hello all, im new to the forum and to the 4stroke world, i bought a 02' yz250f from a guy a few weekends ago and its in great condition and was hardly rode, the only ride time was a few hare scramble/ cross country races he was in. I bought the bike mainly so i could start riding with my son who is 8 and rides a ktm 50 for now and about to move up to something a lil' bigger soon. But anyhow, i see the stock sprockets should be 13/48, well i have been riding dirt bikes(cr80s and cr125s) off and on since i was 7yrs old and now im 28, well once i got on the bike i noticed it didnt feel like it had great power down low but on top she really moves, so i started looking and rememered the guy i bought the bike from said he had just put a new rear sprocket and new chain on the bike, so i looked lastnight and the rear sprocket is a 51.  I guess my question is, how is the 51 different from a 48, i know it has more teeth so it should stretch out the gears right?  thats why its a lil slower down low and fast up top?  I also noticed that the tires are brand new and different sizes than factory, they are 90/90-21 front and 120/80-19 rear..  how is this setup?  thanks for any input or info. on this bike and what i have going on here. also, before anyone asks what kind of riding i will be doing, it will maily be riding through the woods on trails and maybe hitting a mx track here and there so i can try to get my son out there riding and jumping.
thanks,
Danny

Edited by v20fishnboy, 25 February 2012 - 05:54 AM.

Bigger sprocket in the rear equals lower top speed but more power when taking off. Smaller rear equals higher top speed  but lower take off power.

Is the jetting correct? Is your front sprocket a 13 or 14? A big complaint of the guys who race is that the YZf has a lot of bottom to mid power but not enough on top. So it sounds like your bike is the opposite.

Did the previous owner instal an auto decomp exhaust cam and maybe off a tooth?

Gearing depends on what type riding your going to do.  For tracks, 13X48/49/50 seem to be the choices for most riders.  From 48 to 51 is lowering the gearing a great deal for track.  The guy you got the bike from must have been riding woods since you are in VA.  You will find that you are shifting gears quickly and won't get much pull in each gear, giving it a short quick burst, which makes feel flat.  Try a larger front or smaller rear to see if that gets your the desired results.

yes the previous owner did install the auto decopmp. exh. cam but he had raced a few races afterwards and said he loved it. and yes he was racing in the woods. the front is a 13 tooth, but maybe i will look for a stock 48tooth and see how that is.  but i would imagine the jetting is right cause there is no issues with it running at all, i just think the bike didnt feel all that powerful down low, but then again it might be right, i just havent rode in so long, maybe im riding like a sissy  lol   going for a ride tomorrow with my son and a few others so i will see how it does

Go with a 13X49 and try that, 48 might be too tall for mountainous woods.  It should pull your shoulders out of the sockets.  I ran that before I went to WR tranny.  For the lower gearing in tight woods, you can try the GYTR flywheel if its stalling out too much.

I'm not near any mountains but that would be an awesome ride.  I'm actually in Yorktown which is right on The coast but most the woods and trails will be open enough For four wheelers and rhinos to go through also.  I'm thinking I need to go back to a 48 or  to a 49

OK, so you're looking at fast trails then, maybe in a national forrest like I ride in, and possible some deep sand.  You will like 13X49, maybe 13X50 if there's some steep hills. My son likes his at 13X50, he weighs about 150.  I'm 180 and gaining and liked the 13X49 with YZ tranny.

went riding today and the bike does great. maybe i will leave the 51 on it til i get on a mx track and see how it does.  only thing with the bike now is the clutch is acting weird. when im in 1st and about to start riding, when i let the clutch out it grabs fast and hops and stalls. i tried messing with the adjustment but that didnt help.  to get going you have to either give it alot of throttle and almost dump the clutch or barely give it throttle and ease into gear, there is no in between.  not sure whats going on, dont know if i need a clutch or a new clutch cable.  anyone had this problem?  and it is the clutch not the rider cause ive had 2 other guys who ride often try it and it did it to them also and they arent sure what it is either.

im almost wondering if the clutch is messed up from the bike sitting from jan. of 09 til when i bought it a few weeks ago?

Prob is.  Simple replacement, just cost you $$.  I've worked on bikes where the clutch sat up a long time, and the material soaks up oil and will slip under load until replaced.  Someone may have a home brewed repair.  Swede, you got any words of wisdom?

Soak them in root beer

Edited by Swede73, 26 February 2012 - 06:45 PM.

For how long??

About an hour. That's just a last ditch effort thing to get back to the truck or finish the race kind of deal. The sugar and stuff in the soda is supposed to swell up the material just enough to make it work. An old hot rodder from the 50s told me that and I thought he was joking. If you're cheap like me you can give it a try. What's the worst can happen?

well i guess i can start with a new clutch cable and see if that helps, if not it will get a new clutch kit.

I am a little late here but wanted to comment you on you frist questions. I also purchased an 02 yz250f (a few years back) to ride with the kids and here is what I found to worked for me.

1. I ended up running a 51 rear sprocket to help prevent stalling, not 100% but helps 1st gear be a bit slower while you wait for the kids.

2. Tire size won't matter at this point. (your still waiting for your kid)

3. Get a better clutch lever. Moose's ultimate clutch lever systemis a one finger clutch pull. Its hard on clucth cables but it so nice to have.

4. Don't let your bike sit and idle, it will over heat. A better coolant (engine ice or evans) is recommended.


Happy Trails

thanks for the info. Dano, yea i was told by the previous owner that when you start the bike not to let it idle or idle much at all anywhere or anytime. but i will mess around with the bike some and figure it out. my son did great on his ktm50 when we went riding on sunday and i think with him being 8 and almost 50" tall and almost 60lbs he might need to move up to a 65cc but he just started riding so im gonna let him get used to the 50 before i move him up. soon i will be chasing him in the woods cause he likes to fly  lol