Suzuki Ltz250 QuadSport

New Spark plug
New Oil and filter
New gasoline
Cleaned/rebuilt carburetor

The Carb has been completely rebuilt and cleaned several times since initial restart after a few years of sitting. I have gone through all of the troubleshoot issues in regards to the carb.
I am wondering if it's something more severe within the engine itself.

It turns on, idles inconsistently, backfires/sputters and dies when given throttle.

Any advice would be greatly appreciated, thank you.