Hello! Comments on my last post really helped me un-glue my pistons, valves, injectors, etc. Now my big British Bunny takes me deeper. Come along!

I'm not getting fuel injector pulses at the injectors so decided to check the mechanical advance and dizzy condition as that's on the list to do anyway.. In I went. Finger tight plastic screws, one broken. Everything else stuck fast with varnish but in the correct places, except the vacuum advance (Retard?) has a leak. I pulled it off and the numbers on it are:

54427091
6-10-4

This unit is available from British vacuum unit.com but they have it listed for a 1975 xj12. I understand it is important to get the correct one for the specific ignition curve. I'll get my dizzy number off the car and confirm its correct OEM.

The available unit listed for the 1976 says I need a:
54429459
6-10-8

Very different.
Thoughts?



Thanks
Sean

 

The only numbers I can find on my distributor are:
41622D
-4375
That is listed for a 1975 XJ12 and is not the number called for on my 1976, the work shop manual says I need a:
36 DE 12
Unless I'm looking in the wrong place? Drivers side of dizzy just below the cap?

Ahhh I think I see the issue.
The Dizzy numbers are the same for 75 and 76 but the advance unit is different. My bad 54427091 6-10-4 should be a 54429459 6-10-8. someone put the wrong vac unit on or replaced the whole dizzy with a 1975 one.

Yay so problem solved right? And is British vacuum unit a good place to order from?

Thanks
Sean

 

In my world, determining whether the can advances the timing or retards it is the place to start.


Further, the curve is determined more by the mechanical advance rather than vacuum. Vacum advance, in my lexicon is sudden and early. mechanical more progressive.


And, these values are but a compromise and developed decades ago. Things, like driving habits and fuels have changed. Not to mention environments, from the frozen or wet cold to the hot and dry.


Apologies, I seem to be making things more complex and I pride myself on the opposite!!


Carl

 

Hi Carl,

I mentioned the fact it could be a retard unit not advance because I'm a knucklehead who sometimes thinks out loud with his fingers. CCW rotor rotation plus CW movement of pickup = advance. got it.

My understanding is the mechanical advance covers initial advance followed by the vacuum system but concurrently the mechanical continues advancing as the uneven springs are both activated into higher rpm? See here I go again sorry. The Vac advance has a max of 4 degrees on my failed unit but the replacement and original spec one is 8 degrees max. Right? The 6-10-4 or 6-10-8 specification covers that. 4 extra degrees of advance is alot I think?

So my current understanding and plan is...I have a 1975 36 DE 12 Dizzy part number 41622 that I need to update or replace with a 1976 specification dizzy part number 41677 which has a different vacuum unit. Still unknown to me is any other differences between the 41622 and 41677 dizzy besides the Vac unit? Any help understanding this will be appreciated.

Regards,
Sean

 

So scratch everything I said. A petronix with custom vac. unit is on the way.

Thanks all

 

However advanced or retarded incorrectly, it should still be giving a pulse to the EFI unit to fire the injectors !

 

I know they are totally separate systems sharing the inside of the dizzy. I just couldn't bring myself to re-assemble the ignition with the original Byzantine system.
I still need to sort out the ignition pulses. The plastic screws holding down the injection trigger board were loose and one was broken off. I'm hoping it works when I reassemble it. Just another step in the diagnostic process.

 

bthirteen back when i had mine there were many posts and threads about the crank vapors pushing up through the shaft housing and exploding the cap. i put in new bushings and a viton o ring as per suggested by others. the crane flamethrower had a 12 slot wheel in the kit back in 2010.

 

How many wires are there on the trigger board ? 3 or 4 ?

If its 3, you may want to think about changing to a 4 wire board as these are inherently more reliable.

For ignition, the crane is a good option, and cheaper than the RE-OPUS, but the great thing about the RE-OPUS is it goes into the same box as the original circuit board so it looks stock. FWIW as your car is in extremely original condition, I would try and keep it as stock as possible.

We have just finished an XJ12C refurbishment after the car slept for 25 years, so the build thread may be useful.

This post descusses the 4 wire and RE-OPUS for starters

https://www.jaguarforums.com/forum/x...5/#post1402353