I agree that it is probably pinging - detonation - knock. All terms for the same thing.
The key is the Howell EFI kit. I've been tweaking my timing after the Howell install. The engine originally had a knock sensor which would retard the timing as needed to avoid knock.
Now you don't have a knock sensor, and the GM throttle body ported vacuum source connected to the distributor vacuum advance has a different pressure profile than the stock Carter carb. Plus you have eliminated the PCM and the ICM no longer has the ability to adjust timing for you. You have to go old school on it.
I'm still working on exactly where I want mine. But to stop the pinging it is an easy adjustment. Get a 1/8" allen wrench. Pull the vacuum hose off the distributor. Stick the allen wrench in the vacuum nipple and find the set screw. Turn it counterclockwise about 2 turns. Then test drive. Go another turn, test, repeat as necessary until the pinging under load just stops. Count the total number of turns and make a note for yourself so you can reverse it if you don't like the result.
Then research timing profiles over in the CJ forum for more in depth instructions on perfecting the timing mechanical and vacuum advance. The above should be used just to prove that it is indeed pinging, and not something else. The CJ forum has better info on the 4.2 than the YJ forum.
Dodge did not build my Jeep.
1988 Wrangler,4.2L I-6, Howell EFI JP-1,GM HEI Ign, 5 Speed Peugeot, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.