like people have already said Redex, but do the down the spark plug hole treatment, it tells you on the back of the bottle how much to put in, but you basically pour some down each plug hole and let it stand for a bit (i think 10mins but its tells you on the back of the bottle) them cover each plug hole and turn over the engine once, then put your spark plugs back in and start it up, you get loads of smoke for about 5-10 mins, but thats all the crap burning off. I do it to my taxi before each test, 202,0000 miles and its never failed yet! (touch wood)