A tuning question

[vee12eman]

Hi all,

Normally I feel confident enough about tuning the V12, I don’t need to ask. However, with the car off the road due to lockdown, I pursued a lot of jobs, including changing to Vacuum advance. I reset the carbs, putting the carbs to 100 thou as recommended. I rebuilt everything with care regarding air leaks and set timing to 10 btdc static as recommended. The the distributor was in the centre of it’s adjustment range at this point. Car will start and I got the carbs balanced but I can’t get the idle steady below about 1100 rpm. Anything less than that and when the throttle is blipped, the idle slows to around 500rpm, stutters and cuts out. I am going to try the basics again, but my feelings are that I did nothing unusual and at this rpm, the advance doesn’t have much influence, proven when I clamp off the supply to cut it out - no difference is apparent.

Now having difficulty in getting the dynamic timing less than about 16 degrees btdc at 1100 rpm - I simply run out of adjustment. In any case, if I do retard the timing further, the car runs worse and seems happier with an indicated 20 btdc or more.

An obvious thing is to get the distributor moved so I can run less advance, that’s what I am about to do, but what else could you suggest, given that less advance seems to mean poorer running?

One final point is that the petrol is around six months old due to lockdown. How much influence is that likely to have had? We are just coming out of winter, so the petrol has been pretty cool and in a dry environment. I have less than a quarter tank and will I try draining some and refill with new 98 Super - the cars normal diet.

Any thoughts?

Regards,

Simon

[lowact]

Vacuum gauge may give useful info.
Also, at idle, a sheet of paper over any one of the carbs should have negligible impact, otherwise not as balanced as u think ...?

[AussieEtype]

Yes put new fuel in. I am assuming your timing marks are correct - are they?

Get engine up to operating temp.

Set initial timing to about 3 degrees BTDC (temporary only) and set idle to about 600/800rpm - while timing is too retarded, if it will idle it shows fuelling at idle is OK. Many engine start easier on lower advance.

Disconnect vacuum to the dizzy (static timing is exactly that - static no vaccum) - set timing at your required 10 BTDC and at 600/800rpm idle speed. Once set - may need rotating the dizzy, reconnect vacuum and check timing is advancing with increasing revs. If you cannot get the engine down to its idle speed you have other issues.

Take for a drive and make sure it is not pinging (should not be on 98 octane) as 10 BTDC static is getting close to the max advance you can go for your engine. With that advance and a standard high compression Etype V12 you might be able to go back to 95 Octane if it does not ping when driving - but I agree do all your setting up on 98 and then adjust if needed. 95 Octane on a standard carb V12 should be good for everyday use.

Garry

[vee12eman]

Hi guys.

Many thanks for the replies, went and got new fuel, set the distributor to put it in a better central position for adjustments and checked again for air leaks. Found I had forgotten the two manifold tapping for the air pre heat system (fully functional on my car, but not when the air boxes aren’t fitted), so I closed off the flexible pipes. I gave the car a chance to clear the carbs of the old fuel (fortunately I was able to drain the tank of the 15 litres of fuel remaining and replace with 21 litres of Shell V
Power) and tried again. Car will now idle at around 700, with timing at an indicated 10 btdc (I’m a little suspicious of the accuracy of the timing marks but they are at least a starting point). Carbs are balanced with a pretty good meter and seem very close. I might try a colortune in a while.

It still stutters and sometimes stalls on the run down after throttle blips, so I wound speed up to just over 800rpm and it’s better again. I can’t go for a drive - lockdown still applies, but I should be able to in a couple of weeks so I’ll have to wait and tweak it all then. I still think it runs down too slow after revving, but I’ll see how it goes.

If anyone has anything else to ad, I’m still very interested. Thanks again for the replies.

Regards,

Simon.

[malcolm]

Sounds simple, but is your throttle sticking a little open at tickover? So when you do your tuning and set the tickover, it seems fine, but when you blip, the sticking throttle comes back and the "true" (too slow) tickover makes it stall.

[vee12eman]

Hi Malcolm,

Thanks for the response, although I don’t think that’s the problem, as the revs would drop then, on the occasions the engine didn’t stall, it climbed back and settled.

As a further update, I went on and used the Colortune; I use two at a time, I may even look at getting two more so I can do all carbs simultaneously. Anyway, the idle improvement was marked and now I have a more settled engine. I found some of the mixtures a little lean and the V12 does benefit from a richer mix. Setting that actually made a big improvement and I could actually see the revs increase as I tweaked the mixtures. I was then able to reduce the speed and synchronise the carbs again, it did make small differences to the previously well balanced set.

Anyway, I’ll see how it runs when the car eventually gets back on the road.

Regards,

Simon.

[lowact]

Sounds like you still have vacuum leaks. This is why the revs don’t follow the throttle, the leak is providing alternative air flow, most noticeable at idle when air requirement is lowest and vacuum is highest. When driving, the problem will be less noticeable, the leak will be an insignificant portion of the air flow. But idling will always be a problem, you won’t be able to properly balance your carbs, set correct timing or have stable, low idling speed until you fix yr vacuum.
Vacuum gauge highly recommended. Not the oil filled type, not reactive enough. At idle you should have a steady reading close to 20 in.Hg. If not, disconnect and cap (seal off) every hose connection on yr carbs manifolds and balance pipe including from the engine breather (& charcoal canister), brakes servo, carb bypass valves (at carbs and at manifolds) i.e. everything, there are potentially 15, you will be using one for the vacuum gauge. Also ensure that where the balance pipe connects to the manifolds is properly air tight. You should then have good idle vacuum. Then reconnect each service one at a time until you find the culprit.

[PeterCrespin]

What, exactly, did you swap and change when you converted to vac advance - presumably from vac retard? The retard is controlled by the thermostatic valve at the right rear water rail so it cuts vacuum when too hot.

The

[vee12eman]

Hi again,

Thanks for the inputs. I will check everything again, although I was confident about the air sealing. To be honest I suspected vacuum leaks but I can’t find any. I’ll try a more measured approach as suggested and I have ordered a vacuum gauge. Any excuse for more diagnosis tools. The figures are useful for guidance.

Not sure about Pete’s question, on my early UK spec V12, the original retard unit was controlled by a tapping from the left rear carb, on original Strombergs. This is now blocked off and I am using the vacuum advance unit (54405202) recommended by the British Vacuum Unit with a ported tapping in the exact location they recommended. (From email communications with them: the small port hole is .540 from the outer edge inside just past the disc on the air cleaner side.) It comes out adjacent to the throttle plate of the left rear carb, is well sealed and connected to the advance unit with a rigid pipe and two short connections of vacuum hose, similar to the original setup with slightly modified route. At the time of test, this was firmly blanked closed to eliminate its effects during carb balancing at idle, so should have had no influence.

Probably have to wait until the weekend to have time to play again, hopefully the locally ordered gauge will arrive in time.

Thanks again.

Regards,

Simon

[lowact]

Do you have a gulp valve? This thing (C34018), it is a diaphragm valve that pops open on heavy decel, bypassing the carbs.


Include it in the things to be disconnected and connection (to the balance pipe) plugged. If its diaphragm is perished/split (likely) the vacuum leak can be large and being internal you will not know, unless you take it off and try sucking thru it … NLA and not serviceable, to fix you must cut/prise it open, here is pic of mine during restoration using an old rubber covered glove to make a new diaphragm, incredibly it worked.


On some us spec cars (mine) the vacuum to the distributor from the LH rear carb was via the thermostatic valve (C37430), my understanding, it didn’t shut off the vacuum when the engine was “too hot”, it shut it off when the engine was “warm enough”, i.e. vacuum retard was only ever applied when the engine was cold, normally there was no retard?

[MarekH]

The various vacuum connections are documented in the 1974 Parts Manual. Their function is documented in the US edition of the tan coloured Owners Manual (the UK version being blue). A summary is contained at http://www.jag-lovers.org/snaps/snap_vi ... 1190800376 where I have annotated the Parts Manual diagram. For advance, read retard.

kind regards
Marek

[vee12eman]

Hi Colin,

No gulp valve on my early (pre release build) UK car, in fact very little emission control at all except that built in to the carbs themselves.

I’ll see what I can find when the vacuum gauge arrives.

Regards,

Simon

[PeterCrespin]

No gulp valve on my early (pre release build) UK car, in fact very little emission control at all except that built in to the carbs themselves.

Correct. Air injection, gulp valve etc. was a Federal requirement only.

You described your vacuum source but I was talking about the next stage. AFAIK all vac retard ststems used the thermostatic valve. This was to stop the heating effect of retardation, whenever the car got very hot. The valve has four tube nipples and the normal fix is to leave it in place, connect each pair with a loop of rubber hose incase the valve malfunctioned or leaked air. Then strobe for full advance timing and bring the no longer retarded idle under control via the Summer/Winter Strangleberg link and playing with carb settings. I’m hazy on detail after twelve years but probably also fitted an advance dizzy.

[MarekH]

Dear Simon,

From your initial description, you are saying that blipping the throttle means it doesn't settle back to fully closed but instead to the same place it started at. You've set the linkages etc to reflect this, so when it does eventually jump back to where it ought to be, the revs are lower and you stall.

I'd take a look at Roger Bywater's jagweb.com website for how to set the linkages on an XJS and adapt the process for carburettors, by first disconnecting the four carburettors from each other. Then I'd get the desired idle speed, blip them individually and slowly reconnect, checking where your sticking point is.

You may find that some of the linkages don't move together or that one of the 90' bits isn't lined up in the right plane because the bell it pivots on is no longer spherical/circular due to wear. Perhaps just one throttle spindle sticks. And so on.

Typically you balance your idle either on the amount of air getting through or by changing the advance. In this case, the advance is fixed, so you are looking at an inconsistent amount of air before and after blipping - it suggests a mechanical problem with the initial setting.

(Pete - if you look at the Parts Manual link, you'll see that Jaguar eventually just connected the retard unit directly to the lower left hand carburettor port.)

kind regards
Marek

[lowact]

- if you look at the Parts Manual link, you'll see that Jaguar eventually just connected the retard unit directly to the lower left hand carburettor port.)

I'm not seeing this, if true, totally illogical?

[vee12eman]

Hi,

Thanks for more useful and thought provoking answers. I will answer as I can. Firstly, the parts catalog doesn’t seem to show the early Series 3 vacuum connections, at least I could not pin any of them down to my early chassis number, but my engine had no thermostatic vacuum valve, the connection was directly from the retard unit on the distributor to the lower flange of the LH rear carb. No guarantees this is original but I’ve always believed it was. Now it follows the same route initially, but ends near the lh rear carb, but this time the upper flange and the ported tapping is there. Of course the old taping is now blanked off.

Carbs were rebuilt relatively recently, at least mileage wise. There isn’t any wear that I could see and in any case, idling was fine prior to removing one pair to drill the tapping and fit the tube for the take off. The throttles were free to move and balanced with the linkages to the pedestal disconnected and the linkages between the throttles loose initially, later the adjustable linkages were used to give fine balancing. The linkages from the pedestal were set up to be at the same angle, using a smartphone inclinometer app, to ensure each had the same geometry when connected and the short links adjusted to have no influence on the previously set throttle idle positioning.

As I said, I did discover that I had left two vacuum pipes to the air filter boxes open, I closed these off and things improved, this was a source of an air leak to each bank of course. Nevertheless, it still wasn’t right; set the idle to the lowest comfortable idle (higher than it was supposed to be), rev and allow throttles to close under spring pressure, the speed dropped below the previous setting, most times stalling, but occasionally continuing to run, in which case it would climb back to the original idle setting. I used a Colortune device to balance mixtures, setting things a little richer than I would for other engine types, which seems to be the recommendation for the carburettor equipped V12s. Things are much better now, but I still can’t get an idle much below around 820 rpm or so, it fluctuates around this point and isn’t as steady as I would like. Setting the speed lower seems to be too low. I still feel it might be an air leak, but I can’t find one.

Anyway, I’ll see if I can find anything else wrong. Is there a good, safe way of locating vacuum leaks?

Regards,

[MarekH]

You can spray propane at it (from a plumber's torch - unlit...). If there is a vacuum leak, the car should run richer. (Spray it down the carburettor throat to get an idea of what effect it'd have.)

I'd suggest perhaps (one of) the throttle plates is/are sticky and jump back or the choke erroneously enriches the mixture and stifles the engine because the cable or mechanism doesn't more as freely as it should. In any case, with all of the linkages off, you should be able to ascertain whether blipping just one of the carburettors has an effect or rule out which ones don't have an effect.

Page 25.32 of the Parts Manual - engine numbers 7s8444 onwards. Evidently, Jaguar thought it was a waste of space eventually.

kind regards
Marek

[vee12eman]

Hi Marek,

Thanks for the reply, it helped me confirm my car is correct in not having the Thermostatic Vacuum Valve C.37430. The parts catalogue is confusing, as the vacuum system to the distributor is shown in detail on pages 30.28 - 30.32 (that's how the vacuum system for EX/EM car pages are numbered in my FHC 2+2 catalogue), however, none of the engine number series listed covers my car (7S.1388 SA). However, I now find that on page 32.48 IGNITION, (showing the early distributor set up) there is a pipe, part no. C.35610, which is not shown on the later setup on page 32.49. Engine numbers listed (NOT EX.EM - 7S.4879) tally with the ones for the EX/EM system which is only fitted from 7S.4880 - 7S.8443, according to page 30.31 & 30.32. The pipe C.35610 appears to be used with all vacuum systems, also appearing on the Thermostatic Vacuum System pages, but is attached to different hardware.

In short, both early and late UK spec cars have no thermostatic vacuum valve. This was not the intention of this thread, but interesting to me at least, confirming my car was original when purchased.

Colin, this may seem illogical, but my understanding is the retard system is only designed to be active at idle, hence the unusual tapping location on the LH rear carb lower flange, where it emerges directly under the closed throttle. Normal running depends on mechanical advance weights in the distributor.

I elected to use ported vacuum advance, the plethora of info on this and other sites and consultation with many others led me to believe this was best, I am not going into the reasons, please look it up and I'm sorry to those that disagree. The original retard system reminds me somewhat of the "Dieselgate" scandal, where VW made their cars able to pass emission testing, but in normal running, the system was not active. The Jaguar system can't decide the car is being tested - the VW system ECU could detect when the car was being tested in a particular fashion, but nevertheless, the system was, I believe designed to reduce emission only at idle to pass emissions testing (Californian spec in particular?). At least the Jaguar system would be active in congested city streets where the car is stationary more than it is moving. Pity we are all modifying it and even the unmodified system hardly works on any cars these days!

Thanks to all for the advice; Marek, I'll try the propane trick soon, but road testing will have to wait until lockdown is lifted - who knows when that will be?

Regards,

Simon

[lowact]

Clever detective work, imo does indicate early non ex/em cars did not have thermostatic valves. Not convinced about late cars however, 25.32 does show the thermostatic valve. Unless there was an undocumented late model non ex/em that didn’t?

What have you got that plugs the hole where the thermostatic valve would have been? What I’ve put on my engine does not look like it belongs …


Here are some vacuum readings fyi. Like you I’d swapped to vacuum advance, drilled a new 1.0 mm dia. tapping point, was very happy with the position, is nicely in the shadow of the leading edge of the throttle plate, I couldn’t push a fine fuse wire through without it scraping the throttle plate.


Note these measurements are with the car stationary so they don’t correspond to driving rpms, e.g. the throttle opening at 5000 rpm when stationary is only a fraction of what it would be at 5000 rpm under load, and vacuum is ~ due to throttle opening (load) … so my assumption is that when driving I would have the vacuum response as above but compressed into a much narrower rev range, depending on gear, maybe 800-1500 rpm (ish), instead of 800-5000 rpm.

Yellow and green are from this publication, vacuum verification on page 31:
https://1drv.ms/b/s!AiSPBKa26IcchMxt3qy ... g?e=vEeixd

The rest are measurements of my e-type using a vacuum gauge. Orange is my retard tapping point, grey is my advance tapping point, blue is deep manifold vacuum i.e. not throttle edge.
Blue and orange show that I had lousy vacuum at idle (due to leaks via gulp valve and charcoal canister)
Orange shows that the vacuum for distributor retard does not disappear as quickly as you might imagine, so having a thermostatic valve that positively shuts it off as soon as the engine is up to temp. would seem to be the appropriate design?
Grey shows that the vacuum for distributor advance (new throttle top edge tapping) is slow to build, I wonder if this was one reason XJS HE engines used deep manifold vacuum to advance the distributor, with the throttle edge tapping just used, in effect, to switch it off at idle?
Blue line is what you would get if you did as Phil often suggests, don’t bother with throttle edge, use a deep manifold tapping for distributor vacuum advance, no waiting for the vacuum to build ...

[vee12eman]

Hi Colin,

Referring to parts catalogue page 30.18 (FHC), you can find the cooling system casting you have photographed in your post. This confirms that cars up to 7S.4879 (NOT EX/EM) have a casting without the take off point for the Thermostatic Vacuum Valve and that after engine 7S.8444, the NON EX/ EM engines did have your style of casting, but now had a plug, part number C.38358 installed, again confirming these later cars did not have the thermostatic vacuum system. Hence, I have nothing blanking the take off, because there isn’t a take off on my car:



Hope that’s all clear,

Regards,

Simon