Its really quite simple.... it bolts straight in.  Why choose one?  they're not expensive, they fit, they have an appropriate cut in/out/advance spec ..and Aldon didn't make a vac assisted dizzy when I bought mine.

No the results I'm talking about were scatty because its very difficult to emulate a steady, part throttle cruise on the dyno.. so we couldn't get an accurate measure of part throttle fuel consumption... without more time and effort than seemed reasonable.  And we knew the answer would be "better" anyway.  Exactly how much better wasn't obvious until we had a bunch of real world experience.

The Special Tuning HS4 (twin) setup has a conventional port vacuum connection.... on one carb.

I've tried manifold vacuum ('cos IDAs don't have a vacuum port) and it works ...just not as well - maybe needed a different unit?  We really didn't spend as much time and effort on that set up as I decided to swap to the SUs for other reasons. 

Cheers, Ian

Vacuum ports on the air cleaner side of the throttle butterfly are "ported vacuum".  The vacuum is at its highest level when more air is flowing (i.e. WOT).
Vacuum ports on the engine side of the throttle butterfly are "manifold vacuum".  The vacuum is at its highest level when decelerating from high speed with the throttle closed.  The vacuum will measure somewhere between 15 and 20 inches of mercury on a well tuned healthy engine (running a factory cam) at idle.

Some cars use ported vacuum advance, others use manifold vacuum.  I cannot tell you what the Mini uses.  This is a question best addressed to Jeff at Advanced Distributor.  

I have no idea how... or why... you would graft a ford vac advance onto and Aldon dizzy. Might explain why the results are "scatty". Do you mean spark scatter? It may not be activating in a conventional manner. It would also depend on where it is connected to the carb and what type of carb you are running - some have the vac nipple ported before the throttle plate and some after.

Hi Dan, pity you live on the wrong side of Canada ... I'll be in Calgary in a few weeks and this is a discussion that really needs a few beers

I still maintain that at idle ... "who cares??"  

I really don't know how open the throttle is at 70 - I suspect its a bit less than half.  When my car was set up it didn't have the vac unit but the dizzy curve and mixture (needles) were optimised to give a smudge under 100 hp at 5K.  We then fiddled with more needles and kept the hp with better consumption.  After a year or so the expert (not me  decided we could do better so a vac advance unit from a Ford was grafted onto my Aldon Yellow.  We checked the curve ..and surprise.. it was pretty much OK.  Why wouldn't it be at WOT.  

We didn't really do any dyno work with the advance unit because (A) it was the one I had and (B) the results are pretty scatty...

But I did keep fuel consumption records ..and under the same conditions (over a year or so ) the outcome was 30% less fuel.

QED.

You should try it..

Cheers, Ian

A very good discussion on distributor vacuum control for street mini's. Never did hear back from the original poster on testing it's function, I have seen more than a few older collector car's with leaking/ripped vacuum diaphragms.

Ian, the picture is a little different from that.

If you are cruising (steady speed) at 4000 rpm, your throttle is at least half-way open, probably more. Any less and you'd be experiencing engine braking and a loss of speed. At half throttle, even with the engine at 4000 rpm, manifold vacuum is comparatively slight, so any vacuum advance will be pretty much non-existent but your mechanical advance will be full on. Mechanical advance is not dependent on or affected by carb or vacuum - only the speed at which the engine is turning.

Mechanical advance curves max out at around 4000 rpm - it has to do with lighting the fire so it gets fully developed when the piston gets to top-dead-centre.

Without getting into your car's engine specs or state of build, a typical Mini engine's air fuel ratio (SU carb) is controlled by the main jet and needle. As built in the factory, the needle was selected for the particular engine to optimize power. It was found that the idle mixture could be made leaner than optimum (the base end of the needle thicker) if vacuum advance was used to get the lean fire going enough to keep the engine turning. The carb only uses the thickest part of the needle when the throttle is closed down to idle. That is because the dashpot works on vacuum before the throttle plate. With the throttle closed, vacuum before the throttle is very low and the dashpot spring pushes its piston down, shoving the needle as far as it will go into the jet. The amount of needle pulled out is in direct proportion to how much air is being sucked past the throttle. At 4000rpm and the throttle over half open, the needle will be pulled out more than half.

Here's where your improved fuel economy might have come from: Assuming you have not changed the needle or jet, or any other part of the intake or exhaust, if you had set the timing appropriately and then the mixture for optimum power (not necessarily to spec), then you would have better fuel economy. Perhaps before that, if you didn't have the vacuum advance working or you were overdue for an air filter or some other issue, and then you put things right, then your engine would have idled on a leaner mixture (which you tuned in) and consequently improved the mixture in the mid range or upper end.

To answer our questions about doing the timing light test:

When you disconnect the vacuum advance tube you should block the carb side. Otherwise you are creating an air leak into the carb, which will adversely affect idle. The idle speed may then change a bit because it no longer has the benefit of the vacuum advance. You can raise idle speed a little if the engine has problems running smoothly.

Typically, when setting timing, the specs say to disconnect the vacuum advance and raise the idle speed to a certain number, dependent on your engine and the type of distributor it has. See your Haynes manual. The rpm specs range from 600 to an amazing 1500 rpm. With the timing set, you reconnect the vacuum advance and adjust the idle down to your desired rpm. (I'd love 600 rpm on mine but it is a MG Metro 1275 high compression unit and isn't happy below 800 to 900 rpm.)

So in short, I think your improved economy is because you've got it tuned up well!

Hi Dan,I guess what I'm saying is that is doesn't really matter a hill of beans what the vac advance is going at idle.  The throttle is (almost) shut so any improvement is pretty minimal - if there is any.

However, at a 4000rpm cruise - say 70 mph - the vacuum advance will cause a significant difference in the throttle opening needed to produce the necessary 30 odd HP - due to the increased efficiency of the combustion process bought about by the advanced timing.  True, the mechanical advance provides for reduced cylinder filling as the revs rise but the vac unit tips in even more advance as filling drops off as you lift off the pedal.  You will get exactly the same HP but with less air/fuel....

Bingo - improved fuel economy.  

".....Also note that timing, advance (mechanical or vacuum) has absolutely no effect on the air/fuel mixture entering the engine - only the carb does this...."

And I would dispute this as well....  the throttle pedal controls the amount of air entering the engine - the carb just matches the correct fuel flow to air demanded by the driver...  The vacuum advance allows the driver to demand less air (and fuel) ( i.e. lift off the pedal) while maintaining a set pace....

".....Reconnect the vacuum advance and the timing mark should instantly disappear off the scale because the vacuum advance shifts it off the range, maybe 15 to 20 deg. more advance..."

Must admit I've not tried this ... For why? Mr Bridger might say... Do the revs rise?  Do you have to back off the idle stop when you reconnect the advance unit??

One reason others may find it hard to match my fuel savings is that I live in the bush so my car spends most to its time at a 50 to 70 mph cruise.  I don't do traffic lights and congestion - well as much as I can avoid it

Cheers, Ian

Agree, Dan. All it takes is a vacuum gauge to see that idle has high vacuum, as well as sudden closing of throttle and/or decel.


Another tidbit- if you have a decades-old simple timing light, it will not read the timing with the mini electronic distributor. What you end up with is a blurry line about 10-15 degrees wide. You must use a modern timing gun device.

Ian:
What you wrote about significant vacuum at lift-off at 4000rpm is true.  That's because the throttle plate closes to the idle position and the momentum of the spinning engine sucks really hard on the throttle plate. However, with no more air/fuel supply that at idle, the engine would quickly return to idle unless of course you are under engine braking.

Vacuum is high at regular idle (not low as you suggest) because the pistons are trying to pull in air for full displacement but the throttle plate only allows enough air/fuel in to keep it ticking over. To prove this, set up your timing light (no advance compensation) and run the engine at idle with the vacuum advance disconnected. The timing you should see on the marks would typically between 8 deg to 12 deg BTDC. Reconnect the vacuum advance and the timing mark should instantly disappear off the scale because the vacuum advance shifts it off the range, maybe 15 to 20 deg. more advance.

During engine braking, since the engine may be continuing to turn at high rpm, the mechanical advance which typically tops out at or around 4000rpm, will also be at play, providing timing advance on its own.

The fuel economy feature is supposed to work not just during deceleration, but when the car is idling, at stop lights, traffic hold-ups etc.

Also note that timing, advance (mechanical or vacuum) has absolutely no effect on the air/fuel mixture entering the engine - only the carb does this.

While I agree with Dan that the vacuum advance is an economy tool, IMHO it doesn't work as he suggests.  Vacuum at idle is very low and has very little impact on idle consumption (note that timing is set without vacuum and (I suspect) the idle mixture is pretty much unaffected.

However... when you reach cruising speed at, say, 4000 rpm  and lift off the throttle there is significant vacuum developed which the unit translate into a significant timing advance.  The early spark allows a leaner mixture to provide the same power - which means the same speed with less fuel.  My experience was a 30% improvement in overall fuel consumption.

Race cars don't need it because they don't spend much (any? time cruising with the throttle partly closed.....

Cheers, Ian

"alicetaylor" is a spammer and not a very good one.

To add to what Doug wrote, if your existing coil is weak due to old age, you may be not getting enough spark for good combustion.

To clarify what onetim wrote about the vacuum advance, it should not affect power if it is not working: vacuum advance is an economy device that allows a leaner idle mixture. When you open the throttle, the vacuum goes away and the vacuum advance relaxes. That is why timing is set with the vacuum advance disconnected. If the spring or linkage of the vacuum advance is broken (very unlikely), you'd likely get erratic timing. Distributors are made with or without vacuum advance - race cars don't need it as they don't spend much time idling.

When checking the plugs and wires, don't forget to check carefully the distributor cap and rotor - both should be replaced as they are considered wearing parts.

Warning:  Science babble to follow.

Having a performance coil does not automatically mean you will get any change in performance.  The voltage at which the spark plugs fire is dependent on the plug gap and the fuel/air mixture in the cylinders.  If the plug gap is narrow the spark will occur earlier and at a lower voltage.  The spark voltage will not be higher just because your coil has the potential of delivering higher voltages.  If you want a hotter spark, open the plug gap up.  That will take advantage of your performance coil's higher potential voltage.

Before you spend any money on a performance coil, put in a new set of plugs and experiment with the plug gap.  Don't expect a lot though.  Do NOT use resistive spark plug wires in combination with resistor spark plugs.  Use either solid core wires with resistor plugs or resistor wires with standard plugs.  

Regardless, you mentioned doing a complete tune up.  Set your valve clearances and all the ignition settings before you touch the carb.  Do not make carb adjustments first. 

Mine is carbd. Any benefit to upgrading just the coil? Id like to do a tune up and have the carb adjusted properly

You could quickly check that the vacuum advance works, and that the diaphragm does not leak, by sucking on a clean piece of hose with the distributor cap off, to see the base plate move or advance, and that it holds a vacuum. If stuck or leaking you are loosing out on a noticeable bit of power.

The distributor you currently have is a factory electronic one and considered by many to be the best bang for the buck.
If you want to look for a replacement distributor you need to look for an A+ engine, if your engine is an SPI (fuel injected) stay with what you have.

Is your current distributor not working?  Why do you want to replace it?

Each engine is slightly different.  If you want to buy a distributor optimized for you particular engine, I would not buy a "generic" distributor that claims to offer performance. 

If there is a problem with your current distributor, Jeff at Advanced Distributors can rebuild and re-curve your existing distributor for performance.  If you want a distributor that is more flexible than stock and will allow you to change the advance curve, consider buying a 1-2-3 (brand) distributor.

Hey guys. I have a 92 rover 1275. Any idea what distributor this is? Im guessing it and the coil are original. Im thinking of upgrading them to this... https://www.minimania.com/part/MMKT0716/Austin-Mini-Electronic-Ignition-Upgrade-Kit-For-A-Engines

Whats your thoughts/opinions?