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Discussion Starter · #1 · (Edited)
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.


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