UPDATE: I have replaced all vacuum hoses. I found cracks and lots of dry rot. Along with lots of other little repairs while I was in there.

Changed both O2 sensors because I tested it and the check engine light will stay off now while driving. Until you put it in idle the check engine light pops back on. I was almost certain it was the O2 sensors causing the backfire. It runs much better but unfortunately today after 10 minutes of driving it, it backfired once again.

I bought a compression tester and spark plug tester. I've never done a compression test on anything. What am I looking for on compression when I do this test?

Any other thoughts on the back fire issue?

This doosan runs much better(night and day) compared to a year ago. So, all these little things are definitely improving its performance but I still have my main issue. The backfire after it gets hot.