Based on your description of the problem, I suspect a float issue. There was a batch of briggs carbs that had a variance in the posts where the float is attached. As a result of the variance, the drop tab allowed the needle to "drop" to far often resulting in it getting jammed and allowing too much fuel to enter the bowl. The symptoms would be issues would largely be stumbling in corners....mid to exit. Furthermore, the needle is getting bounced around and will sometimes work fine and others get caught and not shut off fuel.