I start up my v12 now and then.
When starting the engine from cold, it runs on less then 12 cyl. and very rough. I would guess 2 or 3 cyl. are out, or maybe even more. And of course, a lot of grey smoke.
It gradually improves warming up, and at normal temperature engine is running fine most of the time, and the exhaust is about invisible.
I have done a temporary fix on the “butterfly” axle bushings (to address worn down bushings), and also adjusted the “butterfly” gap to proper setting (gave 800rpm idle). Also adjusted the idle at the auxiliary air valve (AAV) a couple of turns both way, but did not notice any difference.

The engine seems to run ok when it’s warmed up to normal running temp, but maybe still a notch too rich. If I increase throttle very fast, the engine kind of stumbles a bit before picking up rpm, but when increasing throttle slowly, it pick up rpm as it should. Maybe this is addressed with a mixture adjusting at the ECU.

I guess the coolant temperature sensor is working since the mixture seems to be close to right when warm.
But what about the Air Temperature Sensor. I have not noticed any difference when disconnecting the ATS when cold? Should there bee?

Or is there any way to adjust the cold start mixture?

This is a North American H.E. There is no cold start injectors, nor is there a solenoid air switch (supplemental air valve).

 

Possibly repeated cycles of starting followed by prolonged idle is part of the problem. The spark plugs can easily foul or partially foul under such usage.

 

well its not gonna hurt to go ahead and replace the CTS.
while your at it, swap in some new spark plugs (doesnt have to be special. autolite 103's work fine)
if that doesnt work, you could try tuning in a little extra air via the ECU.

more major things could be a bad spot in your TPS, or uneven injector flow.

 

It should respond to the AAV when warm so you are getting too much air one or more places. Try squirting start gas here and there to see if you get a momentary increase in rpms. Re the stumble start by checking the function of your TPS. And as mentioned the plugs could be fouled. I have found the 12 to be sensitive to plugs, standard GKN plugs have worked best for me, although the flat head also runs well on GKN iridium. All other attempts fouled quickly or simply wouldt run well even when fresh.

 

i think a cold plug is important with such a high compresison ratio. but thats just my opinion

 

Mikey:
I know there is a possible spark plug foul, when repeated cycles of starting is the case. But i always let the engine reach normal running temperature while also changing rpm (not a constant idle) before stopping.

M90power:
There are new plugs (NGK BPR6EFS) which is, i think, the right plug according to spec.

Per:
It stumbles when warm, only if i increase throttle very fast while it's running. Not if i increase throttle more slowly. Well, and of course a lot of stumbling the first minute when cold.

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Well then, as i understand, there is no cold mixture adjusting i can do.
I can live with that for now. The only place this cat is going right now is out from, and back inn to the garage once in a while, making room for working on a Triumph Spitfire.

Will strip down the XJ-S next summer (when the Spitfire is ready) for a complete rebuild (needs some welding). There is a lot of good work to do on the engine as well, like redoing a lot of wiring (have already replaced injector wire harness).

One question though?
Reading between 11,5 and 13 bar on compression test i think the internal health on the engine is ok. So my initial plan is to change all gaskets, hoses, electrical wires and any faulty sensors and adjusting valve gap before putting the engine back in the car. Or, should i do a more serious overhaul like cylinders, pistons, piston rings, bearings....? The engine has clocked 150k miles. Any good thoughts about this?

 

i think its your TPS at this point. the injecto pulse is based on RPM, so it will pick up RPM slowly due to increased ignition signal speed which in turn increases injector pulse which in turn increases rpm and so on. a quick jab will add fuel more quickly via the TPS, and if its not working properly it will stumble. you need to get a vacuum gauge on it and tell us what its pulling.

 

This does not replicate actual operation 'on the road'. Although the coolant is at normal temps, the combustion chamber and especially the plug tips are well below typical temps. Why not clean/change the plugs as an initial step?