Perhaps a chunk of debris has found it's way into the NV seat, so when you turn the needle in, it doesn't shut the flow down. Has anyone ever closed the NV all the way, (perhaps a bit too tight?) This would damage the needle seat, and cause the problem you are describing. Here's the fix, or at least a test: Put a wheel collar on the fuel line between the tank and carb. Once you get her fired up, slowly close the allen screw of the wheel collar down, 'til you are squeezing off the fuel line. it should lean out- if you have a damaged seat, or debris.
I've run planes with this sort of a "field fix" for the season, no problems
If you have debris suspected, use a fuel bulb, sqeeze it, attatch it to the carb nipple then release it, sqeeze it out on a paper towel, or other white paper to check for crud, repeat, this may clear out the seat.
Hope this helps,

Jetts