Just to be clear on the issue. The engine will not start and it backfires through the intake?
It appears you do not have a carb engine, it is actually a throttle body with an injector in it.
So we need to find out if we are getting proper spark and fuel. I suspect we have an issue where the injector is leaking. If the engine is backfiring then it is safe to assume that it has spark at this point. However, if there is too much fuel then it will not run.
Here are some guides that will help with this:
https://youtu.be/dCjmRL3p4Cs
https://www.2carpros.com/articles/engine-backfires-while-running
When you replaced the spark plugs, what did they look like? I attached a chart below that will show what fouled plugs look like. Were they black and burned looking?
Let's start with this info and we can go from there. Thanks
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Sunday, May 30th, 2021 AT 5:16 PM