correct ignition timing 3.4 litre xj6 series 3

[CliveRothwell]

Hi.
1985 car running (poor power and pick up) on new HIF 44 SU carbs. New Jag specialist fitted head gasket last week.

Currently fitted with 123 electronic distributor on the F setting. Can anybody advise if this is definitely the correct setting and also what the best timing figure actually is. The book says 8 degrees BTDC, the engine is actually set to 10 degrees BTDC. (it was set to 10 degrees to minimise emissions and unburnt hydrocarbons.

All knowledgable answers greatfully received!

Thanks

[piman]

Hello Clive,

I think you need to set both the carburettors and the distributor to as close to standard as possible. The needles and springs used on the original (I'm assuming 1 3/4") S.U.s would be the best. you also need to find out the original advance curve and see if your 'F' setting on the 123 matches it. Does your current distributor have vacuum advance, as the engine would have been set up for one. If not it will need a rolling road session to determine the best needles and springs for your current carburettors.

Alec

[CliveRothwell]

Hello Alec

Thanks for your words. It does not use vacuum any longer - the 123 distributor works that out. A rolling road session sounds an interesting option that I have not considered up till now. I think standard may well be the way forward and see how it progresses. Thanks again, Clive.

[piman]

Hello Clive,

"the 123 distributor works that out"

Do you mean that the module is connected to the vacuum pipe from the carburettor?
As I'm not sure what carburettors are standard on your car, but if they originally were 1 3/4 then the HIF is nearly the same so the original needles and springs should do the job. If 2" then it's a bit of trial and error which is where the rolling road comes in.

Alec

[CliveRothwell]

Hi Alec

The 123 distributor is clever and works out the advance required, not using any vacuum at all. However, it's only as clever as the setting entered, in this case at present, setting F, hence my original query. The HIF 44s are 2" (I think- it's been ages since I was able to get outside to check!).

Many thanks

Clive

[piman]

Hello Clive,

I understood that the HIF carburettors gave the choke size in mm and 44 is a shade under 1 3/4", 2" is equal to 50.8mm.

Forgive my scepticism but I cannot see how the electronics of that distributor can 'work out' what vacuum the engine has? I accept it may know the engine rpm but unless you have a throttle position sensor which would give some guide I really can't see it being as effective as a sound mechanical distributor?

Alec

[CliveRothwell]

It never fails to amaze me how little I know! It's all in the name HIF 44 - as in 44mm!
You are correct of course: it does have a vacuum pipe; I've just checked the installation sheet.

Do see the link below, you might find it interesting. It seems to be a good product if s bit costly (and somewhat diffucult to establish which is the correct advance curve).
http://www.123ignition.nl/forum/read.ph ... =954&t=954

Regards

Clive

[piman]

Hello Clive,

I would expect that you get a chart with the distributor giving the actual advance for each curve against engine speed. You also need to get hold of a decent workshop manual which should give the curve for your particular model car and see what 123 curve matches it. I don't like the idea of trial and error with no guidance. What is important is the maximum mechanical advance that Jaguar specify so ensure you don't select a curve that exceeds that. High speed detonation is damaging to engines particularly if the mixture is a bit weak, I'm talking about holes in pistons in extreme cases. What is more worrying is that there is no audible warning of this state.
The vacuum advance can give quite a lot more advance but should only occur at light throttle and cruising speed. Forgive me if I'm teaching you to suck eggs but the car is set to run weak at cruise and requires extra advance to run smoothly.

Alec