I have a BS Intek 344cc 12.5hp (219907 0131 B1) that was allowing gas past the carb to the sump. I attempted to get the needle to seat with no luck so I purchased an aftermarket carb. After installation, engine surged and backfired, which it didn't do with original carb. I removed float assembly from new carb and installed on original but engine still backfires. Can anyone offer any advise? Thanks