Originally Posted by Chas

K/KE-Jetronic is sensitive to vacuum leaks which can be caused by something as simple as the dipstick not seating correctly or the oil filler cap not being on tightly ; pattern caps can be more trouble than they are worth here. On an old XR3i I managed to raise the idle by simply changing the oil filler cap due to the 'new' cap being prematurely worn. Of course, this only applies if the car is running a stock breather system and not an oil separator type ; the crankcase system them no longer influences the running of the car, well, not as much as before.

IMHO without knowing more about the car I would say the issue is the following from what has been said:

1) Poor brake light earth causing the car to run bad (I have had that before on a Ka ; it took an age to diagnose that! The earth actually began to heat up!)
2) Vacuum leak at the brake servo or pipework. Clamping the brake servo line should rule this out.
3) Injection issue - either the mixture is not right or there is an issue with the sensors; be it the TPS (although I doubt it could affect it much, but I have seen this fault affect a car on K-Jet (the throttle switch was not turning on, tweaking the switch made all of the difference) or the electropressure regulator is faulty (this takes place over the overrun shut off valve that is present in K-Jetronic cars). Revving the car and letting it idle again should rule this out but of course it probably not that easy.

I am sure someone else with a better idea could give more conclusive pointers .