need advice on what could cause this slight engine problem

Try the quick double pump when going to WOT. A quick short stab of the pedal, let the throttle close, the go to WOT. Do this as quick as possible. Does it still stumble?

If the pump shot is lean, this very quick stab nearly doubles the volume of gas from the accelerator pump and if it gets rid of the stumble or reduces it, You just proved that the pump shot is too lean.

Remember, these carbs were designed to run on 100% gasoline....not the 10% ethenol that we have now. You need more volume of fuel when using ethanol.