Ignition timing.

[gorwanvfr]

I'm going around in circles trying to find out what is the correct ignition timing for my Mk 2 340. I’m not sure what the compression ratio is but I do know that they engine was overhauled by the previous owner a few years ago and he did re bore and replace the pistons with I think +20 pistons (think that is what he told me)

Depending on which website I check or which manual it seems to go from 10 BTDC to 3 BTDC
The car has a 123 electronic distributor installed.
Any help would be much appreciated.

[piman]

Hello Gorwanvfr,

I wouldn't worry too much about the book figure for static timing, It is better to tune the timing to the way the car runs. Ideally you need to retard the engine a small amount from where the engine pinks. If it does not pink, advance the timing until it does then retard a small amount.

What is important is to select a curve that is close to the original and you'll need a workshop manual to get that figure. Don't rely on 'seat of the pants' tuning of the 123 as at higher engine speeds it's quite easy to set too much overall advance and not be able to feel it. Does the 123 have a vacuum advance as the engine is tuned to run with one as the carburettors are set to run lean at cruise.

Alec

[gorwanvfr]

Hi Alec
Thanks for the reply yes the 123 has vacuum advance and it is connected to the front carb. The car/ engine was running well but I took the engine and gearbox out to fix a few oil leaks mainly the rear crank seal and the input and out put seals on the gearbox.
When I dismantiled the engine I never took note of the ignition timing stupid me. Anyway after the engine was rebuilt and back in the car I went for a start. The first few starts with the coil disconnected so as to get oil up around the top of the engine. Connected the coil wire and it fired straight in to life, so to say I was happy was an understatement.
Anyway gave it a gentle rev and that's when the trouble started. The car coughed and a small flame came out through the rear carb. I guess the battery that had been charging all day had gassed off and you guessed it the battery exploded.
The car was pushed out of the garage so the battery acid could be washed off. They are a heavy car to push on your own.
Back in to the garage and fitted a new battery and started it again. This time I just left it tick over. Too afraid to rev it. It starts perfectly and idles really smoothly.
So hit the books and guessed I don't have the ignition timimg set correctly
Of course had to head back to work the next day and that was three weeks ago. So now I'm here
At the moment the static timimg is set to 5 degrees BTDC.
Should I advance it a bit more or is there something else causing the cough.?
Thanks
Steve

[piman]

Hello Steve,

my manual says 10 degrees BTDC for a 340 (22D6 distributor). As the fuel characteristics have changed since the car was built you can expect that the book setting will no longer be best but I would suggest two or three more degrees advance as a starting point (From your 5 degrees)?

Spitting back through the carburettor could be weak mixture, but if you replace the air cleaner then any spit back will not cause a problem so allowing you to tune the engine.

I just checked my book and there is a big difference between the 8:1 and 9:1 compression 340 distributors:

8:1 41063A 3400 rpm 19 degrees max advance, 1300 rpm 10 - 12 degrees advance 500 rpm 1 - 3 degrees advance, 200 zero advance. vacuum unit 8/18/7

9:1 41064A 2000 rpm 13 degrees max advance, 800 rpm 6 - 8 degrees advance, 400 rpm 0.5
- 2.5 degrees, 225 rpm zero. vacuum unit 5/12/8

Note all figures are distributor figures so double advance and rpm to get engine figures. Vacuum figures are vacuum when advance starts, vacuum maximum (no more advance) and advance maximum in degrees.

There is obviously a reason that jaguar set the distributor to those figures as it seem peculiar that the 8:1 has maximum at 3,400 rpm which will never be reached i.e. 6,800 engine rpm.
Also if using a strobe to set the timing at idle, the original distributors will already be advancing as minimum rpm is in the 200 order (400 engine) so below idle speed so if you set to book speed at idle (10 degrees) you will actually be setting about 1 to 2 degrees retarded from the static setting.

I hope this helps and does not cause too much confusion?

Alec

[gorwanvfr]

Hi Alec
Just to say thanks for the help. I set the ignition timimg at 10 degrees BTDC with a timing light and the car is purring like a cat
Thanks again
Steve