There are a few things that can cause a backfire. For example, a vacuum leak, too much fuel, and so on. Also, if you mixed up the injector wires, it can cause the fuel to be injected into the wrong cylinder at the wrong time.
Make sure there are no vacuum leaks first. If you had things apart, it is easy to miss a hose or to have damaged one.
Next, here is a link that explains how to fix a back fire issue. Take a look through it and let me know if it helps.
Let me know what you find or if you have other questions
Sunday, August 2nd, 2020 AT 7:45 PM