I love this L.A. Sleeve 400cc kit. It is designed to appeal to those who don’t like the big-bike feel of modern 450s but want a more-bulletproof mount rather than a punched-out 250F. This 400 has less power than a 450, but it is definitely not slow. It has a considerable acceleration edge on 250cc two-strokes, and it makes ponies over a longer rpm range. The DR. D exhaust/400cc kit combination produces crisp and smooth power that pulls from idle. The rpm build rapidly and with plenty of energy, but that chuggy, grunty, big-bore feel is gone. Rut berms you would normally bog in third on a 450 you can hammer in second and grab a gear coming out-just like with a 250cc two-stroke. There is ample boost on top, yet at high rpm the bike doesn’t try to rip off your arms. Two other bonus points are super-easy starting and almost no engine-braking. At first I could ride the bike a lot longer without getting tired. After I grew used to how hard I could hammer into turns with control, I found myself pushing harder. With the smoother power, I was better able to handle tricky jumps than when the bike was a 450. The 2007 YZ450F might be a little smooth for this conversion, but on the ’06 it was pure magic. I would make this mod to a ’06 YZ or any Honda CRF450R in a heartbeat.