You most likely have a valve train issue like a bent valve or worn camshaft.
Your compression readings are good because the valve is able to seal but a worn cam lobe will not open the valve enough to allow proper air flow which causes the air fuel mixture to back up in the intake and can ignite causing the backfire.
Here is a guide that talks more about the causes of backfires:
https://www.2carpros.com/articles/engine-backfires-while-running
I would suggest removing the valve covers and inspecting the rocker arms to see if they are all moving evenly. This is not a definitive test but will give evidence of needing to deeper and inspecting the cam lobes.
If this is the case then then we need to measure the lift of the lobe. I attached the spec that you need to get out of the lobe so if you are not close to a quarter of an inch then the lobe is worn and the cam needs to be replaced.
Please reach out with questions. Thanks
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Saturday, April 25th, 2020 AT 3:54 PM