Distributor advance curve with SU versus Stromberg carbs

My 69 OTS was converted from triple SUs from Strombergs but I wonder if the distributor advance curve is different for the SUs?
If it is, and I make the change, perhaps there is better performance?
Dennis

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In my opinion, even if you had kept the Strombergs, the advance curve could have been changed for improved performance (that is what I did).

So yes, you should not assume that the curve you have is ideal.

How can I do this??
Is it as simple as buying a set of weights and springs from somewhere? (which I can install myself?)

Dennis

Dennis, In my opinion getting the advance curve right for your 69 is the key to a happy Jag life. You want to emulate the Series One mechanical advance curve. If you can find a Lucas distributor expert who can recurve yours to S-1 spec that would be one option. A new distributor (Pertronix, 123, etc) is another, perhaps easier way.
Its not about total advance, your distributor has plenty of that. Its how fast the advance comes on from 1500 to 3000 rpm. Just be aware that over the years these engines may have had their heads milled and compression raised. It limits the advance you can add in that rpm range due to detonation/knocking. If your engine compression and/or oil consumption isnā€™t excessive, you should be ok.
Dave

So not as simple as springs and weights changed to a known spec

Whilst playing with springs and weights is one way to go about things (a most troublesome and unscientific way IMHO), another way is to get a 123ignitions distributor.

There basically 3 models to choose from:

  1. basic model with 16 pre-set advance curves to choose from;
  2. the USB programmable version. You need a PC to program the advance curve you want. This model has the added benefit of having a second curve available which can be implemented via a switch in the car. One application for this second curve is to use it for the ā€œracing curveā€ or, if you are driving the car in very mountainous areas, to use it for switching between low altitude (less advance) and high altitude (more advance) curves ;
  3. Top of the range is the Bluetooth programmable version which you can program with your IOS or Android app. I installed one of these into a 4.2 S1 and I was astounded with the results. At one point I had the engine so responsive, it could only work as a race car. I found myself LITERALLY having to detune my own curve to make the car more road drive-able. I did post on this topic: 123 posting

I also helped a friend install the USB version into his '84 Rolls and it has transformed the car.

Obviously, the BT version is my recommendation but it also is the most expensive. No connections etc.

In my case - same weights but experimented with springs to make the ā€˜curveā€™ a bit steeper early on. Fortunately I had a friend with one of these:

If you can buy the springs & weights you could install them. A better bet might be the entire centrifugal weight assembly IF it is available. That would control the curve and limit the total advance to S-1 specs. Keep it all Lucas/original if thatā€™s important.

To Philā€™s point, thatā€™s what makes the 123 so convenient. (Of course you pay for convenience). But you can tune your engine to its individual needs.

A Pertronix is around $210 at Jegs or Summit, so its an economical means to an end: Series 1 spec.

Whatever you do Dennis, you will be rewarded with a more responsive and fun ride. I went about 300 feet and realized I finally found the performance key to my 69 after I put a new distributor in the car.

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I have just ordered springs and weights from the UK fellow ā€œThe Distributor Doctorā€
In addition to installing them, I will need to reduce the maximum advance some degrees by building up the advance stop.

This may not result in as good a curve as custom adjusting on the bench, but it will get my advance curve a lot closer to where it should be!

If anyone wants details, do contact me off line

Dennis

69 OTS

Dennis, I went with the Petronix distributor D177600 (maybe from Summit.) Around $170. For a basic stock '69 4.2 w/ triple SUā€™s. Iā€™m happy. I think it could use a few more degrees of advance, but it is fine for my car & the way I drive. It starts great, idles as slow as I want, & accelerates quickly & smoothly.
Based on Jaglover input, I set the timing at 3500 rpm to 32-34 deg (vacuum plugged). Best advice I ever got & sorry but canā€™t recall who submitted it. Should check archives.

Dennis,

You are probably in a similar situation to me as far as the distributor is concerned (though I still have the Strombergs). I have the 41207 model distributor which has no vacuum advance unit. Later Series 2s had another model which incorporated a vacuum retard unit in a further attempt to reduce emissions. Since the carburetion and ignition timing of the engine are linked, any change to the carburetion is likely to require some changes to the ignition timing to remain optimal. A lot has been written on this topic, but I can do no better than to refer you to this great source of information:


It includes an Excel spreadsheet of the advance profiles of all generations of distributor fitted to the E-Types. I have just (finally) given way and conceded that maybe I donā€™t want to spend any more time messing about with my original worn distributor - there is a new 123 Tune-2 (Bluetooth controlled) sitting in itā€™s packaging on the driverā€™s seat waiting to be installed. I will need to decide whether I want to connect up the vacuum advance (not available on my original distributor) to my Strombergs, as this could affect how much centrifugal advance I program in. In your case, if you stick with the original distributor you will need to decide on whether you want to add a vacuum advance capsule, and what advance curve you want to implement. Then acquire the appropriate weights (which control the maximum centrifugal advance) and springs (which control the rate of advance with Revs). Good luck, and please report back on how it goesā€¦

-David

No need to connect to the Strombergs David. You can connect the vacuum input to manifold vacuum. You can program the equivalent of carb ported vacuum into the vacuum advance curve (0 advance for high values of vacuum).

Philip,

Thanks. Hadnā€™t thought of that - neat idea. I guess that the programmability open up a bunch of possibilities that arenā€™t there with the original set-upā€¦

-David

Here are the curves I settled on for the 65 4.2 I worked on:

Centrifugal advance:

Vacuum advance

Just a note: the 0 at 500 and 29 at 8000 is a bug in the Android app that does not allow changes to those values. My values at 600 and 6000 was how I tried to overcome those bugs.

Iā€™ve been communicating with 123 about this and they have improved the app quite a bit and Iā€™m expecting another issue soon (should have been out already actually).

On my 69, there is a vacuum module that is connected to one of the SUs.

But it advances (not retards) the spark when vacuum is applied

It looks like it is advancing the spark at part throttle which looks correct

Maybe it is possible I have the correct distributor for the SUs?

Denny

Just curious if you could expand on this. What were the negative attributes of it being ā€œtoo responsiveā€?

There is a 5 digit number stamped on the distributor body (possibly begins with a ā€˜4ā€™) that tells what the unit was made for and, if it is as original, details of the curve and vacuum.

I found a Distributor number (tough with it in place)

A0801A

Not sure if it makes sense

Could be 40801A - though that seems unlikely as that is a 4 cylinder distributor. Still anything is possible as a distributor can be build up from assorted parts.

Hereā€™s an example of a number (my S2):

Number below is date of manufacture,