On the original Nissan-Datsun 240Z, if you had every thing set in specs and the carbs perfectly adjusted, at about 50 degrees F, if you did not move the preheat flap to winter, the car was undriveable. Move the flap to winter and it became driveable almost instantly, within a few seconds. No other changes.
Variable venturi carburetors are especially prone to "icing" which is basically due to incomplete atomization of the fuel mixture.
Cold air being more dense will give more power with higher effective compression, but without fuel injection atomization and economy will both suffer. Even my 37 Ford with the original engine had a passage in the intake manifold where exhaust gasses heated the manifold right under the carburetor.
It's ancient efficiency technology.
regards
mech
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