My 1.6 brava is playing up, i think the engine is sending too much fuel because one of the cylinders was down and the spark plugs were all sooted up. When i removed the air filter the engine runs a bit better, probably because the air to fuel ratio has been evened out a bit. But if i replace the air filter the engine starts to struggle. Is there a way to adjust the air/fuel ratio or is it more likely a sensor thats down?

Sorry to state the obvious, but if it runs ok without the air filter but not ok with it shouldn't that mean a new air filter is needed?

Sorry didnt explain properly. The engine ran slighty better with the filter off, but there is nothing wrong with the filter its just restricting the engine more with it on. I replaced the air box with an induction kit to allow more air so i could use the car without it destroying the plugs again. But i just wanted to know if theres a manual way to adjust air/fuel ratio, or if its controlled by a sensor or something.