The transmission and drivetrain is forcing the engine to maintain rpms, preventing the stall out. It will run hell rich, but as you reach the end of the 'deceleration curve' the surging fuel phenomenon is no longer being exaggerated (well, that's my theory...) I would be tempted to try modifying the ignition tune - smaller plug gap/more advance to improve air-fuel charge burn. The Weber is more sensitive to tuning than the factory Mikuni carb.
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