Matt,
What you describe is a classic case of dirt being caught between the needle and the needle jet, causing scratches and wear. Sometimes it can cause the needle to jam and then you go for a real thrill ride.
The dirt makes its way in through the vent hoses and settles in the float bowl. Split them and run the little filters. I've also had the stock GG throttle cable let water in that collected in the top of the slide. If for some reason the needle cannot "float" in the slide there will be a side load on the needle jet and cause wear, it must float. The LTR needles are made exactly like a standard Keihin needle.