Have rebuilt-to-stock-spec 396 L78 and recently experiencing bogging or hesitation (what seems like engine misfire) when accelerating in the 3k to 4k range. Starts, runs, idles well cold. No dieseling or pre ignition noted.

Hesitation begins when engine reaches 190 degrees. Driving a short time afterward, temp rises to 220 degrees. Experienced none of this for 1,000 miles since engine rebuild.

Running stock distributor with mechanical advance, Petronix Ignitor, Flame Thrower coil, Champion N-12Y plugs.

Timing checked at 13 initial and 35 total.
Spark plugs have heavy tan deposits with exception of #4 which was black on half of electrode and insulator and tan on the other half.

Could it be the Petronix Ignitor or coil? Thought of getting another Ignitor but don’t want to spend the $80 if that is not it.

Thought of vacuum leak through carb but no visible signs of this.

Could it be head gasket at # 4? No milky oil or white exhaust smoke. No oil in antifreeze.
See pics of spark plugs.
Lou

Looks like #4 is burning a little oil to me. Have you tried new plugs to see if it fixes the hesitation? Carburetor tuned properly?

Put new plugs in and interestingly, the idle changed from 950 to 750 rpm and the tach needle has stabilized. Got late and have not street tested it for the hesitation or missing. hope it is that simple. As for the touch of oil on #4...we'll see.

The new plugs did the trick. Champion N12Y.

if it happens again, check out that petronix coil. had the same issue with mine and a new coil cured it.