Use a heatgun. to warm up the intake rubbers. when the rubbers is "hand-warm" ie. Ouch.
then spray WD40. and press the carb-unit on.
I guess your problem may have the intake rubber is old and stiff.
try to (when the engine is running at idle) spray on the intake rubbers with Ether (eg. Diesel start) So be sure there are no cracks or leaks.