Needle and seat
Yes I have had this problem. The needle is tapered but the seat should be a square cut edge. When the needle closes it should seat against the square edge so that there is a small contact area against the needle. This very fine line of contact is easily overcome by fuel pump pressure. When the seat area wears it wears in a taper to match the needle. What happens is the needle will now lock into the taper worn into the seat similar to a steering tie rod or track rod end. The carburetor manufacturers in later years used a neoprene tip on the needle to stop the seat wearing into a taper. Hope this is helpful.
Paul
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