You said that the carburetor had been recently rebuilt but it sounds like the accelerator pump may be inactive or giving an insufficient shot of fuel when the throttle is coming off idle.A look down the bore while stroking the throttle just off idle should show a squirt of gas into each barrel.The amount and the point of commencement of delivery is usually adjustable with specs for setting the linkage usually in the carb kit or we can find it in a manual.
Make this observation with the engine off to avoid entertaining the onlookers in case it pops fire in your face.
