New Engine, Jumpy Timing

[colnago]

I got my rebuilt 352 running this weekend. I'm still doing a lot of tweaking, but it's driveable. When setting the initial timing today, the timing would hiccup a couple of degrees every few seconds. This has a new camshaft, new timing chain/gears, and new distributor. What should I be looking into? Could it be the timing light, or "crosstalk" between the spark plug cables? Something else?

Joseph

[markguiver]

Does the timing jump a few degrees, level off, then jump a few more or jump back and forth? If it's doing the former, I would take a look at the harmonic balancer. If I recall, you said the truck was sitting for five years prior to the engine/trans rebuild. If the rubber has dried out or is failing the metal ring could be slipping a little on its way to total failure. That would cause the timing marks to move. Something to consider.
Mark

[colnago]

Hmmm, that's an interesting thought. It holds steady, then hops down, then back up to steady, then hops down, and continues like this. The harmonic balancer looked good, but everything is suspicious on a 50-year-old engine. A new balancer is only about $100 from Jegs; maybe I need to invest in a new one. I'll try to pay close attention to its behavior tomorrow.

Joseph