i've got a 740 200f,it's developed a slight miss fire and seems to chug ,this is also accompanied with exhaust smoke which is'nt too bad but more than normal,i'm going to check and clean the plugs,crank breathers and all the vacuum hoses.what is the black unit below the inlet manifold which has a vacuum pipe going to it?and is there any things worth checking.the engine has always seemed in excelent condition using no oil at all and this problem seems to have come from nowhere,help