2003 YZ125 many questions. forks and low end power...144cc???

[Devin313]

ok...i have a 2003 yz125 that i purchased about a year ago for 1600...the person i bought it from had basically no info on it...i looked it over and didnt find any noticable problems and bought it...now i have three questions...btw i know im a noobie and a lot of my decisions may be dumb but im here to learn so feel free to help...keep in mind, im not a rich guy lol

1. it seemed soooo fast at first but im getting better now and its starting to seem really slow(at the bottom end anyways)...it already had a fmf fatty pipe and i just ordered an fmf shorty pipe and im thinking about getting either a Rad Valve or some power reeds...which?...what do you think? is it worth the money? will i(being a beginner) notice a big difference?...ive also read that with the rad valve i need to change jetting and i have NO idea how to do that...is there a post on how to do that?

2. also is has a severe fork seal leak on both sides...the dealer wanted to charge me like $300 to change them so i found a guy and bought some used forks that had just been serviced for $50...im trying to take off the forks and got off the side without the brakes fine but on the side with the brakes, theres this large nut i cant get off(maybe like 22mm) its so tightim just chewing it up trying to get it off...does this even need to come off? all the other bolts are removed except that one


3. i want to open up the engine and check and most likely replace rings and stuff but im not sure if its bored out to 144cc...the guy i bought it from didnt mention it but he didnt know anything so idk...at the track a few guys have told me it may be a 144 because it has an athena top end and larger rear sprocket, im not sure...is there a measurement i could take of the piston or cylinder to check? it seems like if the previous owner bored it out, he woulda put an aftermarket silencer

[ossagp]

The first thing you need is a manual. Put the pinch bolts back into the side that doesnt have the brake. The pinch bolts seem to be removed from the brake side already. Get a socket the right size to remove the nut and stop using an adjustable open end wrench. The forks should slide out pretty easily. The seals will be pretty easy to change too with a manual and a homemade tool or two.

[ossagp]

Oh, I don't believe Athena made a 144 kit for your year (or mine). I have a 144 that Eric Gorr built from an Athena cylinder. Measure the piston on yours. 54 mm is 125, 58 is 144.

[Devin313]

the the side that doenst have the brake is already completely removed the the fork on the brake side is connected only to the wheel

[ossagp]

Gee, where does that leave you now? Devin, you are liable to have to put something back on there so that you can get some leverage on that nut. As you might have noticed it tries to turn when you have it the way you do.

[Devin313]

oh, thanks...now on to my other questions...anyone? and thanks ahead of time for the help

[the_privateer]

search the knowledge base for jetting, there are hundreds of threads on the subject, I know my yz125 is very picky about the jetting and i find myself adjusting it quite a but. i wouldn't bother with new reeds unless the current reeds need replaced. I don't follow your logic about the big bore and aftermarket silencer but the only way you will find out now is by measuring as ossa said.

[Devin313]

oh i was just saying that if he had money to spend on the big bore kit, it seems like he would have upgraded other parts too

[YZ125motoracer]

athena never made a big bore kit for an '03, only for 05-08. Thats either a replacement cylinder by itself or the athena works kit (which i have). Theres a flat-top piston which is supposed to go with that kit btw, if you plan on doing a top end.

[YZ125motoracer]

oh btw if it is the works kit it will have an athena high compression 2 piece head with a bronze insert, special exhaust valves and a heavier spring. you will need to use athenas top end gaskets which include 3 different thickness base gaskets (use the middle sized one).

[Pretendo]

when you start to lose horsepower its a sign that the piston needs to be changed out. normally you can go 2 ring services per piston. thats the origonal and a replacement. esspecially loss in bottom end, theres too much piston friction now i bet. replace as soon as possible.

[misfitsbob]

Originally Posted by YZ125motoracer

athena never made a big bore kit for an '03, only for 05-08. Thats either a replacement cylinder by itself or the athena works kit (which i have). Theres a flat-top piston which is supposed to go with that kit btw, if you plan on doing a top end.

friend of mine had one on an 03...im pretty sure it was a athena, im positive it wasnt a gorr

[ossagp]

Not an Athena 144. Gorr isnt the only one to build them from an Athena though.

[misfitsbob]

i swear i thought it was athena but it looks like im wrong...i sent him a message asking him what kind it was...he lives around 100miles from me and doesnt have the bike anymore or id already went and looked at it lol

only thing i remember about it was the guy that owns it now let me ride it at a local track by his house and it was running terrible

[ossagp]

Athena made 125's. Still do. since you can get them, head, valves, piston and cylinder for under 500 off ebay they are a good alternative to OE if you are looking for a replacement for a gunnysacked yz125. They seem to have dealt with the exhaust bridge breakage pretty well too.

[YZ125motoracer]

no athena never made a 144 for 03. im a dealer for WPS of long island (east coast athena distributer) and tried to order one for my bike, but found out the only have them for 05 and up. The works fit that ossa is talking about is definately worth the money. ive found that it seems to run the best with vforce reeds, the stock yamaha pipe, and the pc factory sound (long) silencer, as the shorty hits a little hard for my taste down low.

[misfitsbob]

now im really wanting to know what my friend had on his bike lol

[misfitsbob]

he said it was athena......idk

[ClosedCasket]

Anybody notice the left stud holding that jug on?

Looks like a smaller nut close to the top of the stud with a small bead of weld between it and the washer...

[ossagp]

It comes across too blurry for me to tell.