Pinking, Mixture weakness, Vacuum advance, Oil consumption

[rfs1957]

The car drinks oil, did I calculate 400 miles per litre on the 5.500 km trip last month, and I know that whilst VSE in Wales re-did the block and the bottom end 20 years ago, the head never got touched - and judging from the very thin shims, one even below the official thinnest size from memory, it seems clear that the seats are sunken, and from the consumption and the rattles that the guides are xfritzed etc and the winter project is to re-do the top-end completely.

The N12YC plugs were quite furred up after just 2.000km, although the BP6ES I then fitted look much cleaner and have virtually no carbon apparent after the following 3.500 km.

However, I would like to try and ascertain whether the pinking I'm getting between 1.500 and 2.000 rpm is due to something else before I just ascribe it to carbon build-up in the chambers.

I'm assuming that it might also be caused by the wrong vacuum piston springs in the SU's, as the jets and needles are correct, float heights too, and I'll have the right ones fitted sometime next week.

The lambda sensors and meter referred to in a parallel post will show very quickly whether this has made any difference to the combustion.

But could it not also be down to a distributor problem ? I have an Aldon Igniter fitted, but running with the original mechanical advance, and I have no idea what condition this is in. Presumably once this gets sloppy and old it has a tendency to advance more, and earlier, than it is meant to - and excessive advance can cause pinking I believe ?

When I bought the Aldon off Watjag about 15 years ago I was told to run it with no vacuum advance pipe fitted, both ends blocked off obviously, can anyone suggest why ? However if refitted this would only ever INCREASE the advance still further .........

The max advance is easily checked, and it is correct, and the damper markings are precise and TDC certain, but plotting the advance curve with the distributor on the car isn't easy in my experience, and I would have liked to understand better what is going on before I take things to pieces.

Anyone faced similar issues please ? Especially interested in the effects of burnt-oil carbon deposits etc ?

PS I know I'll get the Megajolt proselytisers big-time after this, but I'd like to get the original system working to my satisfaction first !

[Hugo]

Certainly any carbon deposits can cause detonation or pinking, but unless you have some big lumps of carbon in there I would doubt if that is the cause of your problem.
There is a difference between pre-ignition, which, as the name implies, is what happens when the mixture ignites prematurely, and detonation, which is when the mixture explodes rather than burning (relatively) gradually as the flame front moves across the combustion chamber.
I would be very surprised indeed if the carburettor springs had any bearing on this. SU carbs are remarkably un-fussy creatures.
I would put my money on the distributor advance curve being the culprit. You should be able to determine the degree of advance with a graduated timing disc and a decent strobe light - one that is powered from the battery - then watch where the timing goes as you hold it at different speeds.
In fact, if it were mine, I would set the distributor advance to accommodate the pinking. By which I mean ignore the prescriptive settings and just set the advance as far as you can without it quite pinking. You would probably need some sort of programmable ignition to be able to do that though, unless there are any distributor specialists who can tweak the weights and springs to give the desired results.

[christopher storey]

The only function of the carburetter springs is to make sure the piston only reaches full lift at max rpm and full throttle . Since the available springs for any given model of carburetter is very restricted, they are hardly crucial, and in part throttle running it is more the weight of the piston which governs the degree of lift. Therefore as Hugo says I think we can dismiss the springs as the cause. Again as Hugo has said, your symptoms sound much more to me like weak mechanical advance springs, which allow the degree of advance at very moderate rpm to be far in excess of that needed . When vac advance is added to this at small throttle openings the total advance is very considerable. That is tolerated on a steady small throttle opening, but the moment you open the throttle ( and before the vac advance is completely dissipated ) it is no longer tolerated which creates pinking just as the pedal is pressed, but which does not usually persist once a new steady state is reached with little if any vac advance . My experience over some 58 years of playing with XK engines is that they have always pinked in the range of about 1800 to 2000 rpm even when super premium fuels were available, and now that fuels are not as good as they were, the tendency is increased

Your 200 miles per pint is also fairly typical of a well used engine. The real telltale of its causation being top end is a puff of blue smoke on each upward gear change. If this is the case you could try the later XJ6 type guides with valve stem seals, but I would be hesitant about replacing the seats, as there are too many tales of new seats falling out with damaging consequences

PS Transient weakness on opening the throttle is often the result of the dampers being worn or the oil used being too thin. Personally ( and this is a viewed shared by a number of XK mechanics ) I always use engine oil rather than the SAE20 which seems to be used by e.g. Penrite

[Hugo]

.......That is tolerated on a steady small throttle opening, but the moment you open the throttle ( and before the vac advance is completely dissipated ) it is no longer tolerated which creates pinking just as the pedal is pressed, but which does not usually persist once a new steady state is reached with little if any vac advance .....

It's probably me (since it's early in the morning here in Florida and my brain needs a bit of vacuum advance) but I initially mis-interpreted what you were saying here. You are saying that when you crack open the throttle, there is initially a bit of 'residual' vacuum advance which causes or contributes to pinking before it has worked its way out of the system - before the vacuum has escaped if I can put it rather un-scientifically.
That is something I hadn't thought of, but it makes perfect sense.

[christopher storey]

Hugo : yes , the way I see it is that the vacuum persists , albeit in ever decreasing quantities , until such time as the inflow of air " fills the gap". Since air has considerable inertia, this takes time, and of course in addition there is the time which the vacuum capsule takes to allow the baseplate to rotate back to its datum position . Whilst this is probably only of the order of 0.2 to 0.3 seconds, it is that which tends to cause the perceptible rattle on opening the throttle - and even when the vac is gone, if the mechanical springs are weak, one may still have say 10 degrees extra advance than is justified by the engine speed

[rfs1957]

I've cracked it, here's the reasoning.

There exist 10 different SU springs with widely-different lengths and rates, and I would hasard that most of them could - inadvertently - be fitted in an HS8. Their contribution is not negligable, as at around 200 grammes for the correct ones their effect is roughly equivalent to doubling the weight of the floating-piston.

Get the wrong ones and your needles are not sitting where they were meant to.

Since my springs had NO identifying marks it made sense to start by spending £18 and buy 3 new ones. They were, of course, the same as the ones I took out - but you have to eliminate these things.

The float heights hadn't been checked for years but were all still good, and after adjusting the carburettors the three main jets ended up in similar positions, so the basics were sound.

Fuel pressure was about 3 - 4 psi, measured at the carbs, to be sure of no restriction upstream that might be leaning the mixture. Flow judged abundant.

Plotted the timing curve - see below, apologies for scruffy workshop-notes but they beat back-of-an-envelope



which show some disparities, especially at low-rpm. Happy to hear any corrections to my distributor-to-crank-reasoning.

My lower setting was a tad high at 11°, and the max upper a tad low at 32° but overall the curve wasn't outrageously wrong ; dropping the static back to 6° (fully advanced thus dropping to about 27°) took some of the sting out of the pinking but made no difference at all to the Lambda sensors' verdicts and of course took the edge off the performance.

So whilst there's a lot of room for optimisation, I felt the critical mid-range-pinking wasn't down to the distributor curve ...........

The car is stock except for the first baffle in each middle exhaust-box having been taken out with a core-drill, rendering them "straight-through" to the eye - although the exhaust note remains perfectly modest ; race system this most certainly is not.

All this left one easy alternative - the carburettor needles - that I had not wanted to try until I had exhausted the other possibilities, as my current and original UM needle is so widely used and seems to work for most people.

To cut to the chase, this post

viewtopic.php?f=3&p=92050#p92050

shows the truth about the fuelling differences between carb needles, since I thought it unlikely that my engine really needed the extra 27% fuel at Wide Open Throttle provided by the often-mentioned UE needle, for example.

Conclusion : UB needles fitted, cost about £22.

Happily, these hitherto un-sung* needles have at a stroke solved all the problems of pre-ignition and weakness that I had - unwittingly, until I installed the Lambda sensors - been living with for 15 years.

A new car for £22.

The Lambdas are now perfect from idle to thrash, the pinking has gone, at ANY and ALL throttle openings, and the car is ................ a different car.

Goes, and WANTS to go, like S off a stick.

Hallelujah.

PS One the engine has been re-built (this winter) I'll try fine-tuning this still further, with extensions to the SU float pistons, so that the needles' actual positions are visible.

* just one discreet mention on the Forum, from KingRichard, back in 2014 . Respect.

viewtopic.php?f=3&t=6583&p=47740&hilit=needle#p47740

[Hugo]

The great thing about SU's is that they are remarkably tolerant. Springs and even float levels can be way off & they will still run pretty well. I once managed to get the spring half over the damper sleeve and half beside it (half-way up it went off-course!). That was on an Aston DB6. I drove it like that for quite a time without noticing anything amiss.

[abowie]

Hi Rory
I've got pretty much the same car as you and it runs like a scalded cat on the stock needles and springs, with CO at 5% at idle.
I'd be wondering whether a PO has done some headwork or put in a non standard cam on your car.
Mistiming of the cams is another possibility I suppose. Easy to check if you haven't already.
Overly high compression ratio has also stung me like this once on a 4.2 although if your car drinks oil as it does this is perhaps less of a possibility.
WRT SUs Des Hammil's book on SU fettling is actually really good. I built a fast road engine for my MGA last year and it ran like crap on the standard carby setup. I was able to sort it out with advice from that book.

[Hugo]

Just a thought, since it's been on my mind lately - the slight difference in valve duration between early and late cams couldn't account for a different needle preference between different engines, could it? It's so slight I doubt it, and I don't know who's running what cars, but it's just a thought that crossed my mind.

[rfs1957]

Andrew, G'day down under.

My point about the lambda information is that, until I had it, I too thought my car ran like a scalded cat (albeit with some mid-range pinking) and it too had satisfactory CO on tick-over.

UM to UB needles wouldn't change that as station 1 and 3 at least are identical, and station 2 is actually 9% leaner on the UB. Swapping from M to B didn't even require any carburettor adjustment.

From all that I know about the history of the car it has never had anything trick done to it, mechanically I believe it's even pretty flakey, and the cams are standard, and I know their timing is correct. Obviously when I strip it I'll be looking for any deviances and will report back.

Is the exhaust a bit more free-flowing with the baffle drilled out in the middle boxes ? Maybe ? But it sounds like everyone else's E-Type to me.

Since most road cars seem to get tested Wide Open Throttle on rolling road dynos which are actually inertia dynos (and NOT water-brakes or eddy-current equipment which allow intermediate throttle openings to be "frozen") the combustion information obtained during these runs bears very little relation to where we spend 95% of our time when we're driving these cars.

The shots below shows pages from a catalogue I wrote regarding fuel-injection on Ducatis, quite different perhaps, but the point regarding the irrelevancy of the WOT zones that are tested by inertia dynos remains valid for our cars. It's in French and English, you just have to search a bit.

The plot is 16 throttle-opening points against 16 rpm values, which is how the Magneti Marelli alpha-N stuff reasons. Nice parallel with the 16 stations every 1/8" on a 2" SU needle in our HD8's eh ?!





I'd bet there are some cars out there where a £22 punt on 3 x UB needles might be money well spent.

[Hugo]

Is the exhaust a bit more free-flowing with the baffle drilled out in the middle boxes ? Maybe ? But it sounds like everyone else's E-Type to me.

That is a very good question - these things can often be counter-intuitive. You can sometimes get better gas flow - if that is actually what you want - WITH a baffle than without.

[abowie]

I shall order some :-)

[Gfhug]

Rory, I agree with your point about dyno testing being somewhat irrelevant to our normal driving, often cruising at 2,000 - 2,500 rpm on typical roads.
Without the benefit of Lambda sensors, how would we notice the benefit of changing to UB needles? And how would we set up the car?

As you say the scalded cat is an impression without formal acceleration tests.

Asking as I feel my engine is probably a bit on the rich side. Though that is only an impression, mainly formed by thinking my fuel consumption of circa 22mpg is a bit low (maybe a heavy right foot?)

At the MOT, yesterday, the guys did an idle check on the exhaust giving:
CO 4.8 / 5.7%
HC 140 / 175 ppm
CO2 4.41 / 4.46%
Lambda 1.06 / 1.02

Numbers bouncing about a bit as no electronic ignition. But the dizzy was overhauled by the Dr. and carbs overhauled by me, including new springs, UM needles, etc.

Maybe a bit academic, as the car runs nicely with no real need to change anything. But interesting to learn more about our cars and engines.

Thanks for any comments and it'll be interesting to hear what Andrew Bowie thinks with the change.

Geoff

[Hugo]

I wouldn't pay too much attention to the numbers. It's what it feels like on the road that matters. I once rebuilt a Harley engine with a 'hot set-up' kit (bigger pistons & cylinders, cams etc). I warned him that it might not deliver the performance he was expecting, since this set-up was designed for max power high up the rev band, where you don't normally need it. You could tell just by looking at the great big lumps on the camshaft instead of little pointy things like the Jag. Anyway, I did the job and was amazed at the power increase right through the rev range - more than I thought possible. Then my friend went & put it on a dyno and never stopped moaning that it didn't make the numbers S&S had specified. No matter that it went like you know what, he was obsessed with the numbers.
I'm a great believer in trial & error - just try something & see how it feels on the road.

[Gfhug]

[rfs1957]

Dear Hugo,

If achieving your record rapid ascension in the postings had not been at the price of hundreds of hours spent on the Forum, a casual observer might have thought that your intention was to probe at what point your audience of anoraks would grasp that you were winding them up.

Fortunately for you, and unfortunately for the rest of us, there generally reigns amongst E-Type owners a certain probity, a reticence at raising the tone, rising to the bait, or rocking the boat, and slight irony or understatement is about the limit of what we stretch to when getting an even strongly-felt view across.

I say fortunately for you, in that you have managed - unchecked - to write some of the dumbest nonsense it has ever been given for me to read on the Internet, and certainly the most half-cocked cod-science written anywhere on this Forum.

When other members have used the irony / understatement approach, it has just gone right over your head.

The result is that you have filled and clouded hundreds of posts with irrelevant and incorrect twaddle, through which the rest of us have to wade.

It is highly telling that you have visibly never actually spent any time researching or calculating what you spout, still less actually taking your own photographs, making your own sketches, downloading them, in short making any EFFORT.

It's far easier - and, somewhat worryingly, apparently more rewarding for you - to just inject keyboard-diarrhea.

As I look through the dross that you have posted, I am hard-pressed to find a single contribution where it could be said that you have actually added anything of any sense or consequence that furthers our understanding of these cars.

This sentiment is shared by a great many of us here, who have compared notes by PM, and were hoping that you might "get the message" and belt up.

Since you haven't picked up the signs, I have elected it to give it to you from the hip.

Please just READ the Forum, and keep your comments to your self.

Stop your irrelevant, boring, distracting, incorrect and frankly tedious additions to the exchanges that we have here, as they were frankly a lot more enjoyable and useful before you joined our ranks.

If you want to write about Buses and Diesels, do it somewhere else please.


PS Your latest twaddle here

viewtopic.php?f=3&t=10855&p=92403#p92403

is just that ; reflection requires visibility. Geddit ? The aluminium inner surface is not silver at all, it is blindingly obviously BLACK as it is in the dark hard against the bitumen that it's joined to, and if anything - being an excellent conductor - some other Hugo might even write a post to tell us that it makes things worse, and we ought to use a poorer conductor like stainless steel.


PPS If you want to write puffery about motorcycles, especially S+S in Wisconsin, choose your audience,

and if you want to spout cod-science, avoid discussions with Chartered Mechanical Enginners that are considered, by the owners of some of the most rarified kit ever, to have hands that can be trusted to actually do the work too.




Theory and practice under one roof ?

I would hasard that you have neither.

[Gfhug]

Though, Rory, if you feel up to coming back on topic, I hope my question wasn't dithyrambic

Thanks

Geoff

[rfs1957]

Hi Geoff, missed your

"Without the benefit of Lambda sensors, how would we notice the benefit of changing to UB needles? And how would we set up the car?"

I think setting up would be identical as the 1st and 3rd stations are anyway the same diameter ; I haven't changed anything on mine, I just swapped them over.

Since nobody, anywhere - from what I can glean from around the Intellect - actually has a clue what stations are in use at such and such a zone of the operating envelope (which in itself I find fascinating !), it's all guesswork at the moment.

But anyone who has studied SU needle profiles will know that very few actually have a "standard" profile with a regular, linear taper - they jump quite wildly sometimes from station to station, and I believe these will have been designed on a water-brake dyno where needle position is known, manifold depression measured, and combustion optimised for that station.

To be honest, I last adjusted the carbs 4 years and maybe 8.000 km ago, and the tick-over could be improved on - it's a bit splishy-splashy rich and smelly, and close to the red-rich LED up top, but as I'm now about to strip the motor I was more concerned about finding out why it was running so weak higher up.

So I would have to answer your question backwards, as it were - putting UBs in made a massive difference to my car that I wouldn't have needed the Lambdas to spot.

I may yet discover something else that explains my car's need for a richer needle.

The decision to publish any of this only stemmed from my concern when I realised that changes that were apparently quite modest at 7% - UE - and 11% - UO - were actually 23% and 38% respectively, whereas the UB is just a 4% diameter change to give an 11% change in fuelling, a more plausible modification.

Bit of a rambling answer and I promise to do better when I aim for the best set-up with the new motor !

[Gfhug]

Thanks, Rory. I think you've hit it on the head regarding some of the changes and uses of needles.
I had my car on a dyno and without doing multiple runs it was concluded the carbs were ok at idle and maybe lean at higher revs. Never did get written results, but they never sent me a bill either .

Keep up the good work.
ATB

Geoff

[chrisfell]

There have been many times when I’ve thought that what I’d read here (and on certain other E-Type related sites) was less than optimal advice. However, as those who have read what I have occasionally written will already know, I am no expert and can not denounce the nonsense with any authority. If I had such authority, however, I would hope to have used a more diplomatic voice to express it. Despite some posts being arrant nonsense, I value all contributions of this forum’s Hugos as well as its Rorys and others of superior knowledge.

I hope that Hugo has a thick enough skin to except the criticism and that the rest of us have enough forbearance to accept his occasional expressions of misunderstanding.