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Auxiliary Enriching Device

Auxiliary Enriching Device

Larry Lee

Dirk Luesebrink wrote:

The problem:

The AED is for enrichment of the mixture while warming up. Engine starts, AED is on, runs fine for a while. Getting warm, AED shuts down, but then, after let's say 15 minutes, AED suddenly kicks in again and of course immediatly shuts down the engine (flooded condition).

I am assuming that you actually have an SU AED (Auxiliary Enrichment Device), and not the earlier TSC (Thermostatic Starting Carburettor). Unlike the TSC, which used an electric solenoid controlled by a thermostatic switch on the engine and took its fuel from one of the main carb bowls, the AED is a completely self-contained device. The AED draws its inlet air through a pipe that runs from a fitting on the exhaust manifold. On a Jaguar 4.2l engine, you can feel the inlet filter on the back (toward the block) side of the rear portion of the manifold. The pipe normally runs behind the engine and under the carbs, terminating in a short, vertical hose connection to the AED. A fuel hose feeds the AED's float bowl through a needle valve (under the large hex plug). The enriched mixture passes through the tee-shaped supply tube to the intake manifold. On Jaguars, AEDs were used with dual SU HS-8 and HIF-7 carb setups (possibly other setups as well). I have a dual HIF-7/AED setup on my 1979 XJ4.2, SII. Engine vacuum draws air through the unit. Most of the air to the AED is enriched and passed to the engine when the unit is cold, but a little is drawn into the upper, plastic chamber of the unit, where it flows around two bi-metal springs. One controls the mixture, and the other controls the overall air flow through the AED. As air to the AED heats up, the bi-metals close the mixture needle and the main valve that sends enriched air to the engine, and the AED effectively "shuts down." But the main valve has two small holes in it that allow a small amount of air to continue to flow through the top chamber to keep the bi-metals hot as long as the engine continues to run.

Your problem - the AED shutting down, and then starting back up -- is a bit unusual. The more common problem is for an AED to not shut down at all. I would suspect one of three problems for starters:

  1. Check to be sure your hot air delivery pipe, including the filter, is open and allowing free air flow. The filter can be cleaned, but it is a b#&*h to get off with the exhaust manifold on the engine. Disconnect it from the AED, insert a piece of hose, and see if you can blow through it without much resistance. It is very important that this pipe and the connection at the AED not leak. If it does, cold air will be sucked in, and it is possible for leakage air to dominate the flow when only the small amount of temperature-maintenance air is flowing through. This could cool the bi-metals and cause them to partially open back up, over-enriching the warmed-up engine. The pipe should be covered -- at least the portion under the carbs -- with foam insullation to retain heat, too.
  2. Check the two rubber elbows that connect the supply tube to the inlet manifold and the hose connection to the AED itself. Leaks here could cause incorrect flow through the AED.
  3. If the hot air delivery pipe and supply tube check out okay, look at the main valve disk to be sure the two small holes are open. If they are plugged, no hot air can flow through the unit after it shuts down. You get to it by removing the two plastic valve body pieces (four screws), but you must be very careful not to damage the thin, fragile gaskets, or have a repair kit on hand with new ones. (Kits can be purchased from Burlen Fuel Systems in England or Joe Curto in New York.) Also be careful to note how the metal ring fits around the main valve diaphragm. It can be tricky to refit with the AED right side up, but you can carefully rest it on the metal body and lower the valve body down over it. Looking at the aluminum main valve disk, you can see the two holes. Be sure they are clear.

These appear to be the most likely causes for your symptoms. Check them out and let me know what happens. Most of the manuals-even those available from Burlen-say very little about working on AEDs. They are a bit delicate to adjust. But some additional information is available here if you want to dig deeper into your AED and feel comfortable doing so. But don't start tinkering with any AED adjustments until you are certain that the above items check out.

AEDs have become somewhat rare, but a few of us on the list have them. Let us know what you find.


I saw in a later post that your mechanic said the supply tube (mixture delivery pipe) was freezing cold. This is exactly what happens when an AED does not shut down as it should, particularly where the main valve closes, or partially closes, but the metering needle does not. The AED can then flood with liquid gasoline. This gasoline evaporates into the air flowing through the AED and into the supply tube. Since evaporation is a change of state that requires heat absorbtion, the mixture-and the supply tube-get very cold. (This is exactly what happens in an air conditioner's evaporator.)


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