Timing Jumping Drastically
I have a 66 Coupe with a 1980 302 and 4 speed. Anyways, the car runs fairly well except that I have been chasing a problem with the timing for weeks now. Long story short, it turns out the PO had installed the MSD Ready to Run distributor without installing the advance limit bushing, thus the mechanical advance was adding about 36 degrees all in around 3000RPM. Not good. I have since rectified that situation with the 21 degree advance limiter bushing. So, I set my timing at 14 degrees initial, with one blue spring and a light silver one, hoping and expecting to have about 35 degrees total at 3300ish RPM. This is not the case. In reality, the timing rises with RPM as it should, until it nears 30 degrees btdc, which is when it very quickly, i.e faster than you can follow, jumps itself upwards of 40 degrees where it stays until i get disgusted and let it go back to idle. I took it out for a drive like this, and it runs like a scalded dog, better than it ever has. And so I come to you wonderful folks, has anyone ever seen anything like this before? The engine is an unknown, seems pretty stock though it does have a cam with the 351 firing order in it. I just tonight replaced the spark plug wires. The only thing that makes any sense to me is that there's something wonky with the timing chain? Though I wouldn't imagine stretch to be so abrupt and precisely at the same RPM each time, and I don't think it's slipping a tooth because it always returns right to where it belongs at idle.
Any help would be appreciated greatly,