Hi I'm pretty new to this forum and just rebuilt my 400EX engine. I went too http://www.robbsracingengines.com/400EXPAGE for a race motor. The kit I put in has the 440cc mod w/ a fcr 41mm carb, high rpm cam, HD valve springs, shortened valve guides, hard welded rocker arms, oversize stainless valves, Porting, HD head studs, HD crank & rod, lightened flywheel, and a +6 degree timing key. The engine is very strong, pulls hard and revs very quick (450 quick). Power out of the corners are smooth and fast, and it has that extra blip of power off the lips of jumps. Reliability is claimed to be very good but mine is too new to really see. If you have more questions about my new engine, feel free to PM me I'll get back to this review when I get to ride it some more. Later.