I think this is why I always disconnect the vacuum... 3000 rpm under no load neutral in garage), reverse load (rolling down a hill), cruising load or full acceleration are going to produce totally different throttle plate settings - and hence vacuum.  If you're running on a dyno then maybe but unless you know exactly where the vacuum is at particular load settings you're just whistling in the wind....

Best to remove it from all consideration IMHO.

However, something is going on here... most vac cans I've seen specs for only provide a max advance of less than 10degrees... 11 deg of static advance + 25 dizzie advance + 8-9 vac advance still leaves 10 degrees or so for distributor wear???? 

Cheers, Ian

Shop manuals make the assumption that the dizzy installed is the right one for the engine and has the right curve for the build, including intake and exhaust. That curve would detemine a specific advance at the 1000rpm (or whatever), most likely at the bottom of the mechanical advance curve. As designed, the engine wants "X" advance at the specified tuning rpm. Because the vac advance would be in paly at low rpms, it needs to be disconnected for the setting. Change the spec of the engine or the dizzy, and the manual process becomes relativve.

123 ignitions:

They have 16 preset curves and a guide for initial selection, but as Marcel Chichak (who was involved in its development for application in Minis) says - pointedly - don't just accept what the chart says, but test other curves; the engine will tell you what it prefers. On my car, the curve it preferred was not close to the chart curve, but the engine is very happy with it. Maybe not perfectly spec'd but I don't have a custom-built engine where a curve could be 'engineered'.

One nice feature is that, once a curve is selected, turn the engine to TDC, plop the system into the engine and set static timing at "0". They even include LEDs inside to make it exact. You don't even need to get your timing light out.

Another feature is that the curves provide "0" advance until the engine reaches 500rpm, when the curve jumps instantly to the base idle advance. This makes starting incredibly easy and smooth, with no kick-back against the starter. Once my engine is warmed, it will restart with just a short click of the starter motor - enough to fire that first cylinder. (It makes modern, electronically controlled fuel injected engines seem sick by comparison because they need to be cranked enough for sensers to get their readings.) Probably easier on the starter and ring geaar too.

I understand that those wanting more top end advance for higher performace etc. can advance the static timing. (I'm not trying this!)

Good info on the 123 on this site by Marcel Chichak

//www.tdcperformance.ca/index.htm

On the Tuning Guide link, you can find the 4 curve families for the Mini 28, 30, 332 and 34 degrees total advance.

This is interesting - maybe I'll give this 11° setup a test drive after all.

//www.minimania.com/msgThread/117684/1/1/Mechanical_Advance__maybe_did_something_wrong

Ok, I checked my timing today and was quite surprised to find the following:

1000 rpm with vacuum disconnected :  11deg BTDC    (this is close to what the books say for 1275A+ engine)

3000 rpm with vacuum connected:   54 deg BTDC!  

I'm very surprised by 54 deg.  My car doesn't run optimally (I lose power on the highway and occasional stutter  and backfire which I thought was from running too lean), but it's not that bad.  I don't hear any kind of pinging or detonation typically.  Is there a way to adjust the high rpm advance without screwing up the idle advance?  I have a Ducellier dizzy.  Haynes doesn't even suggest checking timing at high rpm.   I do know that the previous owner said something about my cams being modified for more power.  Would that require the larger advance? 

Doug, I run 38° max advance in mine, with a 9° plate in the dizzy- so static it is about 20°. I get away with a stock Lucas starter but only because I have a kill switch on the ignition (before doing this I busted about 4 starter pinions). I suspect some of these guys with fixed dizzys may use a kill switch too. But their starters as I said are Heavy Duty, they would crank a V8.

Kevin, those guys running between 30 and 38 degrees of advance....  Apart from the gear reduction starter, how do they get the engine started?

Here in Australia, many historic racers DO run with locked (no advance) distributors. And some use up to 38° BTDC advance (if their head has been bathtubbed, like mine). But 30-32° is common.
They get around the starting problem by using a heavy duty (non-Lucas) pre-engaged starter. Some of these are a modified Bosch planetary geared type. Bear in mind that on the track many of these 1275-1330 motors are off cam much below 4000rpm, so idle timing is unimportant

Doug, one car make that comes to mind for using the maximum advance point timing is Alfa Romeo's 1600, 1750 and 2000GTV etc motors, with twin Webers or Dellortos. The factory manual detailed the procedure, at 4000rpm.
BTW, these performance motors and many others using Webers or Dellortos (eg Fiat) have no vacuum advance. It is an economy device, nothing more.

NO advance?  I know that you can set up an ignition kill switch so you get no spark until the engine is turning over fast on the starter (to prevent kick back with too much advance).  BUT, I'd be very surprised if even that would work on an engine set for a fixed 30 degrees of advance. 

I don't even know how I'd start Googling that topic.

Hi Doug, I think you'll find articles by people like Keith calver who advocate maximum advance timing.  However I would suggest that this is pretty much a racer issue - some racers don't use any advance mechanism but I'll bet such cars are a bu&*^er to start... I think DV suggests that about 30 is the beneficial max for A series timing - but then all cars are different.

But I really don't think it matters...

Cheers, Ian

I do not doubt what you have found in books.  However, I have never found a manual that discusses setting the maximum advance as I described above.   The manuals I have read always talk about setting the timing at idle.  I agree that at idle you would remove and plug the vacuum advance tubing during timing. 

I would suggest any road car needs a vacuum advance.  As stated - its a fuel economy device - up to 30% improvement is possible (probable).  I don't see (and haven't found in may years using a VA) that high speed advance will cause any problem - it will only happen with a closed throttle.  If you want to check out what's happening just connect up a vac gauge.  The advance unit will have specs that tell you how much advance you're getting per " of vacuum.  If you're really worried about excess advance during track time just disconnect the vacuum line - it won't affect anything else. 

I don't know about "playing with multiple curves of a 123".  However, a competent operator won't take long - maybe 30 minutes (I don't remeber it taking even that long) to find the required advance curve (required by your specific engine that is) on a dyno.  Basically you lock off the dizzies advance mechanism, run the engine at set revs (say 1500) and physically twist the dizzie to give max power (measure the advance at this point).  Increase revs to next required step (2000?) and measure max power advance.  You only need to do this up to about 4.5 maybe 5K max.  Once you have the required curve you take the dizzie to the test bench and fiddle with springs and weights to get as close to the curve you're just measured. A final WOT/full load run to check for detonation and off you go...

This is a scary and dangerous process - doesn't take much when fiddling with the dizzie at high revs to inadvertantly over advance with resulting damage....(remember - we're under maximum load at this point)

Presumably, with a 123 you could use another dizzie for the discovery phase - anything that physically fits would do as the advance is fixed.  Once you know what you (your engine) want you just choose the closest avaible from the 123 spec sheet. 

BTW every tuning guide I've ever seen says to disconnect the vac advance line when setting the timing.

Cheers, Ian

I do the same as Doug, but with vacuum advance disconnected just in case.

I go for 28 degrees max, though, as suggested by Dave Evans who operates a mobile chassis dyno here in So Cal. He says he's done hundreds of minis and says 28 degrees is where the max power is. He also advised me to not use the vacuum advance on my sometimes track/sometimes road car. His argument is it's a fuel saving and emissions device designed for the parameters for the specific engine it was attached to at the factory. Once you mod an engine, that vacuum is no longer designed for it. His concern is high rpm and intermittent lean conditions at throttle liftoff. Says that's the fastest way to burn up a motor.

That said, he said it takes hours on a dyno to find the correct and optimal tuning for a dizzy+motor combo and didn't really want to "play" with multiple curves of a 123 dizzy. Said it'll take about 1 hour per curve to test on the dyno and at his rates and for my intended use, it wasn't worth it in his opinion. I said I wanted to try 1 different curve and we ended up losing 2 hp at the top and gaining 2 hp of torque in the middle by using a more aggressive curve. His claim overlaps with Doug's suggestion: Playing with advance curves (once setting max advance) is more economically done with a "butt dyno" on the street with a long hill. It's the changes in pull on accelleration and listening for detonation.

My vacuum comes from the carb, near where the carb attaches to the intake manifold.  Would I still leave it connected when doing the test?

On the other hand, some engines, from some manufacturers, take the distributor vacuum advance from the carburetor, just in front of the butterfly, which changes when the vacuum is max/min, for specific emissions reduction targets.

Norm

I leave the vacuum advance hooked up when setting the timing to 32 BTDC at 4k RPM.  I'm not sure at that engine speed how much contribution to the advance the vacuum canister provides but I don't want "more" advance when I hook the canister back up.

People have told me the engine probably came out of a 1980s MG metro.  However, I think there have been some mods done to the cams (just based on what the previous owner said).  For starters, I was going to set the timing to what Hayne's suggests (13 deg BTDC), however, it does not explain what the rpms should be when doing the test.  Is it idle or some other rpm.

do you have the vacuum line plugged in to the distributor when you do the 4000rpm test.

On A and A+ engines, cylinder #1 is the cylinder closest to the water pump. 

I do not use factory published settings on timing.  Instead, I bring the engine up to 4000 RPM using the idle screws, set my advance timing light to 32 BTDC, and line up the ZERO timing mark and pointer.  After the engine speed is returned to idle, I test drive the car (accelerate in too high a gear going uphill) listening for pinging or knocking.  If I hear any bad sounds I retard the timing in 2 degree steps and repeat the test drive until the bad sounds are gone.  This will give you the most advance your engine can handle based on its particular build, condition, and the grade of fuel you use.

#1 is at the front end of the motor. You need to tell us what car the motor came out of before we can say anything about the timing.

I have yet to see a diagram of a 1275 A+ engine showing which is cylinder 1, 2, 3, 4.  Is 1 on the radiator side or clutch side?  The other question: what should the rpm be when setting the timing with a strobe?