Oh dear oh dear oh dear.
You have my deepest and most profound sympathies; my Guzzi Le Mans self imolated some time ago after a backfire, so I know what it's like. Mine was a total, abject, complete and utter loss.
What sort of ignition system did you have?
My Guzzi had a very exotic and expensive ignition system that ran at crankshaft speed, providing a lovely fat spark at the fixed advance point before the top of the exhaust stroke, as well as on the compression stroke. Of course "valve overlap" means that the inlet valve opens before the end of the exhaust stroke. In my case, I primed it, cranked it, and it didn't start, but came to rest with the left cylinder at the top of the exhaust stroke. The spark ignited the fuel-air mix in the chamber, which blew back through the part open inlet valve, causing the very rich mixture in and around the carb to ignite, blowing the filter off and spreading flaming mixture everywhere. Furious engine cranking would not come close to sucking the inferno into the hole, so I hastily dismounted. Ultimately it burnt the tyres off the rims, and the only casting apparently unaffected was the rear bevel box. The engine and gearbox, castings melted, could be described as comprehensively rooted.
This is not a new phenomenon. It is a special feature of crankshaft speed ignitions when used in conjunction with the Otto cycle. No, Bob, this is not supposed to be a Villiers advertisement!! My 1933 New Imperial uses (as did many lightweights of the thirties) a crankshaft speed ignition, but was also fitted with a flame trap from new. Although technically called a "Strangler" for starting purposes, it is in fact a Davy Lamp.
JFerg
