HD8 carb issue

[christopher storey]

I am interested to still get an answer about how far down the jet the fuel should be seen and settle.
MarkRado say 5mm ? Is this correct ? Now you can see why I comment about needle thickness now relevant to the initial problem as described.

John : the flooding was not perhaps what you and I understand by the term, that is of flooding with the engine stationary, but was excessively rich mixture when the engine was cranked over, resulting from excessive quantities being sucked out of the ( virtually unoccluded) jets . This is why it has been cured by the 125 needles . Incidentally I have always used UMs , and have steered clear of the fashion for going to UE,UO etc

[John ball]

Good afternoon Steve and Chris, I realise I am on the E Type forum so my car being an XK120 is not quite the same. It is a 3.4 with twin HD8’s. So, in relation to the question of am I trying to discuss or set up my car, it is a bit of both. I went into the garage this morning to have a look at all the suggested parameters again. I have found that with 2 1/2 turns down from the bridge the mixture jets are both 0.073” down. I have been running at 3 1/2 turns down as slightly smoother tick over. This could be because I am running open throats without filters ? I could tell before it was too lean, as with two fingers over one carb throat it smoothed out. Also this morning I wound the Jets right down to where the fuel was level with the jet top. It was not your suggested 5/16” but pretty close to the Heuer suggested 5mm. One was 4.56mm 0.183” and the other 5.73mm 0.225”. So, I am wondering if I should be adjusting fuel bowl tang to produce a lower fuel level in the jet. I have measured the float adjustment and it is currently correct with the 7/16 bar.

To get a consistent 750rpm idle the air volume screws have to be open 5 turns, with the butterflies totally closed on the stops.

So, after all this long ramble, I only have two HD8’s so assume I need slightly richer mixture than your 3 HD8’s?
I have tried UM needles in my two carbs and the engine is reluctant to rev past 3,500. So now using UN’s, but my fuel consumption is fairly poor.

The ignition timing is set at 10 BTDC and a max of 34 degrees.

[mgcjag]

Hi John....are you sure you have HD8s....dont think they were standard on a 120....even so S1 XJ6 ran twin HD8s with UM needles....so if you have them you could follow the set up details in the XJ service manual....do you have an aed unit fitted as well.....you could also join the Jag lovers forum in the US who have forums for all Jag models...Steve https://forums.jag-lovers.com/?mobile_view=1

[cooper66]

I am interested to still get an answer about how far down the jet the fuel should be seen and settle.
MarkRado say 5mm ? Is this correct ? Now you can see why I comment about needle thickness now relevant to the initial problem as described.

John : the flooding was not perhaps what you and I understand by the term, that is of flooding with the engine stationary, but was excessively rich mixture when the engine was cranked over, resulting from excessive quantities being sucked out of the ( virtually unoccluded) jets . This is why it has been cured by the 125 needles . Incidentally I have always used UMs , and have steered clear of the fashion for going to UE,UO etc

Absolutely sorry for the late feedback. Indeed engine not running there was no flooding, but trying to start the engine with the starter you can imagine the gap between the needle and the jet. This was the root cause of the flooding.

[John ball]

Hello Steve, yes I do have HD8’s. I have owned cars with SU carbs since 1973 and had my first E Type aged 23 in 1977. The change to 2” carbs on a 120 is a quite often fitted modification. I know the H6’s well as they are fitted to my Healey 100 and have the 0.100 jets as apposed to the 0.125 jets. I made a mistake earlier as the mixture screws are not down 3 1/2 turns but 3 turns 0.088” I do have slightly higher lift cams supplied by Rob Beere. They are designed for SU’s to give good torque and are 0.41 lift instead of 0.385 lift. I did have D type cams but sold them as too long duration, so produced power high up to the detriment of low down torque.
Yes, I do know the Jaguar lovers forum and have posted on several topics. The reason to discuss on here was for different opinions from mostly U.K. based people and people who know and use the HD8.

Anyway, what else to do on a wet miserable Sunday afternoon, when I can’t get either car out !!!

Much appreciate the conversations.

John

[christopher storey]

So, after all this long ramble, I only have two HD8’s so assume I need slightly richer mixture than your 3 HD8’s?
I have tried UM needles in my two carbs and the engine is reluctant to rev past 3,500. So now using UN’s, but my fuel consumption is fairly poor.

The ignition timing is set at 10 BTDC and a max of 34 degrees.

John - it's an interesting question whether you need a richer mixture on 2 carbs using the same needle as would be used in a 3 carb setup, because of course any given engine at any given speed sucks harder through 2 carbs than through 3 of the same size, and thus the actual mixture strength may not be all that much weaker. Nonetheless, every car is different , and if yours behaves better with the jets set a bit further down, then I think you are quite correct in using that as the setting. As you say also, filtration differences affect matters, and freer flow filters than originally used probably require a richer setting.

PS also a cam change will affect mixture requirement . However, I would not depart from the 7/16 bar method of setting the float level

[mgcjag]

Hi John.....so has this problem recently started or are you in a new set up situation...have the carbs been recently rebuilt.....if not they should be looked at....do they have manual choke bodies or are you useing an aed.....re the filter you could trial with a secondhand S1 xj6 type filter for HD8,s adapted to fit.....then once you carbs running ok you can try different filter types...Steve

[PeterCrespin]

This is indeed an interesting thread and points to a notable property of CV /CD carbs - namely their ability to self-compensate for variable demand over large parts of the fueling range.

Remember that Jaguar used the exact same carbs on 3.8 and 4.2 E-types - a capacity increase of over 10%.

[John ball]

Peter, Steve and Chris, interesting point about the 3.8 and 4.2 using same carbs and needles. I think the truth is that I do need to run a richer needle with only two HD8’s. For a long time I had been running the car with the jets too high at 2 turns down, but was more focused on ignition corrections. So, with the jets set slightly lower it does make quite a difference. It just shows to me how sensitive these setting are. It is worth mentioning again that all the needles are virtually the same at the tops, so my only adjustment for richer idle is to lower the jet. I had the car on a rolling road last year and it did improve the power with thinner needles lower down the needle - thus I am using UN. In fact as I mentioned before, the well respected Jaguar rolling road team said that even with E types on three UM’s to get more power they needed thinning out. If you look at the SU book the UM is one of the fattest. Very noticeable on a bench with a UO or UN.

Also to answer other questions, yes the carbs were rebuilt by myself, but with Burlen putting in new shafts.
Yes, it does have the electric choke, but with a manual override switch.

I took the plugs out this morning and they are fine, not sooty just slightly off the tan colour and clean gapped.

I have enjoyed this discussion as really I do not have a problem, just fine tuning for the optimum!!

What else can you do on a rainy January day !!!

[mgcjag]

Hi John....dont forget to look at the air filter....running with non isnt the best idea....have a look at the ITG foam filters and other similar makes.....Steve

[John ball]

Yes, I know you are right about the air filter. Just not had one on while fiddling. Never had a proper one as with the HD8’s sticking out more than H6’s as standard the steering column is too close. So had just trumpets gauze and with foam cover. So, now going to investigate the proper asymmetrical type as fitted to the C type.

Also, just a comment. You all might be thinking why did not the rolling road sort out tick over mixture.
Well, when the car was being tested, the engine was stinking hot and oil at its thinnest. They were testing it for max power and torque. The tickover problem was really only when it is in the period of off choke but before fully hot, but with the slightly lower jet by half to one turn it has cured that period of stalling.

[MarkRado]

Indeed very interesting. I fully agree with Christopher #46 that the engine with two carbs is sucking harder therefore lifting the piston more thus increasing mixture strenght. The jet height difference between 3.8 and 4.2 (my Etype/ my friends Mk10/- both 3 HD8) is considerable: 1,1mm (3.8) vs 1,6mm (4.2)

[PeterCrespin]

“.. the engine with two carbs is sucking harder therefore lifting the piston more thus increasing mixture strength..”

Not as clear cut as that Mark. I’m sure Chris agrees the whole idea of tapered needles is that the mixture ratio stays more or less the same over a range of air flows. The needle may be shaped to fatten the mixture or lean it at various points, but the fact that more petrol passes through the jet as the piston slides upwards does not automatically mean the mixture gets richer.

[christopher storey]

Ah , Peter you are quite correct in that what I should have said was that with 2 carbs it probably needs a richer mixture setting to achieve the same result

Incidentally , the UM needle was used with the HD8s not only on 3.8 and 4.2, but also on the original 3 carb setup in the 3.4 litre XK150S

[MarkRado]

Absolutely right Peter. I was thinking in terms of quantity- the same amount of air has to pass the carbs, no matter if there are two or three of them. So more air and therefore more fuel per unit in an installation with two carbs compared to three, but not richer.

[John ball]

This is fantastic, total international conversations - UK, USA and Austria !!

You three guys are far more technical than me.

With my experiments, is there anything else I can try to further improve the performance ?

P.s. Chris, I have here in front of me the official SU book. It states the needle on the 3 HD8 XK150S is the UE.
Looking in the SU needle profile chart, the UE is richer than the UM right from stage 2 but not as rich as the UN that I am currently trying. So, since the UM does not work in my 2 HD8’s I might try UE’s, as my fuel consumption is not too good. When the car was on the rolling road, it had UO’s. They filed them slightly and the power and torque increase was very good. They told me to get slightly richer needle. Maybe I have gone too far with the UN’s ?

Pps it was interesting at the Jaguar rolling road to see on their log of other cars, how little power the cars produce compared to the original specifications. Also how much with some carb adjustments they could increase power and torque.

Ppps without starting yet another long post of ignition. What do you three set your static, idle and max ignition.
I have 10 BTDC up 1000 and max of 34. I have a vacuum connected

[mgcjag]

Hi John...yes please start another thread on ignition.....this is an HD8 issue thread and will get " off topic" very quickely...thanks....Re UE needles see post 655 here on the last page viewtopic.php?f=8&t=8874&p=120219&hilit ... ad#p120219

[John ball]

Hi Steve, I have looked at that post about the rolling road and UE needles. I must say I am a little bit surprised by the first run with UM needles, as the power is very low and drops off after 3114 revs. Even the second run with the UE needles only produces 195 BHP and the power drops off after 3924 revs ! Seems quite low for a 4.2 engine. I assume being a new engine it was tight and they did not rev it too much. The torque of 378Nm/242ftlbs seems ok. I think that after the first run on UM’s they must have done an ignition adjustment, as the jump from UM to UE seems too much.

My engine is only a 3.4 and gave 180bhp at 4,800 and 284nm/210ftlbs at 3500 and that is with UN’s.
I would expect the 4.2 to have much more torque than the 3.4, which clearly it has in these figures.

According to both the Rolling road specialists I tried over a period of four years, they said most E types in standard form were nearer to 230bhp and XK120’s in standard form were nearer 140 bhp not 160 bhp.

I may write about ignition trials in a new post at a later date - currently running a 123 with Bluetooth on an iPad and iPhone. Have two curves set up, one E Type and one a restoration company recommendation.

[Tom W]

The concept that with two carbs instead of three, the 2 must flow more air isn’t entirely true, particularly at idle. The engine doesn’t consume air constantly, more in discrete bursts that correspond to the intake strokes on each piston’s induction stroke. With an in-line 6 engine, there are 3 induction strokes per revolution. With an intake valve duration up to 120 degrees, there is only one cylinder on its induction stroke at any one time. In practice, 120 degrees is ridiculous short. With an intake duration beyond 120 degrees, 2 or more cylinders can be on the induction stroke, but one will be finishing as the other is starting. Because of the firing order, the two that are on the induction stroke are being fed by different carbs. So, on a multi carb engine, the carburettor needs only to be sized to suit one cylinder, until intake duration exceeds 240 degrees. More carburettors means they’re sat idle some of the time whilst others do the work. On a 2 carb engine, the intake stroke alternates evenly between the front and rear carburettor. On a triple carb engine, the intake is even across the centre carb, and uneven across the outer two, but because of the firing order, one carb never has to feed consecutive intake strokes.

The balance pipe across the manifold does mean that all carburettors “see” all the intake strokes, but this pipe is there to use the positive benefits of manifold pressure to improve the filling of the cylinders, rather than to provide a supplementary air path to feed the engine.

The advantages of 3 carburettors comes from the improvements in volumetric efficiency due to the improved intake tract design possible. These advantages usually outweigh the disadvantages of the uneven firing across the outer carbs.

[John ball]

Hello Tom, your post is very interesting and obviously your knowledge on the subject is immense.
Please can you explain some thing to me that I don’t totally understand. You say a 2 carb engine, the intake stroke alternates evenly between the front and rear carburettor. But if the two carbs both go into one big manifold, does not the air get sucked through both carbs as they are only 9 inches apart, as the whole internal of the manifold from front cylinder to back is hollow and about 20 inches long. On my XK I have the two carbs going onto the large manifold, but on my Healey 100 the two H6 carbs go straight to the head, but there is a small tube in between to two carbs just before the head.