My 63 Mk2 has twin HD6 and AED like in the picture


Occasionally, very often infact, fuel pours out at the yellow hole in the drawing, which is part of the AED. Since it's fed of the front carb float chamber I thought it would be because a faulty float/dirt in needle/seat. Bought two repair sets for the HD6s, and today replaced needle, seat, adjusted float level (and yes, the float floats) on the front carb. Still, just turning on the electric pump, gasoline pours out of the hole in the AED still. Did I look the wrong place?

 

First the MK2 pulls vac. from the manifold not a carb body. Not sure where this diagram comes from. That aside are you sure your fuel pump is correct. If the floats seats and needles are working properly it can't pump out (unless float level is very off) but too much fuel pressure will do it.

 

SU carburetors work well with fuel pressure in the range of 1.5 psi to 3.5 psi max at the inlet to the float chamber.

 

Its worth checking that the floats haven't failed and filled with petrol and also that they are the correct weight, I had issues with one that was too heavy and caused the fuel level in one carb to be too high.

It should be easy to tell because the fuel level in the float chamber will be too high and it may flood out of the jet if you take the dashpot and damper piston assembly off.

 

Is the front carby float chamber "breathing" to atmosphere. This is done via the overflow vent pipe.
if the vent pipe system is not fitted with the special cut out fibre washer between the top of the float chamber and the vent pipe, it will not allow the float level to stabilise inside the float chamber and will induce flooding.
I have encountered this on a couple of occasions where people had substituted a plain flat fibre washer instead of the special washer with the internal cut outs.
Another clue to this is obvious carby flooding but nothing coming out of the overflow vent pipe.
Bill Mac
MK1
MK2
S3XJ6
X300 (X2-don't ask!)
15 previous Jags MK5 to X308

 

That's interesting. I'm not near the car now, but I do remember the washer on fast inspection filled with sealer. Can you show me a picture of what the special washer should look like, so I can find that in the kit. And yes, when the overflow happens at the center "carb", no overflow on front float chamber

 

It has the spaces for it to breath.

 

JeffR!
Thanks. That is the washer i was trying to describe.!!!
No Quarter
If you can't find the actual washer then attack a plain washer internal radius with a small triangular file.cutting a couple of grooves towards the outside perimeter.
Just make sure that there is some opening to allow airflow between the carby float bowl and the vent/overflow pipe outlet.
Bill Mac

 

https://www.ebay.co.uk/sch/i.html?_f...sher&_osacat=0

 

One more thing to check when making sure the washer is in place is that the vent pipe is open. We have seen them crushed and even small "critters" that have made a nest and blocked the tube. The end result is the same as the improper washer.

 

I endorse Coventry's remarks on checking the vent pipe for blockages especially for "critters"
In my part of the world we have continuous battles with mud wasps of all shapes and sizes especially in the summer wet season.
Some of their favourite nesting sites are in the water drains hoses of the fuel tanks fillers of XJ6 cars.
They also love the SU vent pipes. On my MK1 and MK2 I have "belled" the bottom end of the vent pipe which is easily done with the nose of long nose pliers.
I have then wrapped the end in fly wire gauze which is held in place with a tie-wrap or tie wire. The "belled" end stops it from falling off
That keeps the little b-----s out!
Bill Mac
MK1
MK2
S3XJ6
X300 (X2 don't ask)
15 previous Jags Mk5 to X308

 

Exactly. On my front SU was the gasket below, making sure that the float couldn't get up and close the fuel coming in. It now runs beautifully:-)

 

For some reason even “specialists” seem to miss the fact that these SU AUC 1928 special fibre washers are intended to be fitted in pairs. That is a stack of two per carburetter.

SU made this change from a single AUC 1928 to two AUC 1928 around 1962/63. I have known owners fit three carbs on a former two carb machine because of a lack of power at high RPM. All that just because they fitted single washers and proceeded to overtighten the chamber centre bolts on the original two carbs. This sort of silliness is still practiced today.

Only six weeks prior to 911 I flew to Chicago to consult on a 4.9 ltr engine (not Jaguar) which had a RPM restriction………..all that it needed was the extra washer in the stack.

 

Norman would love to see some documentation of this "change" from 1962/63. Evidently Jaguar did not get the memo as there is no bulletin we can find in this area. Evidently "specialist" as you say, did not find the memo either as they continue to supply only one in the rebuild kits. Let's help everyone (even Burlin) with some documents. Thanks

 

That makes no sense to fit two special fibre washers per carburetor, only one per carb is necessary.
Number 29 is an aluminum washer that seals the top of the long nut 28 to the banjo float vent "30". (it does not breath there, and even if one did fit a special washer there, it still can't breath in that area because there is a lip at the top of the banjo breather 31 to prevent breathing)
Number 31 is the special vented washer that seals to the banjo float vent and the float lid.
The photo is incorrect in the fact that number 31 is not the special washer, but there is still only one in needed per carburetor.
If one installs a special washer where 29 is, it won't do anything.

I don't believe there is a memo found because there is none to be found, no change was necessary.
One can fit two special washers if one wants, it won't hurt anything, but the second one where the aluminum washer is won't do anything.

 

I have approximately 58 years of Jag ownership and have had just about every workshop manual for Jag carby cars from MK5 to S2XJ6. (I still have quite a few.)
I have never seen or noted any requirement for two AUC 1928 washers in any manual.
I have to agree with JeffR1 that it simply does not make sense to fit two washers.
Bill Mac
MK1
MK2
S3XJ6
X300
15 previous Jags MK5 to X308

 

Nothing about two AUC 1928 washers in this.

 

I re-read NWG's post and he said to stack the special washers.
I guess one would have to glue them together so the spaces would stay in line, otherwise if they moved, it would end up defeating the purpose.
With that said, one special washer should suffice to act as a breather, I can't see the car taking up so much fuel that two would be required.

 

JeffR1

You are exactly correct, one special washer should suffice, but if you read my post, you will see the trouble arises when the float chamber bolt is overtightened. In that instance the fibre washer is crushed and the chamber can come under the influence of a depression. At that point the serrated washer will become no more than a normal fibre washer sealing off the entire chamber. Stacking another washer provides extra opportunity to retain breathing capability.

Following on from single and double serrated fibre washers, S.U introduced an alternative steel cap. These caps sported a tunnel shaped impression to allow breathing ability. At S.U Service School, which I attended a few times, we were instructed to either fit 2 x AUC 1928 or fit a steel cap. If the carburetter already had a steel cap then the instruction was to reshape the tunnel impression if necessary to allow good breathing ability. These instructions were always carried out at any workshop at which I was working, but mostly we would convert to steel caps.

Today the cars are probably maintained to a better standard than in the past when the cars really had to work and over tightening float chamber bolts and crushing washers is not so common. That is possibly why these modifications are not so well known.

Even companies like Rolls- Royce or Bentley did not issue a bulletin on this AUC 1928 subject even though their post war 4.25 ltr and 4.5 ltr engines fitted with H and H Thermo carburetters had two AUC 1928 Fibres on each carburetter. I am therefore not surprised if Jaguar did not issue a bulletin. No doubt they also did not issue bulletins, for instance, on alterations to depths of S.U counter bores of the piston guide rods, which are just as important.

I assume from your comment that you were not aware that your own Bentley had a total of four AUC 1928, some surprise! They are known in this format by Burlen under at least carburetters AUC 551 and AUC 594 …….and Oh yes, I have no Jaguar documents but I have copy documentation of the R-R and Bentley units. Can you really see R-R fitting two washers when a single washer would suffice, what made them think they needed two?

Whether Burlen have Jaguar documentation I have no idea but given the gap and changes between the old S.U company going into demise and Burlen filling the gap I would be surprised if many documents have not been lost. Evidently they found the memo in respect of Bentley, perhaps they mislaid the one for Jaguar.

Glyn

The official Workshop manuals for MKII and S Types do not mention even one washer, never mind an external publication, good as that is.

That publication however does show the alterative cover cap on page 41 Fig 36. It also describes removing the float chamber lid, banjo and washer / cover cap page 42 The official manual again mentions nothing about even removing the drain pipe.

The short answer here is that it makes very good sense to fit two washers for the cost and trouble involved. If a flooding problem occurs on the road and the driver /mechanic needs to remove the float chamber lid more than once he / she may quickly realise the benefit of that extra fibre washer.

I have relayed all the data I know and each owner makes his own choice, as I have nothing to add as far as I am concerned the subject has closed.

Thank you all for your interest.

 

Always happy to learn!. It's not that long ago that constant depression carbs were a mystery to me. I'd never had a car with them & was a multiple Side draught Webber DCOE & Dellorto DHLA man pre fuel injection. SU's are dead simple once you get your head around them. Clever devices. I made the book above my Bible then rebuilt my HD6 thermos on my S type. Runs beautifully.