Hi all,

I wanted to create a discussion thread to really get my head around timing and start-up process on the V12.

Start-up - When started at cold, the cold start injectors fire, some cars only while actually cranking, others until the thermotime switch de-energises it. My first question, how cold id cold? In Aus, I'm never starting mine in less than say 5C, does that count as cold? Do I need the cold start injectors, or can I remove them. It just means one less system to go wrong and two less fuel hoses to replace.

During initial running - OK, so it's started. For the first 15 min, no vacuum is applied (too much vacuum on a cold engine might be a problem). Also, the Auxillary Air Valve is open to allow more air and a higher idle.

The V12 has a complex but clever and effective system for applying vacuum advance at idle. This relies on a vacuum regulator which limits the maximum vacuum applied to the module irrespective of inlet manifold vacuum (past the butterflies). This applies a nice steady vacuum signal, and so a nice steady fuel efficient idle. If anyone knows where I can get one of these, my car did not have one when I bought it (actually, all the vacuum advance had been plugged up - no idea why, I have reconnected the vacuum advance to the throttle edge so no vacuum at idle but vacuum correct everywhere else. Works fine, but causes bad fuel economy and more heat in stop-start traffic)

All comments welcome

 

For the cars having the cold start injectors (they were discontinued at some point in the early-mid 80s.....I'm a bit surprised your car has 'em) they operate only when cranking and only if the coolant temp is below 35ēC. The maximum "on" time is 12 seconds...which would be in freezing conditions. The "on" time decreases as temperature increases.


In warm-ish weather you'd probably never miss them if they were gone. In cold weather you'd have extended cranking before the engine starts. I'd unplug 'em and see what happens...then decide.








It just 45 seconds on the USA cars....but the vacuum system was a bit different and I never fully got my head around it myself. It's one of those things I understsood as I was reading the details of how it operates but then lost my grasp of it a minute later :-)
Also, the Auxillary Air Valve is open to allow more air and a higher idle.







If nobody else chimes in I'll try to find a part number for the vacuum regulator...which might help in your search.





Mine was plumbed that way and I didn't have any heating or economy issues <shrug>

Cheers
DD

 

Matt,

Doug has pretty much nailed it.

The Oz and NZ cars never got that vac regulator. I tried to get one waaaay back when I was Parts Manager for Jaguar, and NLA was the reply.

The cold starts are subjective, mine had them, 3 days later, gone. All you need do IF really cold, and/or worried, is ignition ON, one pump of the gas pedal, which fires all 12 injectors, ONCE, and then proceed to "start" the beast. Works EVERY time.

Mine is vac advance via the underside of the throttle housing, and has vac at idle, and NONE on accelerate, and vac again at cruise. Nice STABLE idle and heaps of Mumbo, and great economy. The vac dump valve is retained, as is the vac full fuel valve, all in the same vac line.

Mine has no AAV, balance pipe, overrun valves, or anything else for that matter, right back to basics, and 10+ years on, still sweet as you could ever want. It also has the 71mm throttle discs, and cold air intakes, freeflow exhaust (2" ALL the way), and a 16CU ECU (in lieu of the 6CU).

Last 4500km trip in it returned 11.2L/100kms, so not toooooo worried.

 

Hi, my first post. I recently became an owner of a 1980 XJS that had been stored for about 17 years. Obtained a roadworthy a month and a bit ago after the predictable repairs needed. One of which was very hard to start, traced to a non-functioning cold start injector, once replaced it started albeit with a rough idle (since traced to a rust hole in my brake booster, a clean out of the AAV, and blanking off other areas allowing air in). So far my very limited experience in Australia is a cold start injector is required for a pre HE model, particularly if there are also other factors afffecting the idle. Im interested in this thread as I am now looking at the vacuum lines and its effect on improving further the idle. So far a steep learning curve.

 

OK, dammit, got logged off by mistake, so don't feel like re-typing everything.
Grant and Doug - thanks guys, love to get the technical details.
Interesting to hear I never had a vacuum regulator - so whats the dump valve for?
Disconnected my cold starts - one less thing to go wrong, and I'll probably never need them.
Sorry Grant, your right, my vacuum advance is the same, full advance at idle, drops off as the butterflies pass the throttle tapping. Idle wanders a bit sometimes. Obviously, if the idle increases, it increases the vacuum, which further increases the idle. This is what the regulator fixed?
I recommend the balance pipe, doesn't this just correct any minor difference in the butterflies on A and B bank?
Love to see pics of your cold air intakes - still planning to add these.
Cheers

 

OK, The dump valve vents the vac in the dizzy line to atmosphere INSTANTLY, as apposed to a few seconds delay that could occur by natural bleed off. This bleed off on the HE is too dangerous, and pinging reeks havoc, so an instant dump when the throttle is opened rapidly, drops the timing back to mechanical only for that instant. These valves do fail, and leak vac to atmosphere all the time. This prevents the vac advance from working in the first place, and is another vac leak that upsets idle.

As best I can establish that regulator was linked to USA emissions requirements, and we never got them here. If your idle gets unstable as you increase engine speed then something else is off. TPS adjustment, throttle rod adjustment is not quite "spot on", dirty throttle discs, mechanical advance sticking, vac advance capsule failing, etc etc.

The balance pipe does exactly that. Since I dont run an AAV, or PCV, etc, the ECU vac take offs are from 2 spigots from the old cold start holes, to a "T" at the rear of the fuel rail. This gives a significantly "snappier" vac signal to the ECU, and works sweet for me. The throttle disc screws are used to set the base idle, and are balanced "spot on" with a modified carby balancer, and I am obsolutely **** about those rod adjustments, and it takes time to get then right, and that is just fact. Once the discs crack idle and the engine gets to about 1200rpm, the balance of them is not so great, and as the discs open further the balance factor is way less critical. I use a PreHE inlet end plate on the LH side, so I got 2 vac take offs for the brake booster. So with the 2 hoses for the ECU, and the 2 hoses for the brake booster, I have a balance pipe of sorts I suppose, just not so large.

I also run an XJ40 3.6ltr TPS on it, and it is just so much more accurate then the original thing, split voltage is easy to set, and it is still the same 10 years on.

Got heaps of right ups of the alterations to the "Beast", and will PM them to anyone who wants them, who sends me an email address. If, and when I find out/learn how, to post then here I will do so.

 

Mate, would love to see all the info you have. I'm at mrbras7@bigpond.com. Like the sound of "The Beast"