In my case, at peak rpm, there is still about 2mm to go before the barrel is fully opened.

Now I set the engine servo to travel to this limit, otherwise during take-off and if I fully opened the throttle, the engine spluttered and the take-off is aborted just before lift-off.

I did disconnected the whole carb assembly and blow directly through the inlet opening while slowing opening the throttle. I noticed that the air flow increased progressively as throttle is opened, and just beyond the 2mm-to-fully-open mark, there is a "whoosh" and air rushed freely through. Maybe this causes the splutter as the air mixture becomes too lean?

I will remove the crowl and do a pinch test again as Bill suggested.

Thanks,

Chris