from what i can ascertain, the top of "section 8" is at about the height of the float in it's parent float bowl. and if so, would put the level of gasoline in "section 8" preciously close to the vent hole and make is susceptible to overflow on acceleration. which might make one want to lower the normal height of gasoline in the parent's float bowl, if that's possible. in fact, i think i remember reading something about lowering the level in one or the other floatbowls on these engines....

i also suspect that the overflow on the parent carb is at a slightly higher level than that of the vent in "section 8". which would cause any overflow to be discharged from it rather than the parent carb's overflow tube.

this brings up another point that confuses me. not that there aren't enough already! and that is: if "section 8" is pretty much filled with gasoline when the electro-magnet opens the conduit(s), after it was closed, to the intake manifold (something that would, i think, only happen maybe once or twice during a drive cycle), then the first blast will consist almost entirely of gasoline and not a mixture of air and gasoline. but, i assume, that the vacuum coming from the intake stroke of all six pistons would soon eat up all the gasoline in that relatively small reservoir that is "section 8" and begin supplying a mixture of air and gasoline to the intake manifold which in turn would make the adjustable needle actually perform as intended. as an air/gas mixture control. if there's only gasoline going through there, then it's serving as only a stopper, no matter where it is set. i would think.

anyway... food for thought.

 

Good food for thought. The heated manifold would vapourise the fuel with the B head set up. With the B head set up it would take a little longer for the fuel to change from droplets to vapour. It is a more crude set up but it works. On the XJ6 they have a pipe that draws hot air from the exhaust manifolds which is a good idea as they heat more quickly than the coolant. See Video above.

You can lower the float level in the parent carb.

I would not let this worry you. Carbs with throttle pumps blast raw gasoline into the airstream.

I'm used to multiple banks of these. You should see the action of the throttle pumps. Multiple twin side draught Weber DCOE 45's. Aston Martin's Tadek Marek 4 litre 6 uses 3 of them. Choke per cylinder. Aston engines were square or close to square. They did not suffer Jaguars long stroke & high piston speeds. They had Aluminium blocks & ran dry sleeves sealed top & bottom for efficiency.






Aston Martin Vantage engine. Single plug version.

 

I think this might show the fuel levels better. Line 21 is the fuel level in the float chamber and it shows the level just below the main jet in the carb body but as the tapered needle 2 in the AED at 14 is closed it blocks off any fuel being allowed to rise any higher in the AED to space 1 in this diagram (previous post space 8). When the solenoid is switched on it lifts the valve cap 11 and this allows the engine to suck both air from space 8 and fuel up past the tapered needle jet 2 at 14. You can see that the needle 2 is lifted by the vacuum created by the engine when the AED is switch on when the engine is running. Needle 2s height can be adjusted by nut 5. This allows you to control the amount of fuel being drawn up passed the needle 2 for summer or winter temps. .
If space 1 in this diagram (previous post space 8) is being filled with fuel when the engine is running then there is a possibility that the needle 2 is worn or not seating properly at 14 because of dirt and that might be where the smell of fuel is coming from.

 

Good pic Rob.


I suggest that Huey buy this book. It is not expensive but most enlightening.

 

Rob, Cass, That's an ace drawing and very helpful. Are you sure about vacuum pulling the needle up? That seems the wrong way to me, but I may well be mistaken; all too often I am. It looks more that vacuum would pull it down against the spring. That would also give the right characteristics for fuel delivery, more as the throttle opens and vacuum goes down?

One thing that surprises me about the design is that SU/Jaguar didn't take filtered air from the plenum.

 

Cass, space 8 is the solenoid windings, maybe you mean space 1 ?
Something I don't agree with in your last sentence; the needle 2 never really "seats" against 14, it's always in a given position depending on the adjustment of nut 5, but it never seats.
If one assumes that nut 5 is adjusted to where the needle is fully seated, then no fuel would flow at all.
This would cause no fuel getting to the engine even with the solenoid open.

What does happen on a very hot day is chamber 1 fills with fuel to the point where it overflows because the fuel is very hot and expands beyond the capacity of chamber 1.
This gets worse if the float valve that is controlling the fuel level to the AED is worn causing fuel to seep into chamber 1
This happens even when the solenoid is fully seated.

Another issue if the 11 brass valve plate is worn and not seating properly allowing a minute amount of vacuum enters into the AED causing rich running and fuel to enter chamber 1 beyond its capacity _ this was happening in my car.

 

Jeff ~ I think Cass is referring to my diagram space 8 that Huey was querying. His diagram is to illustrate fuel level.

 

That's confusing, that's totally different from the one on post 23 ???
What post is your diagram in ?
Why do we need two different diagrams, Cass refers to space 8 and in his same post shows the solenoid as 8 ?

EDIT, never mind, post 6...

 

Sorry it was late at night I have edited my post to cross reference the numbers which are confused. I used space 8 as someone mentioned it in a post referring to the first diagram. The second diagram I posted was to illustrate the fuel level and had different numbers. Sorry for the confusion.

 

I came across this site. It's quite interesting with a lot of old Jaguar stuff. Incidentally, the Peter isn't me.

Interests

It has a good page on the starting carb with a clear explanation. I think that it largely agrees with how I thought the device works:

The SU Starting Carburetter

.... and has a simpler diagram (yes, yet another!) of an old version, which is much easier to follow. And has yet another labelling system (fortunately letters):


​​​​​

 

Very Good Peter ~ excellent article ~ No need for input from my Bible. I'm going to try their/your method of setting the needle in future ~ I think I get away with my method from cold because I live in such a moderate climate. If one does not understand the hisser after all the above buy the book & read. It makes matters very clear even for meI had not had a car with SU's. Until the Jag they were a mystery to me. I was a twin side draught Weber man until fuel injection that I learnt very quickly in all it's forms. SU's/thermo are simple & very clever devices. After reading & tearing down my first set I had a whole new respect for them. (I don't know why I disliked them. Probably my brother-in-law's Moms modded high bumper Triumph Spitfire with large tyres on it & the effects of a hot cam. It always had the most awful idle nevermind what you did to the carbs & tore out it's diff with the extra power. She drove everywhere with the accelerator hard to the floor and boy she could drive. Like my sister a very capable driver)

 

My final suggestion is to take a set of carbs apart, hisser and all. Look for shaft to body wear & replace the diaphragms & jets. Then reassemble & tune, including jet centering etc. Then you will fully understand every nook & cranny of a Thermo SU. Trusty book above at your side.

 

I endorse Glyn's suggestion.

I have just rebuilt/resurrected six HD6 SU carbies out of an assortment of bits and pieces I had lying around after 40 years of MK1/Mk2.
​​​​​​​These carbies are already reserved for different friend's projects

Unfortunately, I could only assemble one starting carby (hisser) out of the lot although I did end up with one that used a manual lowering of the main jet for starting.

When dealing with a "hisser' be very, very careful as it is a slightly complex device and if you do damage it or lose bits parts are phenomenally expensive.

 

... and a carefully rebuilt old SU will come in much cheaper and most likely function better than a new one.