Carrot Cruncher

I've exhausted my normal diagnostic routes with a Stihl TS410 disc cutter. Handed to me by my boss as an unknown it ran ok but with a tonne of smoke, the engine then backfired and cut out like the ignition had been cut and hasn't started again since! It has good compression and fuel which I have changed for a fresh mix. The spark is good so to my mind it should go or at least fire! I've had the flywheel off in case the woodruff key had moved or semi sheared but all is well. I've also disconnected the stop switch in case that was causing a fault. I think it has reed valves and I have no knowledge of these. I've also removed the very dirty air filter which made no difference and I've tried lobbing some fuel directly in the bore. It hasn't even had the courtesy to fire once. Ideas please and tell me what I've missed. Phil.

Welcome to tip logs / cord! Ring in advance and the Mrs will supply Tea!