Hi. I have a B&S 18.5 engine (31N707 0301 B1) that will start but backfires real bad. So I took carb off and cleaned it real good then put back on, no change. Adjusted the valves and no change. Cleaned coil contacts, no change. Took Flywheel off and checked the Key. No damage to key. I then took off the carb and put a new carb on, still no change. Backfires and blows mist out of air filter intake. This engine is on a Poulan riding mower. Would appreciate any help. Thanks