Thanks for the replies Dave and Timmy.

From Dave's recommendation to lock off the vac advance and Timmy's comments about no downsides to running with vac advance, I'm thinking it might be best to get the vac advance working (as it was originally intended) and I'll investigate if the bits in the dizzy do move when I apply a vacuum to it. I don't have a vacuum pump at the moment and will see what happens when I suck on the hose and see if anything moves in the dizzy. If no movement, I'll go and grab a pump and see what happens. It's the original dizzy so maybe the diaphragm is gone?

I can't remember how I plugged off the vacuum port at the carby end so I'll go and see what I can find tomorrow.

Most of Minnie's miles are country driving on fairly level roads and I sit on about 40 - 45 mph as I can avoid major highways most of the time so I'm guessing a working vacuum advance would be beneficial under those conditions. Is that correct?

Cheers Rocky

Get the 5568. Dwell is important with proper electronic ignition systems too.

Despite what many people say, sucking on the diaphragm doesn't do much. You'll have to use a proper vacuum pump or the engine.

The vacuum advance will be beneficial whenever you aren't at full throttle. Full throttle equals a rich mixture and low vacuum. Low throttle (cruise) equals a lean mixture and high vacuum. The lean mixture works better with more advance since the leaner mixture requires longer to burn. If you were to tune it very rich like many people do, you wouldn't notice much difference with vacuum advance.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

An update on what I found out today when checking out the vacuum situation. When I restored Minnie, I put the old copper vacuum line back on, including that little cannister thing that bolts up on near the thermostat housing and ran the line under the air cleaner so it looked original. Blocked off with a piece of rubber tube, as is the tit on the carby.

Tried with a small amount of vacuum on the end of the copper tube and there's a leak somewhere as it just sucks air. Haven't found out where yet though and it's a job for next time. Will have to pull it all out and see if I can find where it's leaking as I can't easily identify where it's leaking when it's in the car. Once apart, I'll blow into it and see what happens. The copper tube screws into the vacuum module on the dizzy so not quite as easy as a rubber tube connection that end.

Maybe that's why the bloke who tuned it up for me didn't worry about the vacuum advance not working?

Can you undo the pipe at the dissy then block it off to try the pipework for a leak. Also jury rig something onto the vacuum advance unit to see if it is good or leaking. It may save you some work.

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Cheers Dave.

A couple of good suggestions Dave - thanks. I hadn't thought of trying that first. I was just thinking that I'd have to undo all my nice neat handiwork in pulling it all apart

Well I've ordered an Innova 5568 timing light and will see how things are set once I get it.

As an aside, I borrowed a basic light and as far as I can tell, the timing at about 650rpm is about 10 or 11 degrees BTDC. Hard to tell for sure as the timing light was not flashing consistently as the magnet insider the clamp that goes over the plug lead was broken. According to the book, this might be a bit too much. The book has different specs for standard fuel of premium and I'm running either 95 or 98 petrol. I can't recall what the exact numbers are as I don't have the book handy at the moment.

I've also been playing around with the vacuum advance inside the dissy to see if the plate actually moves or it's seized. When I unhook the spring on the vacuum advance unit, the plate moved but not real smooth so I put just a few drops of oil between the plates and it's freed up ok.

The spring on the vacuum unit is very hard to push up and there's no way it will move just sucking on it. I'm going to buy a vacuum pump and see if it works with a bit more suck that what I can do by mouth.

Will post results after I get all the tools I need and see what's happening.

Cheers Rocky

It will move by "mouth vacuum " if it is working properly and you can feel the pressure changing so if it is not I would suggest it is NFG.
10 btdc at 650 is not far off either and whoever set it up may have compensated for no vac a bit aswell. When you get your new timing light check max advance as that is the best way, 30ish btdc at 3500 rpm.

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Cheers Dave.

As stated, vacuum advance is for part throttle cruising.. basically its a fuel consumption improvement device. At a steady cruise at around 110 kph it's worth a 30% (yes 30) improvement in fuel consumption for my 1330. 110 in an 850 is no longer "part throttle" (BTDT) so I guess your not going to see much... Your 850 will be quite happy cruising at 100kph... as long as you avoid hills (As a student in the 60s I did several Sydney/FNQ trips in my 850 - all without issue (except for the flattened butt).

The VA allows a leaner mixture when cruising so might make some improvement in your previously mentioned "overfuelling" issue.

Vac advance units are pretty generic... mine was made for a 90s Ford Escort. You just need to know what advance you need it to provide and the vacuum required to provide same. Specs are (usually) stamped on the actual unit (I think mine is all in at 8psi). Probably cheaper to buy a new unit than a test kit. Maybe one of the local shops has a dizzy test machine (pretty old fashioned these daze) and will let you run yours up. It only takes a few minutes to check all the moving bits out.

You can get a rough idea if the vac advance is working by running the engine and sucking on the advance tube.... if it's working (but you won't know how well) revs will rise a bit.

BTW...the "little canister thing" is a flame trap (presumably in case of a back fire). I know the factory thought you should have one but mine was deleted about 30 years ago..

Good luck

PS I wouldn't be too fussed about finding a unit that measures dwell... That's what feeler gauges are for With most (all?) electronic units its buried in the firmware and there's nothing you can do about it.

With many electronic modules, a failure mode is the loss of dwell control. Impossible to test without a dwell meter.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

While this may be true it's indicative of the problem with electronic ignitions. If they have an issue there's really no way of knowing what without the help of an expert... and then it's probably a case of "get a new one". When one of mine died I sent it to Melbourne for repair. The expert found nothing wrong, diagnosed an earth issue..... and sent it back with a spare module.

So if the dwell meter tells you that you have problem, the solution is the same... send it to a pro to repair (who will need to test it anyway to confirm your diagnosis), or buy a new one.

Cheers, Ian

I just carry a spare 'advance modified' Chinese dizzy with the same Powerspark module as my Lucas 43D, it's pre-timed to drop in and work.
Murphy insurance...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

When specifying vacuum advance data, the first figure is the vacuum level (“Hg) upon which vacuum advance begins, the second figure is the vacuum level at which vacuum advance finishes, and the third figure is the distributor degrees advance attained at maximum vacuum level. Those of you who have my “Distributor Guide” book will know that the vacuum advance specifications differ quite a bit between models and the distributor fitted. It seems the factory was engaged in a constant battle between having as much vacuum as possible vs the quality of the fuel available.

The cannister in the vacuum line in early models is a fuel trap. In those days, the vacuum diaphragm was no doubt more susceptible to degradation from petrol than would be the case now. Plus, it served as a vacuum reservoir that dampens out the pulses from the carburettor thus minimising wear at the distributor. Evidently a cost saving measure meant that later models just had a plastic tube and rubber hose connectors.

It is possible replace the diaphragm but it is not an easy job. The vacuum unit can be opened up with a suitable jig and a can opener (see picture), and closed again afterwards with another jig using a press. This can be done a couple of times before metal fatigue becomes an issue. Once opened, there is a rubber-impregnated cloth (probably canvas) which is nearly always brittle and cracked. It is quite thin. I have found a modern replacement (but thicker material) made from neoprene. Unfortunately, the original diaphragm is riveted in position and this has to be drilled out. I tapped the hub and screwed the new diaphragm on and secured with Loctite on the thread. So far it has worked very well. It is possible to suck with the mouth and hold in position to test, but of course a vacuum gauge is better.

I don't see any reason not to use vacuum advance, I may do some testing to demonstrate why it is used. The following is how to identify Lucas advance diaphragms. The degrees are of course in distributor degrees, not crankshaft.






This is how I've done it too.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

Sounds like a lot of trouble to gain a possible 10 to 15% fuel economy at certain times.
There is a theory that would suggest that if you need a vac advance you are not driving the car hard enough.
There was a reason the Coopers and S were not fitted with one.

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Cheers Dave.

I don’t think adding a rubber hose is a lot of trouble

Even buying a brand new vacuum canister is cheap enough and will definitely pay itself back. I don’t know about most people, but I spend at least 90% of the time at part throttle where vac advance is a benefit

Cooper engines did have them fitted. Cooper S didn’t.

Cooper S engines were also noted in new road tests to have poor advance curves for road use, and they did have issues with valve seats cracking. One contributing factor is the low part throttle timing causing super hot exhaust going past the valves

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