The guy that tuned my engine said without an insert the air wouldn't flow smooth into the carb and your much better off with any of the three supplied with the mayhem. I am currently running the 7.5 and plan to move to the 9 later

I picked up my truck today and while the guy was showing me outside how well it runs the clutch bell came loose and it lost a bearing and the screw he fixed it and explained to me that they didn't put lock tight on the screw that holds it in and it needed like 5 shims or washer to take out the slack you guys may want to check yours for play and put some lock tite on that screw, But anyways this thing hauls ass I mean it really moves all the guys watching that have revo's and savages where impressed with how the truck gets down and squats when it takes off now I just have to learn to drive it lol