I have an 89, has a 347 stroker w HCI. When I start it up it backfires, real little but noticible and then starts knocking right away! new plugs, but the wires are a little old, the timing is set at 12 degrees, it ran ok before, but it started to not have much power in between 2 and 3 rpms it felt like it would kinda bog. then i let it sit for a few days and when I started it back up, and it was knocking ugly!! any advice would be greatly appreciated!!