AED/ASC Device Differences

I’ve been messing with these things since 2000 and always learning something but I’m stumped on this one. As I understand AED is Automatic Enrichment Device and ASC is Auxiliary Starting Carburetor both solenoid operated with thermostatic Otter switches. What is the difference? I believe my 59 3.4 liter has an ASC which feeds a rich fuel/air mixture directly into the intake manifold through a large metal tube. The 1960 3.8 I’m working on has a different set-up with four steel tubes routed along and into the manifold. I’ve been looking through all my books and can not find anything.

I always thought the two terms were synonymous.

Burlen Fuel systems give some useful info on their site Servicing - SU Carburetters


The nomenclature is confusing, but there are two completely different devices, both referred to by SU as “auxillary starting devices.”

One of them is the AED shown in the above Burlen URL. It’s very complicated, and comprises a thermostatic sensor, a complete carburetor including a float chamber, and a bimetallic control for its jet. With a cold engine, it delivers a very rich mixture whilst starting, reduces the mixture when it senses manifold vacuum, then reduces it continuously as the engine warms up.

The second, to which you refer I believe, is the ASC, also called the auxilary enrichment device by SU and known by many as the “hisser.” It’s a solenoid valve operated by an Otter switch. When actuated, it sucks air through an unfiltered opening, mixes it with fuel taken from the float chamber of a nearby carb (not it’s own float chamber) and passes the mixture into the manifold. It is unmodulated by vacuum or by temperature and remains on until the Otter shuts it off. There are at least two versions that I know of. In the earlier, mixture exits horizontally, at a right angle to the casting; In the later, mixture exits downward or axially with respect to the casting. The installation also differs among applications–various pipe configurations into the manifold, some with anti-belchback valves and some without. IMHO.


Here is the Burlen link to the ASC/AEC or Hisser/Thermo Robert Wilkinson is describing Auxiliary Enrichment Carburetter (Thermo) - SU Carburetters

Terminology often used interchangeably - therefore understandably confusing.


Thanks. Very helpful. Like I said…always learning something. I judged a lot of cars but have never seen one with the Enrichment Carburettor. Any idea which years/models were fitted with them? The hissers on my engines are certainly the simpler of the two.
Pat H.

Aha…synonymous in purpose, utterly different in execution of said purpose.

AFAIK, the AED came into service with XJ6 engines, previous used either the hisser, or, in some cases ( E Type) manual, cable operated chokes that dropped the main jet to enrich the mixture. ASC is electrically operated and can easily have a bypass switch included in the circuitry, I don’t think the AED is easily modified, but the carbs equipped with AED can have a manual choke conversion fitted, which, again, drops the mainjet. The ASC in early cars had a single feed into the underside of the inlet manifold, later cars ( about Mk2 onward) used a branch with 4 or 6 ( can’t remember exactly) ports into the manifold as it spread the mixture more evenly. Both AED and ASC work perfectly well when correctly adjusted, according to contemporary roadtests.

And now “we have the rest of the story” to quote Paul Harvey. That explains why the hisser on my ‘59 XK150 (Sep 58 build date) is configured differently than the hisser on my ‘60 MK IX.
Pat H

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A little more to the story. The horizontal was used on the pushrod engines. The vertical on the XK engines.

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Interesting, Rob. It appears that the hisser casting is integral with (or attached to) the float chamber of the SU carb to its right? IOW, not via a banjo to the bottom of the carb. If that’s not the case, ignore the following.

I have (but cannot seem to fine, or else I’d attach a photo) a hisser casting with the horizontal exit that is otherwise like the XK version IIRC. I bought this casting (NOS, without solenoid coil or any other parts) at a cheap price from some mail order source, maybe eBay. My intention was to spiff up my existing hisser, unaware of the difference. I wonder if the SS design was carried to the XK, or perhaps there was another pushrod engine with an unattached hisser?

Yes, and attached to the carb body with a banjo bolt on the opposite side of the bowl from the solenoid, all-in-one. You will likely find the number 3258 cast into the side of the float bowl. In which case you can sell it to any pushrod owner 1938-51. In fact me if you want to, contact me offline.

The solenoid/bowl/banjo all-in-one but with vertical outlet was used on early XK engines up to about 1952, and will have the number 3256 cast into the bowl.

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I wish I could find my bit. It had the horizontal outlet, but was otherwise interchangeable with a “modern” hisser–like on the 420G for example. No float bowl attached. Or so it is remembered by this 75-year-old. :slight_smile:

If somehow I have a combo hisser-float bowl it will yours at no charge, Rob.

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Found it! Its part number is AUC2761, different from that mentioned by @Rob_Reilly. Below is a photo.

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It’s supposed to fit several XK engine models.

Too new for me. It appears to be correct for Mark 1 & 9.
The Mark IX SPC gives an assembly number.
AUC2761 is perhaps a casting number?

Nothing strange nor unusual about about your Starting Carburetter (correct spelling by both SU and Jaguar). AUC2761 is a BMC Part No, so tells you it is post 1952 when Nuffield Corporation bought out SU Carburetter Co, but reorganized things under BMC part numbering protocol (another company owned by Nuffield) . But is simply the common Starting Carburetter as fitted to all SU carburetter assemblies with this Thermo Choke option, with Jaguar indeed one of the main customers as fitted on all twin H6-Thermo carburetter assemblies as fitted to XK120/140, Mark VII/VIIM/Mark VIII with H6 carburetters…, noting XK120s built pre 1952 had same unit, but did not have the BMC Part Numbers with AUC prefix molded in…

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“Carburetor” is also a correct spelling…:smirk:

Thanks Roger. I never thought otherwise, but there was some discussion in this thread suggesting that horizontal (not downward) mixture feed to the manifold was unique to starting carbs on pushrod engines.

Maybe in America only - but then we all know Americans don’t speak/spell ‘proper’ English anyway… :crazy_face:
Jaguars and SU Carburetters are/were actually MADE IN ENGLAND