[jhv48]

You're just starting to exit the idle circuit when you lightly touch the throttle.

Try this first. Open the idle air adjustment screws an additional 1/4 turn and see if the stumble goes away. Some engines just don't like to run at the highest vacuum level and require a slighly richer idle circuit to transition properly.

Also, make sure the accellerator pump arm is starting to move the same time the throttle is being cracked. Look down the bore at the squirters and make sure they start squirting the moment the throttle is cracked.