Dirty ignition switch. Squirt loads of contact cleaner down the key hole, stick key in and turn on/off a few times. Had the same with mine before after it sat out in the rain all day. Span over fine but wouldn't try to fire.
Bang from exhaust is because even though it's not getting spark, the air flowing through is still pulling in fuel and it ignites in the exhaust when it finally fires up.
Bang from exhaust is because even though it's not getting spark, the air flowing through is still pulling in fuel and it ignites in the exhaust when it finally fires up.