The starter isn't the fastest spinning, but the bike fires right up. It then takes a bit longer to warm than a YZ. In Costa Rica (where the bike was unveiled before the media) every tank of gas was a new surprise-the quality was random, making it hard to truly feel how the bike was running-but these WRs proved they could and would run on low-octane fuel. They hiccuped and detonated, but they survived. We waited till we got a test bike back to California, where we could put some hard miles on it, to see what the new motor was really made of. Plainly put, it rips. It is much more lively than the old WR but at the same time still stays very smooth and progressive in the power buildup. The jetting we finally settled on is in the included chart. And if it is ever finicky, especially at idle or low throttle positions, just play with the fuel screw. The best thing about the bike is the big power that still comes with such a quiet exhaust note. With only the smallest muffler end-tip insert removed, the bike is whisper quiet and never has that snap-crack-burst of sound nor really roars decibels, even in the upper Rs. The bike is very quick and the power delivery is great for low-traction situations; it grabs traction when there is very little available. We have nothing but praise for this motor.The handling isn't as light and flickable feeling as the new WR250F. But when compared with last year's steel-framed bikes, the 450 definitely has a lighter feel that is most notable in side-to-side movements with the bike and again in the steering feel through the bar. It still has some girth to it, a good weight that helps the bike stay planted and makes it stick to the ground rather than deflect and dance over things, though some of its bite is from the power. It isn't heavy feeling by any means; it's definitely lost some hulk compared with the last couple of years of WR450Fs. You will notice this. Kiss top-heavy feel good-bye and good riddance!Suspensionwise, the Yamie has to find a balance between the plush and very linear suspension needed for Eastern conditions and the stiffness and holdup needed for faster conditions or sand riding. Yamaha went with a more-rigid setup compared with the softer-suspended WR250F so that the 450 will work in the faster stuff and carry its weight and speed. It still lets you feel the small bumps, like most lighter-weight, larger-displacement bikes or competition-focused 450cc four-strokes, especially at slower trail speeds. But if you pick up the pace and hit stuff aggressively, the bike feels a lot plusher and doesn't dance or deflect any longer. And it has great bottoming resistance at the end of the stroke without being too springy in the middle of the travel. For heavier riders, the initial bump feel isn't an issue at all, but riders lighter than 170 pounds will notice it. This bike (or any 450cc four-stroke) will never feel as light as a two-stroke in rocks and roots, but it will stick and absorb before it will kick or deflect.The bits and pieces are all first-rate, and we think Yamaha has found a great balance between what an off-road rider needs and the added weight that comes with that stuff. Bonus points for the long-wearing steel rear sprocket, the coolant-catch tank, the digital odometer and the lighting that really works. Nice ProTaper, too! The new airbox door makes the old one seem like a pain in the rear, and we can live with the dual oil drain plugs, even if all the oil comes from the same place. The gearing on the bike is adequate for most everything, and there's never a gap in the spacing: Top speed is in the high 80s, low 90s on dirt. The clutch stood up to plenty of abuse and isn't really needed to coax out power, just to calm it sometimes. Or it's sometimes needed for the really slow and nasty stuff, where the pull is just right. Quick-adjust, too, if you really get it hot! (Remember to readjust after it cools.) The Yamaha brakes give all you ask of them and, along with the O-ring chain and wheel spacers and bearings, have stood up to mud in the past like they are supposed to. The tank runs a little small for our tastes-getting 60 miles out of a tank on a trail ride or more than 45 in racing conditions is asking a lot-but it is narrow!What do we really think of the new WR450F? Well, we love it! It begs the question, Which is the new king of the 450cc off-road wars? With Kawasaki coming into the game really soon, we're bound to find out in some sort of a shootout as soon as we gather all the bikes together. But if you bleed blue and have been trying to liven up your steel-framed bike without much success, this is the immediate answer.