I have a vc with a 253 that is not running right. It was sluggish on take off with a rochester on it, so I replaced that with a 465 holley(ran better but still sluggish) fitted a dizzy which I know is working fine & added some timing to it 10 degree advance(ran good for 2 days). now the car won't idle,back fires through carby when accelerating,blows lots of black smoke & cannot time as as soon as you move the dizzy it stalls & won't start. First time I have had this problem with any commodore I have owned. Can anyone help as the only thing that I can think of is the timing chain has streched or slipped