1985 XJ6 3.4 Series 3 Starting / choke problem

This car is standard apart from a 123 distributor and a manual choke conversion.
 
Symptom: from cold, the car will not idle evenly on choke, revs rising quickly to 2000 rpm then dropping quickly to 450 rpm repeatedly.  It will do this for 2 minutes or so until the choke is put back in where upon it may just about tick over if you use a little throttle.  If you put the choke back out, the revs drop off risking a stall.  During this warming up period, it is undrivable.  Many hundreds of pounds and hours have now been spent on this problem at a well known Jaguar Specialist garage.  A "new" AED did not start the car.  ALL electrics have been replaced.  New carb gaskets, pipework, Carb overhaul: main jets, plenum springs etc etc.
 
Once warm, the car runs fine with good emissions, restarting easily.
 
So, if anybody has experienced such a problem like this and knows the cure, I'd be very pleased to hear from them.  Ask your friends!  It must be a simple fault that has been overlooked.  Remember,from cold, with choke, rising and falling revs down to almost stalling / excessive reaction to slight throttle when on choke is what you need to think about!
 
Many thanks in anticipation - I am getting desperate now after a year of this problem!
 
Clive

I will throw some hints at you but I am not a Technician, instead I am a TAENICIAN, a Trial And Error Nician. 

I have 17 years experience with my '84 and you could say I've "been there, and done most of that".  I am familiar with the 4.2 fuel injected engine, and with the 3.8 carburetted engine.

In the XJ-6,  the "hunting" (up and down) of rpm's when cold, is usually the classic symptom of a sticking Auxiliary Air Valve (AAV), which of course, un-sticks itself after the engine is warm, just like your pain dissappears as soon as you get to the doctor's office.  
 
Another related component, but not necesarily the culprit in your case, it's the Cold Start Injector dripping fuel when it should shut itself off.  But I assume your car does not have the CSI, since you say you have a "manual choke".  

I suspect it's the AAV. The attached picture shows the AAV to the left of the red arrow head. Removal is not easy, and even then, cleaning it is another story in itself, I hope you have a Factory Shop Manual and a new gasket for reassembly.  (not a Haynes manual, those are useless for the XJ-6).

Otherwise I'm sure someone with a different experience than I,  can contribute another opinion.

Jose 


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Thanks for the advice, unfortunately, your comments relate to a fuel injected car - my car has 2 HIF7 SU Carburetters and therefore does not have an AAV.  Good try though!
 
Many thanks

Greetings Clive,
 
That  sounds like a vacuum leak to me, perhaps intake manifold gasket? You won't likely get too much help from U.S. posters, I don't think we've seen S.U. carbs since 1974 MGB...and those were manual choke.