[Tim Ayers]

I also run a Petronix unit in a crab distributor. Without timing marks, this is what was taught to me is spark knock sounds like a dull thud coming from the back of the motor. From a rolling start in first, shift into second and floor it listening for the knock. If you hear it, the timing is off. There is a metal tab with marks and a hold down screw on the side of the distributor.

If you have a crab distributor, it is fixed to the timing cover. The tab on the side is what adjusts it. I forget which way advances or retards the spark. Someone will hopefully chime in with which way is which.

Make a series of adjustments doing the same thing with the engine. When the knock goes away, you are within time. Finite adjustments can be made from there.

That said, like others mentioned, you also need to know what jets and power valve sizes you are running. If known, report back with what you've got.