CaptainToyota,
I bought my son a 2004 YFZ right after they came out. We put an HMF full exhaust on it, and removed the air box lid for more performance, but did not do any internal engine mods at all. He flogged the YFZ for two years, including a few hundred races up Sand Mountain at little sahara (Utah), countless drag races against dad (most of which he won), and his first full season of USRA sanctioned desert racing. Maintenance was not the best, with only three or four oil changes, and not one single valve adjustment. He also had a hard time knowing when to slow down, and managed to suck muddy water into the engine at least a couple of times when he hit water crossings wide open in a desert race. He managed a top ten points finish for ATV desert racing his first year (age 14). So basically, the YFZ was used hard, and we never had one single problem with it. I think YFZs are tough as hell, and we bought a new 2005 model to replace his 2004. Read any dirt bike magazine shootout on the 450cc motocross bikes, the YZ450F is always described as being rock solid reliable (unlike the Honda).

DV

 

The latest, greatest 4-pukes are very high strung and therefore you have to watch them very closely, especially the valves. We are still pretty much in the beginning of the much more high tech 4-stroke era, Yamaha brags that the YFZ450 will rev to 10,750 rpm, the price for that kind of stress is less reliability. If you had a '90s 4-stroke, 10,750 rpm would have been National type performance and unheard of in a stocker.

As usual, Honda seems to be more reliable from what I have seen, although I rode one and don't think it has near the high rev capability of the YFZ.

 

Not sure if I agree with Honda being more reliable. A couple of my friends have TRX450Rs and they've already blown them up twice. They both say that if they werent such die-hard Honda guys theyd be riding YFZs. Last weekend I went with them to a local drag race and they were all pumped-up afterwards because they actually managed to beat a couple YFZs.
Cap'n