340 max advance query…48 degrees!

[KrisGrim]

Hi all,

I have a stock 340 8:1 comp ratio and have been looking into max advance on the engine. I have stripped the standard Lucas distributor 41063/A which is the original as per Haynes. This is the same dizzy as the 8:1 3.4 mk2. This has a 19 degree cam in it and according to multiple documents the 340 has a 10 degree static timing.

If my calculations are correct, and this is the nub of the question, the calculation for max advance is mechanical advance x2 plus static timing, giving me 19x2 + 10 = 48 degrees all in at 3400rpm https://mgaguru.com/mgtech/ignition/pdf ... _specs.pdf

Surely that is too much? I must be missing something. The 3.4 mk2 8:1 had the same dizzy but static of 2 degrees so 40 degrees max advance, less than the 340 but still higher than the generally agreed 32-34 max advance.

I’m calculating this wrong, what am I doing? Or is 48degrees OK?

Thanks all

[piman]

Hello Kris,

your calculations are correct as is the 19 degree rotor for that distributor.
I have a little book that gives distributor specifications and the one that is unclear, as all the rest are distributor figures, is that the maximum advance at 3,400 for the distributor, i.e. 6,800 rpm engine, which it will not do normally?
48 degrees does seem a lot but that is what Jaguar set the engine to?

Alec

[piman]

Hello Kris,

just to add, looking closer at the specifications.
No advance below 200 rpm so there will be some advance at idle and at 500 rpm advance will be between 1 to 3 degrees, all distributor figures.
I wonder why the static setting is now 10 degrees as the 3.4 8:1 was 2 degrees. It also used a different specification distributor but with the same maximum of 19 degrees mechanical.

Are you having a problem or is it just academic interest?

Alec

[KrisGrim]

I am having trouble with the car at low revs when hot, it stutters and eventually will not go at all but will tick over roughly. So decided to strip out the distributor as it felt like the car was heavily retarded in low revs but was fine at higher revs. It is then I noticed the cam and the amount of advance. It may not be the distributor I have a newish accuspark module which may be giving problems when hot or it could be a vacuum leak from my brake servo. I have swapped the coil and no change.



I was thinking about a 123 unit and the 123 Jag goes to 32 degrees at between 3000-3900 engine revs. The original Lucas was 42 degrees (16 degrees x2 + 10 ) at 2600 engine revs. All this according the to link in my original post and the 123 docs. It also states 18 advance (4x2 +10) at 1000 revs. It is one of the most advanced curves of any Lucas distributor in that list.

So seems the Jag 123 can’t match the original distributor advance for the 340. Maybe the usb/ Bluetooth ones where you can map the whole curve would be needed.

The engine does seem to take a lot of advance I can’t make it pink. Just seem such high figures for an engine I thought took 32-34 max advance.

[piman]

Hello Kris,

I'm guessing that Jaguar were trying to make the car more economical by then and set the carburettors to run leaner than before Does the 340 have HS6 rather than HD6? A lean mixture requires more advance so it could be why the change?

Is the distributor worn at all, and is the rotor free on the drive spindle and returns back to zero with the springs? A good clue to poor running is the appearance of the plugs, they should be a pale brown at the tip of the elctrode.
Do you have a strobe light to see what the timing is doing? Possibly the vacuum advance is not operating? Another avenue is if one of the carburettors flooding slightly, that would be less noticeable at higher rpm?
I personally don't like the switchable 123 distributors, although the more expensive models can be programmed via a lap top.
I had my distributor overhauled and re calibrated by The Distributor Doctor who specialises in Lucas distributors and their spares.

Alec

[KrisGrim]

They are the HD6 variety, I did strip them when I bought the car(just to check them) a year or so ago and couldn’t see any issues. I may well need to check the diaphragms and float levels again. Yes I have a strobe light and the timing does seem erratic, I need to take a more systematic approach with the checks at certain revs and temperatures. Everything in the distributor seems good and returns as it should but everything is checked when it is colder when she runs OK.

I have a 123 in my Triumph 2.5 gt6 and never had a problem with it so I’m going to swap it to the Jag to hopefully rule that out. I’ll then move on to other areas. She is going to the Silverstone Festival so need to get her sorted!

Good to know my advance calcs were correct.Thanks for your help.

[piman]

Hello Kris,

if you do use the 123 here are the specifications for the 41063A distributor in case you don't have them:-
Vacuum module 8/18/7 (Start and max vacuum and maximum flywheel advance), Run up to 3,400 rpm, (Which seems to indicate that is distributor rpm), 19 degrees maximum, 1300, 10 to 12 degrees, 500, 1 to 3 degrees, no advance below 200 rpm.

I also have Triumph, a 2000 saloon but fitted with a 2.5 P.I. engine plus quite a few other modifications, still on points by the way.

Alec

[KrisGrim]

Hi again!

I have now purchased the 123 usb distributor and set the curves based on the original distributor. I have set the static to 10 degrees. I think the distributor centrifugal advance is correct but I’m a bit unsure about the curve of the vacuum advance. A bit of conversion needed as 123 deal in Kpascals and the vacuum advance I guess is in inhg.
So I figured based on the vacuum unit of 8/18/7 (7x2)14 degrees advance at 61 kpascals = 18inhg. The start would be 8inhg = 27 kpascals. Just not sure of the curve from 8 to 18 inhg, is it all in straight away? I’m not sure I have represented this correctly in the attached 123 image.

A second opinion would be much appreciated! I’m not sure the vacuum is set right.

Does anybody have any curves that they can share for reference?