If you have a Bing Carb I would try adjusting the needle jet. To get to it, take the top of the carb off. remove the throttle slide and you will see a tapered pin coming out the bottom. This is the needle jet.
It has four adjustments at the top. You will see a clip on it in one of the four grooves. If the clip is near the top groove it is set where the tapered needle will allow a lean mixture during low speeds. If you drop the clip to the bottom groove it will raise the needlejet and make a richer mixture.
The symptom you describe is a bog and is caused when the air does not get enough fuel mixed with it. When you are idling, the throttle slide is closed and very little air passes through the Ventura area of the carb.. When you open the throttle your engine sucks a greater amount of air. This requires a rush of fuel to atomize with it. Raising the needle will allow that extra fuel at the initial opening of the slide.