Have a Husqvarna AWD with a Honda engine on it. The carb is clean, the flywheel key is intact, and the blade and mount are tight and right. An initial view under the valve cover shows nothing remarkable (nothing loose or slipped off). It has spark, compression and air, but won't start. When you pull it over, it sounds like it's backfiring through the carb a bit or the head gasket is blown. I don't see a seam for where the head attaches. It acts like the flywheel key is sheared or the blade is loose. Neither is true. Ideas? This is the Honda that has the plastic, geared wheel visible when you pull the valve inspection cover off. School me please.