Spitting carb

LM1000 with bellmouths.
It’s now happened 3 times: turn the ignition on for a few seconds, just on nothing else didn’t touch starter or throttle, turn it off and the left hand carbs spits itself off the manifold.
Nice dents in the ally bellmouth, if it was an original plastic one I imagine it would have broken. It’s a testament to the powder coating - frame not damaged.
Bike runs well and ticks over well so I assume the timing is ok - it was checked by a very ugly Guzzi maestro mechanic in Sarf East London (no points for guessing who)
Any ideas??

I had that with the Spada III, in my case the carbs were way out of adjustment.

What ignition system does it have? The RITA system fires the plugs (both together) as the key is turned off. I can hear mine chuff sometimes, must depend on how much mixture is left in a cylinder.

Dyna III