A back fire is just a tune issue. In my case I had a YS too lean, it fired up OK but on take off there was a slight pop and parts started falling off right after lift off, about 10 feet in the air. Spinner, two nuts, and washer. The prop was still on the shaft. The engine was an AC series and you tune by adjusting the high end then the regulator screw, no low end needle on this engine. It's a true balancing act but once you have it set nothing ever changes except the high end a little come the winter months. I was only a touch off but that's all it takes. I think that was my only back fire so far when in the air. I had one once on the test stand but that turned out to be the needle valve assembly. The prop about took my head off, missed me, went into my shop and right through the wing of a brand new plane. Missed the ribs but a patch on a new plane???[&o]