Hi all....first post on jaguar forums. I have a 1975 XJ12C and am in the process of troubleshooting backfiring that occurs on light acceleration from low speed. Have tried a few things and now wondering about the vacuum advance. Two questions:

1) Can a broken vacuum advance cause backfiring on acceleration

2) Should the vacuum advance on my engine have a bleed hole and not hold vacuum? I have read posts on this and other forums that some V12 variants had a bleed hole in the vacuum advance diaphragm. I do get advance of 6 degrees when vacuum is increased to the module from 7 to 9 inHg, but the module does not hold vacuum. Here is a video of its operation http://xj12c.org/wp-content/uploads/...904_092435.3gp

And Sarc....if you are out there....it would be great if you could check your XJ12C to see if the module holds its vacuum as your engine would be identical to mine.

thanks,

John

 

No help on the vacuum advance question. Not familiar with the older systems. And some of those older models had vacuum retard systems rather than vacuum advance.

However, a backfire in the intake system (is that what you're experiencing?) usually suggests lack of fuel

Cheers
DD

 

Yes, a few things in play here.


1. A bleed in the vacumn advance is unknown to me. But, I suppose it might serve to slow the advance !.


2. Use a smart strobe riming light to plot the advance at increasing RPM's. Or determine, there is none. Resulting retarded timing might be the answer.


3. Although, to me back fire denotes, in the exhaust, that might nt be the issue here.


4. If in the intake, as Doug mentions, I agree, not enough fuel in the mixture. Why ? to be determined.


Carl

 

Thanks both of you

Typically backfire in the intake is lean and in the exhaust is running rich. The pops seem to be coming from under me which would indicate exhaust miss.

What I didn't mention is that this was the first drive after some major surgery to the fuel system after swapping the original rail with one from a later HE. Oh...while in there I also removed the air injection and egr. If interested....all the gory details are on my blog which is the link in my signature.

Have tried
- pickup gap re-gapping to .020"
- timing and advance checking
- new spark plugs and wires
- rotor and cap relatively new and both look OK
- throttle switch and ballast resistor checking
- capped of the overrun valves for now as they were leaking and I haven't taken time to re-adjust them yet

thanks for the ideas

John

 

John:


The clarification means a lot. Back fire in the exhaust is a timing issue.
It ties in with the "vented" vacum advance bellows.


That is where I would do more research and tests. A crude test would be to advance the static timing and see if the back fires lessen or go away.


But, the correct fix is a "proper" diaphragm and a compatible static timing setting.


Carl
l

 

+1 to Carl's advice. Jaguar may have used a "vented" diaphragm but, if so, never seen or heard it reported, and cannot understand why the design would use a vacuum induced advance, then negate it by "bleeding off" the enabling vacuum. Weird, and I can find no reference in Jaguar service manuals. Much more likely that the "bleed hole" is a sign of failure and should be replaced. Suggest you raid the parts catalogs and start comparing . . . or jump straight to Carl's suggestion to just replace with new.

Best wishes and do let us know your progress,

Ken

 

Thanks all for the advice....will move forward with replacing the vacuum advance module
Removed the distributor tonight and its a 41622A model. The numbers on the vacuum module are hard to decipher but my best guess is part number 554427091 with operation of 6610044.

Will order my advance from these guys as their ebay store looks really knowledgeable on Lucas vacuum advance modules.

The reason I was confused about there being a bleed hole in the diaphragm is that Kirby's book notes that some units have an orifice to provide a deliberate leak.

I'm sure it will take a while to get the module, but will update this thread once I install and try it out.

thanks again
John

 

John!
Since it is a lean mixture problem, try adjusting the micture on the
flow meter, richer.

Note the position before you change it, then move it inwards or clockwiise
to make it richer.

Go a half a turn at a time.
Walter

 

Thanks for the idea Walter. I think as the backfire is in the exhaust I may be running rich not lean. These early D Jetronic cars run with a constant pressure in the fuel rail and I have installed a pressure transducer connected to a gauge in the car so I am able to monitor real time to ensure I always have the right pressure in the fuel rail.

 

If you upgraded to the square HE fuel rails you now have different injectors and your mixture is off. The later style injectors run at a higher fuel pressure. You may be able to fudge the system by running a higher fuel pressure

 

I changed the rail but kept the original injectors. I sent the injectors out to get checked out, services and flowed and even went to the effort to put the slightly higher flow injectors in the center positions where the intakes have slightly higher flow. I used one of the original pressure regulators at the exit of the rail and verified that I have a constant 30psi in all running conditions. Was trying to eliminate vapour lock so only changed out the rail to the larger and much simpler HE rail. Thanks

 

Thanks for all of the ideas. The vacuum advance module was shipped quickly. After re-installing distributor and checking timing again there is no more backfiring on acceleration. So to confirm.....the vacuum advance for this V12 application should not have a bleed hole and should hold vacuum.

thanks,

John

 

Thanks John. I have been away and only now, read your update. My take on the minutae of all versions of the V12 ignition/induction setups is not as comprehensive as many here (Grant, Greg, Doug - take a bow) but the idea of a "diaphragm bleed hole" on your engine raised eyebrows here. Glad to add your findings to the brew of accumulated knowledge.

Beyond several V12, Daimler Dble Six & XJ-S beasties back in the day, most of my experience was focused on full race spec V12s - notably, no less than 4 in total in the Unlimited Hydro "Jagged" which was a 6.0L Twin Turbo . . . but . . . there is always a but . . . that was also many years ago, and I am now up to my eyebrows in bringing a tired but very desirable pre HE back to life as a modsports road car. So . . . the learning continues . . .

Best wishes,

Ken