The YZ125 makes slightly more peak horsepower, but the YZ250F makes its power over a greater portion of the rev range.
Designed for the same mission by the same company, it's no big surprise to find many similarities between the YZ duo. Both share the same suspension, swingarm, brakes, fuel tank and tires.
Similarities end when it comes time to fire up these screamin' tiddlers. A light prod or two on the 125s kickstarter has the stroker belching Yamalube-tinged exhaust out its expansion chamber and stinger muffler.
Revs rise and fall respond in direct and immediate response to the throttle and 38mm Mikuni TMX carburetor, as the 2-stroke exhibits almost no flywheel effect that slows engine response.
Things aren't quite as simple for a 250F rider, as lighting its fire is a lesson in patience and technique. Four-strokes, especially high-compression ones like the 12.5:1 YZ250F, can be a bear to start. Do it the wrong way and the lever will kick back like a mule. To ease starting, Yamaha (and others) incorporates a compression release (activated by a handlebar-mounted lever) that raises an exhaust valve to let the kicker get the piston easily past top dead center, where the compression would otherwise be at its maximum. Once past TDC, raise the kickstart lever all the way up, let out the compression release lever, and stand on it. You can have a look at our detailed explanation of 4-stroke starting techniques if you still have issues with the procedure.
The YZ125 feels light in and out of the corners, but it lacks the grunt of the bigger 4-stroke.
(For 2003, Yamaha has added an automatic compression release to its 250cc and 450cc YZ and WR lines, making this sometimes-frustrating procedure a thing of the past.)
Do it right, and your ears are greeted with a deep, revvy roar as three intake valves and two exhaust valves respire fuel and air. Throttle response is incredible for a 4-stroke engine, rising quickly with a quick yank on the Keihin 37mm flat-slide. A throttle position sensor synchronizes carburetion with the optimum ignition map for a clean power curve up to the lofty 13,500 rpm redline.
Generally speaking, more rpm equals more power. To accommodate a high rev limit at safe piston speeds, Yamaha built the 250F motor very oversquare (meaning the bore is much larger than the stroke). At bore and stroke dimensions of 77.0 x 53.6mm, the 249cc mill cranks out 32.1 rear-wheel horsepower on White Brothers' Dynojet 200 dyno at its peak.
It's abundantly clear which bike has the edge in torque: the 250F takes the cake with a long, burly torque curve.
In comparison, the YZ125 beats out the twice-bigger 250F in peak power, pumping out just one pony more at 33.0 hp. Those of you who stopped reading after the previous sentence won't get the full story, though.
A quick glance at the dyno chart clearly shows an obvious advantage for the bigger-lunged 4-stroke. Power builds much earlier, and the torque advantage it has until 9000 rpm is massive. Once past 6000 rpm, the 250F doubles its output just 2000 rpm later.
Just as important to the 4-stroke's advantage is its relatively expansive powerband. It's pulling hard at its torque peak of 18.3 ft.-lbs. at 7750 rpm, and it doesn't really slow down until it hits the rev limiter at 13,500. That's a power spread of more than 5000 rpm! Immense by 125cc standards.
There was little question which bike came out of the corners better. The YZ250F was the king of of blasting out of a berm, especially in the loose stuff.
Conversely, the YZ125 is a much peakier animal, though it is blessed with a remarkably linear powerband for a 125cc 2-stroke. Credit the Yamaha Power Valve System (YPVS) and the "square" dimensions of the 54.0 x 54.5mm bore/stroke cylinder. It's advantage over the 250 is a short one, from about 10,000 rpm to 11,500, after which power falls flat. Meanwhile, the 250F still has 2000 rpm still to play with. Keep the 125 on the boil, and it'll run with anything in the class. Ride outside that sweet zone and you'll be eating 4-stroke roost.
Our first playground with the YZ duo was Saddleback Motocross Park near Irvine Lake in Orange County, California. Its vet track, a hard-surfaced course with small- to medium-sized jumps was a good warmup for the pro circuit. It didn't take many laps before the 250F became our favorite. Its much wider spread of power was much easier to transfer into speed, as there's ample twist to get hooked up no matter how lazy we were with the gearbox.