Rochester 4 Jet Carburettor – Problem

Good morning you loved one ! the following problem with my 63er Impala, he does without problems no matter warm or cold, in N he takes on the gas super as soon as I go on D or R it drains off. But can find here no regularity so no matter whether cold warm – sometimes he runs net. Freu mich über Hilfe – schöne Greetings