Right mate this happened to me on saturday when i did a service, i changed the plugs and leads and oil, turned it on and it run like crap, misfiring, and smoking like mad. Ive solved my problem. What colour is the smoke firstly? Mine was white, normally means water in the engine BUT mine turned out to be the ht leads, they where in the correct order but 2 where not pushed into the coilpack properly! pushed these in and it was fine. You have got to think that surely you cant have done anything that bad if you have only removed the plugs and leads. It took us over 2 hours to solve the problem.