pre-programmed curves on 123 ignition - help please?

[Tom W]

Yes, your summary is correct.

I picked 10 degrees as a figure for the test as it was an easy test and the distributor was already installed at 10 degrees. I haven’t settled on the exact advance value I want at idle yet, there’s probably another round of checking the carbs, then checking if more or less advance improves things. Everything is interconnected.

There are two fundamental differences in the two methods, and advantages and disadvantages to both. I guess which is better depends on your engine and setup.

With my settings, if I want a different static value, I have to physically adjust the distributor. And when I programme the curve in the app, I have to deduct the static setting from the programmed values. This is extra faffing. The advantage is the actual advance can never drop below the static setting, even if the RPM is below 500rpm.

With your method, once the distributor is accurately set to TDC it can be left that way and all changes to the curves are made in the app. There’s no arithmetic to be done, and all values programmed in the curve are total crankshaft values. You don’t need to enable the static advance setting, or enter any static value in the app. The disadvantage with this approach is if the revs drop below 500rpm, then your timing drops to TDC, irrespective of what’s programmed in the app.

In my experience with my car, at 500rpm, the engine won’t idle and dies if the timing is 0 degrees, so I have to use a method with some static advance built in. I chose 10 degrees as I know this allows the car to start and run and makes the maths straightforward. I think I will be adding any additional advance at idle in the app.

For my vacuum curve, its based on the values from the E-type workshop manual, converted to the absolute units used by the 123. I don’t know if it’s correct yet. The Webers aren’t drilled for ported vacuum, so the take off has to come from the manifold. I don’t know if for a given load state the vacuum measured at the manifold is the same value as would be measured at a ported source, so the curve may be wrong for my engine. I think I’ll only get this correct with a rolling road session.

My vacuum curve is:

16deg at 0kP
16deg at 32kP
14.4deg at 56kP
6deg at 70kP
3deg at 75kP
0.4deg at 80Kp
0deg at 84kP
0deg at 100kP
0deg at 200kP

[John ball]

Hello Tom, I believe there is also another difference between our two versions.
With crank pulley marked at 10 BTDC, then the strobe will be correct at the 0 position. But with the mark set at 0 BTDC, then the strobe will be correct at the 10 position. This is assuming you have the strobe with the rotating wheel on the dial.

Although in your previous post you confirmed to me, that basically our two methods are the same and yours is more logical and standard. I do wonder how accurate the Dashboard page is on the 123 on my iPhone. It appears ok at the lower rpm for crankshaft advance, but seems to go very high advance when I rev the engine to 4000 rpm in the garage. Do you think this is because the engine is under light load and the butterflies more closed than they would be on the road, thus on the road more open and less vacuum ?

A couple of other comments -
One of the people that tune, said they use higher static of 15. This may we’ll be good for your car that is running Weber’s and slightly more powerful than with 3 SU’s.
I think the 123 instructions are very poor. It would be very easy to set the car at 10 BTDC on the distributor and look at the screen and set the static as 10 and also program a normal curve up to 34, unless you totally concentrate. The original pre programmed 123 was far simpler. The instructions for the Bluetooth version are not comprehensive enough. It is only with help on these forums that a full understanding can be gained.

My engine is a bit of a combination - XK 3.4, two HD8’s with slightly richer needles, slightly higher lift camshaft, the old electric hisser choke. The car feels powerful with my settings as described, but which ever way I set carbs either by moving jets up or down or changing needles, I cannot get the car to idle at 750 until it is absolutely stinking hot. Ok on the choke at 1100 rpm but once I switch off choke with manual switch it will not idle. Makes initial driving annoying unless I sit at home warming engine with foot on pedal !!

[Tom W]

My readings off the crankshaft are all with a standard timing light, not a dial back light. That adds an extra dimension, and potential confusion.

Basically the advance measured on the crank pulley at any given time is the actual advance value the engine is experiencing at that point. The difference with the dial back light is you adjust the dial until the pointer matches TDC when frozen by the strobe, and then read off the degrees on the dial to find the total advance. With a standard light, you read the advance by seeing which timing mark is aligned with the pointer. The dial back light is better as it allows advance to be measured at higher RPM. With a standard light you can only measure as high as the marks go on the crank. Usually not much more than the idle advance.

I think there’s some lag over the Bluetooth connection between the distributor and app, but haven’t compared the accuracy of the dashboard against known calibrated gauges. I don’t trust the tacho in the car any more than the app.

[mgman]

Hello Gentlemen,

I have recently installed a Bluetooth 123 Distributor and have set the RPM advance as per the manual for Stromberg Carburettors. I have blanked off the throttle by-pass so there is normal manifold on the over-run.

I now wish to setup the vacuum advance. As I understand high vacuum advances the ignition as as the weaker mixture on "cruising" throttle settings takes longer the burn. Is this correct?

I have looked at a specimen file and I am not sure I understand it. Are the settings additive ?

If anyone can supply a proven file for a 4.2 engine with a pair of Stromberg Carburettor I would greatly appreciate it. (I have yet to drill to hole by the butterfly)

[mgcjag]

Hi Stuart....Strombergs arnt very common on this forum but you may get answers....however plenty on the US JagLovers have 123 with Strombergs....here's one thread..plenty of others if you search..Steve https://forums.jag-lovers.com/t/more-12 ... ons/445251

[johnetype]

The Stromberg carburettors are linked to meeting US emissions specifications with distributor timing curves also dictated by meeting emissions specs.

Running Strombergs here in the UK is just the same as running SUs assuming you've drilled for a vacuum advance take-off, removed or permanently wired open the secondary throttle arrangement on the inlet manifold and use decent petrol.

Set the 123 distributor up with the same curves for both RPM and vacuum advance as you would for SUs, ie the UK car timing curves and the car will run very well.