from yz125 to crf250r or yz250f

You will have more maintenence with the 4 stroke. the oil will need changed more + an oil filter, you have to keep a close eye on the valve clearance, and then when it comes to the time when you have to rebuild your cost will be about 3x more expensive to do a top end if you have to replace the valvetrain compared to a piston and ring for a 2 stroke. I think you should probably be looking for a 250 two stroke if you almost always ride harescrambles and woods.

I guess i should probably add a 4 stroke won't need rebuilt as often as a 4 stroke, but the average rider is probably only doing a top end rebuild on the regular only once a season or once every 2 years ( unless they blow it up in between) so that might now matter for you.

And also, if you buy a 250f that has been beat it's whole life and has a poor head/valvetrain, you are going to have nothing but problems and reliability issues unless you drop lots of money for all new top end parts, including new valve seats.

Something else to consider: if you start racing take a look at which mfg has better incentive for the events in your area. The Honda is a great bike if the valves don't cost ya $$$$.

like mxtech1 said if you are going to buy a used 4stroke you have to be very careful. Also a 250 2 stroke sounds like the bike you want for the offroad races, a ktm 250exc would suit you well. If you are fixed on either the yzf or the crf id say the yzf. Its heavier and taller but the crf will chew through valves and is lighter. The crf has a powerband much like a 2 stroke.

Whats your budget? that will help you determine what to look for.

Depending on whether you want to ride on tight bush trails, supercross tracks or wide open fast motocross tracks you may want to adjust your sprockets to alter the low-end power or top speed of your bike.

The easiest and cheapest way to adjust the final drive, or gearing of your bike is to change the front sprocket. However, it is recommended you change both to get a better result. As a general guide, changing 1 tooth size on the front is equivalent to changing around 4 teeth on the rear.

If you replace the front sprocket with a smaller one (less teeth) you will increase low end power but decrease the top-end speed - good for tight motocross or supercross tracks. If you replace the rear sprocket with a larger one (more teeth) this will do the same.