There was a similar accident in NZ a few years ago and this was their comment on the carburettor:
”The Gypsy Major engine employs automatic carburettor heat, in the form of a spring-loaded butterfly valve, interconnected with the throttle control so that at full throttle cold air is admitted, but at cruise, warm air from the engine bay is admitted through a flame trap. The spring applies 4-5 kg of load to keep the valve in the warm air position in cruise. If the throttle friction nut is not firmly applied before take-off, the spring will cause the throttle to back off if the pilot’s hand is removed from it.”
https://www.caa.govt.nz/Accidents_an..._18Oct2003.pdf