What Motocross Bike To Buy...help

I think you'll be just fine on a 250F. I'm a little heavier than you and still find that a 250cc two-stroke is a little too much. Since you like Yamaha, I think a YZ250F will fit you fine.

You need to keep a close eye on the valve clearances. The YZF is supposed to be the most reliable of the bunch but after a while the titanium valves fatigue and will eventually break at the stem, causing catastrophic engine failure. This is called dropping a valve. You can help prevent this by checking the valve clearances periodically, and changing the oil and cleaning the air filter frequently. Fresh oil is your engine's best friend.

A 4 stroke 250 MX bike has a lot more low rpm and mid rpm torque than a comparable 125 2 stroke - but they both have about the same peak horsepower - the 250f is easier to ride fast. The 125 requires more effort to keep it moving fast because you have to keep it high in the rpm range.

Advantages of thumpers:
More tractable power.
Makes power over a longer range.

Disadvantages:
Weighs more than a two-stroke.
More moving parts.
Valve trains are pretty fragile on the high-tech bikes.
Have you tried starting one? Ugh.


If I raced motocross in the 125 class I would be on a 250F. But I don't. I race offroad and find a two-stroke way better for my riding. Since getting on my GasGas my speed has atleast doubled. I also ride really tight trails where it's not a matter of if you stall, it's when. I'm not a very big guy (6' tall but I'm pretty skinny at ~155lbs) so starting a hot thumper is an issue for me. I also don't like the added weight and gyroscopic feel on tight trails. The maintenance issue is a trade off. Two-strokes will go through top ends more often and it's not strange to hear of them needing new cranks after many years of riding. The new thumpers are race bikes and require a race bike maintenance schedule. If I owned one I'd check the valves atleast every 10 hours, maybe less. You can probably get a couple seasons out of a top end in the thumper if you change the oil frequently (after every race, every 2-3 rides) and clean the air filter often. If you get a 125cc two-stroke motocross bike you'll probably do a top end atleast once a season and probably do new rings half way through the season, but a top end is cheap and easy to do on a two-stroke. A four-stroke is a lot more complicated. If you search the forums there are lots of threads covering the two-stroke versus four-stroke debate.

SpeedyManiac said:

. . . it's not strange to hear of them needing new cranks after many years of riding. . .

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At 12000+ rpm I suspect we'll find the same in all MX bikes in the long run - it would be surprising not to.

Anybody ever think about 12000rpm. Thats 200 revolutions a second on the crank! Geez.

To get on equal footing with a 250 4 stroke, the 125 2 stroke basically needs to be overbored to 144cc or more.

The Suzuki team manager recently said that the switch from 2 stroke to 4 stroke required that they have three mechanics for each rider instead of the one mechanic they had been using prior to the change.