Hi,
First, are you certain the spark plugs wires on going to the correct cylinders? I attached two pics below. Check to make sure you have the correct firing order and cylinders selected.
Next, have you checked for spark to the plugs? There could be an issue with the ignition control module. Here is a link showing how to check:
https://www.2carpros.com/articles/how-to-test-an-ignition-system
The idea that it did backfire indicates either a build-up of fuel, intermittent spark to the plugs, or timing issue.
Let me know about the things I mentioned.
Joe
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Wednesday, March 16th, 2022 AT 4:26 PM