My 68 LTD has been running great for 10 years / 30k miles on rebuilt FE engine, stock, nothing fancy. Very consistent Barry Grant 650 carb, Pertronix ignition.

On same full tank of gas, ran great for the first 5/8, and within 20 miles slowly developed extreme pre ignition, low power. I almost had it towed home.

First was to check timing, right were it has been, 8 deg static, 34 full.
All spark plugs look good, I was hoping to see some crud lodged in 2 or 3 plugs, but none.

I am in process of drawing old gas and put in fresh. I always use name brand gas, no 7/11 or shettz crap.

Thoughts??