1994 Jaguar Vanden Plas Smokes black, and spark bulbs smell

Hi, I am having problems with my Jaguar 1994 V6, some months ago I changed the air filter and the car started smoking very black (it was having the problem before, but got bigger after I changed filter) so, some day did not started and I did change all the spark bulbs, the car turned on, (but smoking yet), so I let the car for one more week at the garage, I went again to turn it on and it did not start, so I sew the sparks and the almost new ones were black again, which could be the problem and the possible solution? sensors, tune up?
thank you!