455 problems

[mad_mike06]

I have a 72 delta 88 with the 0riginal 455 and sometimes when i go to cut the car off, the engine makes a clunk noise and wont cut off. I can turn the key back to the on position and the car comes on like it wasnt off. does anybody know why and what i can do to fix the problem. 1 more thing... when i start my car warm or cold, grey smoke comes out the tail pipe for like 5 seconds. any suggestions...