i have a 454 in my 72 and every week or two the timing keeps getting screwed up and my car backfires thorugh the carb under heavy acceleration, whenever i set the timing back it seems ot be fine. my question is why is my timing keep going off? i have gear drive, could one of the teeth be off on it or something? i have MSD ignition also

Funny that you mention that. I have been having the same problem with my stock 70 El Camino with the backfiring through the carb under heavy acceleration. Hopefully someone has the answer as my problem is driving me nuts.

If the timing is changing on it's own, I suspect the distributor hold down is bent. Do you have a chrome one? If so, I suggest taking $5 with you and going to your local Chevy dealer and getting one from them. That's what fixed my wandering timing issue.