles" and then "stumble at cruize"

He talks of mismatch of advance diaphrams...How to read the little numbers on the unit itself. Timing wieghts etc etc...

Im sure you have been researching a lot already but hopefully you can glean something out of this guys site.

You may be on to something new with your set up Lawrence...Youve definately done your homework
already... Just trying to help and good luck!

:-)

Steven Devine

Cheers Steven.

I couldn't find the "stumble at cruise section". But anyway there's no mismatch of parts on mine, only a wrong vacuum advance take-off connection.

However, I suspect when I get around to connecting the vacuum advance to the air filter side, it will improve not only idle, but maybe also performance in higher revs. I'll have to see.

Lawrence Slater

Lawrence,
When I fitted my HIF it worked fine straight "out of the box". - Actually it was straight out of the scrapyard. Other than a clean up and replacing the needle, I made no other special adjustments. And it worked fine. Its been on there for a good many years now but as I recall it was smoother than the twins, accelerates just as well, if not better and certainly gave much better fuel economy. I don't believe I purposefully had to retard the ignition although I would have almost certainly checked the timing as I just like fiddling with things!
But I am using it with a Metro dizzy, so this throws in another variable factor

Guy W

Hi Lawrence,

when you make the change, the only difference will be at idle

As has already been established, the vacuum at everything other than idle is the same, for manifold or ported take off.

Norm

Norm Kerr

Hi Guy.
Chris at Octarine Services explained a while back, that the Metro distributor vac advance, works effectively in reverse when compared to a 25D4.

Norm, I don't agree that it has been established. That explanation is on a B, not a 1275 Spridget. And I can tell you, as I've just observed it (30 minutes ago), that my manifold vacuum doesn't drop below 10Hg, as I pull away from a standstill at idle. Hence as I explained, I'm too far advanced.

Lawrence Slater

Yes, that is what I meant when I said "this throws in another variable factor". Meaning that my set-up varies from yours.

Why would yours be "too far advanced" "as you pull away". Surely as you pull away you want it advanced as you accelerate?

Guy W

Guy.

Acceleration is different to starting from standstill. There is far more engine load when starting from standstill.

I borrowed this bit of text.

"As an engine comes under a load it cannot tolerate as much spark advance. Also, when an engine comes under a load, intake manifold vacuum decreases. So the vacuum advance works out well by retarding the spark (less advance) when the engine comes under a load."

As I just said, the manifold vacuum doesn't drop below 10hg, on my engine as I pull away from stationary, unless I were to floor the pedal. That's enough to add an extra 6 degrees of advance to the ignition timing, when it shouldn't be there.

If it was connected "ported" there would be little or no vacuum at idle, and as far as I can remember, but can't test at the moment, it doesn't instantly jump to above 8Hg, because again, when pulling away, I don't floor the pedal. The vacuum would increase as the rpm rises, and the throttle is gradually opened more.

But as I've said before.
"However, I suspect when I get around to connecting the vacuum advance to the air filter side, it will improve not only idle, but maybe also performance in higher revs. I'll have to see"

Lawrence Slater

And, what I should also have added, but forgot, is that --

When I re-connect "ported", I will be able to restore my static advance to where it should be, and that will affect the ignition advance across the whole rpm and load range. I'll no longer be running "retarded", in order to get a decent idle.

Lawrence Slater

There might be an impression, that I'm trying and desire, to improve my engine's hp performance by altering the vacuum advance. I'm not per se. This thread is really more about my curiosity. I wasn't particularly unhappy with the performance of the HIF44 as it was. Equally, I wasn't overly impressed with the HIF when compared to the HS2's. On the plus side, I was happy enough to never have to balance twin carbs again, although it's not really much effort, and not often required. I just happened to discover that for a 25D4 distributor, the HIF has the vacuum advance take off in the wrong place. This made me wonder what the consequences of that might be.

I don't expect to suddenly gain noticeable hp by connecting the vacuum advance to where should be connected to, when coupled to a 25D4 distributor. Clearly though, there must be some "theoretical" gain, not only in terms of a decent idle with the ignition timing set where it should be, but in terms of better fuel economy, and as the fuel is burned more efficiently, in terms of hp.

I used the term theoretical, because as it says in the link Norm posted, -- http://mgaguru.com/mgtech/ignition/pdf/tuning_lucas_dist.pdf ---

" --- anyone professing to know what curve is suitable for your particular engine formula, without doing dynamometer testing, is offering nothing more than an educated guess, --- ".

So even when I re-connect my 25D4 vacuum advance to a ported take-off, and set the ignition timing to the book quoted static figure of 7 degrees btdc, unless I put it on a RR session I'll still be guessing that I'm getting the most out of it.

I'm happy with that, since there is little effort involved and it will cost me nothing monetarily either.

Lawrence Slater

Lawrence,

I don't know much about the HP change, but if you have been running retarded at cruise because of the timing you set for the idle, then I think getting your advance back up at cruise should improve you mileage.

I had an old Saab 99 that was getting about 24 mpg (U.S.) with the factory timing setting. But, it tended to spark knock even with premium. I stared adding ethanol to my fuel (about 20 - 25 %) and I doubled the static timing from 9 to 18 BTDC. The result was no spark knock and 29 mpg.

Charley

C R Huff

Cheers Charlie. I agree. :).

Lawrence Slater

" --- anyone professing to know what curve is suitable for your particular engine formula, without doing dynamometer testing, is offering nothing more than an educated guess, --- ".

So even when I re-connect my 25D4 vacuum advance to a ported take-off, and set the ignition timing to the book quoted static figure of 7 degrees btdc, unless I put it on a RR session I'll still be guessing that I'm getting the most out of it"

CORRECT! Which is why the only practical recourse is trial and error by rotating the distributor!

Mark O

NO MARK!!

The practical recourse, as you put it, is to reinstate the ported vacuum advance take-off, --- to match the operation of the vacuum advance on a 25D4 distributor, because that's what I've got --- , and then to set the ignition timing to the static book value, and then to fine tune it by experiment(guessing) or RR session. I'll experiment(guess), it's cheaper.

Lawrence Slater

wait, perhaps Mark and Lawrence are both meaning the same thing, but misunderstanding each other:

Sure, set the car back to factory spec first (which is what Lawrence is doing) as a good base line. Then, if your engine is modified from standard (different cam, or ported head, or increased CR), you want to confirm that curve is still going to be OK, rotate the distributor to verify that there is no detonation at various load settings/throttle positions, either on a RR, or by carefully doing it on an empty country road.

If you are systematic about it, you can check each scenario, at each key RPM to study and confirm, and then based on your notes, have the distributor re-curved and/or buy a different vac unit, based on what you find. Or, based on the results, decide that what you have is good and be satisfied at knowing a job well done!

Of course, if your engine is stock, then the stock curve should be fine and no need for the experimentation.

"I love it when a plan comes together" - Hannibal

Norm

Norm Kerr

Hanibal who? Lecter? lol.