I often switch between 63 and 65 depending on track location. Since 65 is 7mm carb it uses less fuel to mix with air to get perfect mixture (63 is 9mm) Given the same track (45 seconds track) i can get 14 min with 65 compared to 12 min with 63. However, when i race at high altitude, its very difficult for me to get the tune right with 65. The room for error is very small. 63 is a lot more forgiving. 65 is smoother at the low end. And the low end needle with 65 is almost all the way in. Hope its useful. I havent tried 69 yet so i cant compare.