Jump to content
Sign in to follow this  

yz 125top end problem:/

Recommended Posts

Its a 2002 yz 125, it started bogging when I got to top end power.

I went through the carb a gazillion times and cleaned it out. come to find out it was the the piston and rings( not an experienced mechanic) I started to realize it when it got harder to start and realized the compression was close to nothing with the kickstart.

So I got a new top end its the 2nd one ive done (1st on an rm 125) the 1st was a success this one, not soo much...

I broke it in the night it was put together following a procedure I believe was posted on here, the second day i took it took a riding spot just corners and singles so throttle was pretty steady not wide open the whole time, riding time was maybe an hour total.

second day the type of riding was a combonation of corners , straights, deep sand sections, whoops, and there was alot of wide open sections on our track we had. we were practicing starts also numerous times.after about 4hours total maybe on the bike it started bogging out as if it were fouling a plug. so i took it back hoping the gas was a little rich and called it a day. but that wasnt the case it was doing what it was before with the old top end.. so heres some pictures maybe you can tell me whats going wrong and causing this and if theres anything I can do to save this... the plan is getting a 144 built but in good time i just want this for now. so any input would help alot thanks :/

IMG_8183.jpg

IMG_2454.jpg

IMG_1752.jpg

Share this post


Link to post
Share on other sites

it looks like your cylinder needs to be replated, you can see where the piston has worn down to the aluminum around your exhaust ports, at WOT i would imagine you probably get enough friction to have some the aluminum smear on the cylinder wall probably wrecking your rings/piston in the process

Share this post


Link to post
Share on other sites

^^^^ Yeah, Looks like it's time for a re-plate, or just go for the 144 at the same time.

It also looks like there may be a crack at the bottom of your exhaust bridge. Hard to tell in that pic.

Share this post


Link to post
Share on other sites

Is this normal wear? OR is there something that i need to adjust

To prevent this wear? And do you guys know an approximate

Price for replating would cost? After thats done Can i get away

With another top end kit and call it a day?

Share this post


Link to post
Share on other sites

Looks to me like you just need a replate. Also I haveto agree with cwtoyota. Looks like there is a crack at the exhaust bridge.

What do the head and top and bottom of the piston look like? That will give us a little more info.

Share this post


Link to post
Share on other sites

Top of the piston is black, im pretty sure its normal? Just a bit like charcoal color

Bottom looks fine, likes just like it did before it went in. And there were no unusual marks on the hhead. Good eye fellas i couldnt tell but i think its a crack. Does that mean i gotta get a new jug or can that be taken care of woth replating?

Share this post


Link to post
Share on other sites

Did you atleast get your cylinder honed before the rebuild? I don't know if that could be an issue or not.

Share this post


Link to post
Share on other sites

The plating usually wears around the exhaust port first. Post a picture of the top and bottom of your piston.

Most replating shops can weld cracks or low spots in the aluminum.

I had a 250cc jug welded and plated by PowerSeal a few years ago. The work looked perfect, the turn around time was short and the price was fair at just under $250 with welding and shipping. That seems to be the rate for most plating shops if welding is involved. Without welding they are $180-200.

There are a few other very reputable shops doing that work in the same price range. My only experience has been with PowerSeal.

Share this post


Link to post
Share on other sites

I went with Millennium Technologies when I needed a replate. They found a crack that needed welding as well, and like CW, mine was also around 250ish. Their work looked real good. I couldn't tell where the weld was, and it looks like someone took their time chamfering the ports really well.

Share this post


Link to post
Share on other sites

Thanks alot guys. Im gonna take it to a local mx shop here

After worK that also does machine work. Good to know it CAN be fixed.

A local mechanic here in town said it didnt need to be honed as its not always needed, i know less then he does so i took his word. So no i did not hone it.

Share this post


Link to post
Share on other sites

Did ur mechanic measure the bore? Nikasil does wear and can be honed, i have mine lightly honed every time i rebuild. I also get the bore measured with a proper bore guage to fit the new piston properly. If you were low on compression to the point of top end bogging, Ur mechanic should have at LEAST measured and honed it. Most shops dont hone them because you need a diamond hone.

Share this post


Link to post
Share on other sites
Did ur mechanic measure the bore? Nikasil does wear and can be honed, i have mine lightly honed every time i rebuild. I also get the bore measured with a proper bore guage to fit the new piston properly. If you were low on compression to the point of top end bogging, Ur mechanic should have at LEAST measured and honed it. Most shops dont hone them because you need a diamond hone.

Thats exactly the opposite of what my local ex pro mechanic/ riding buddy told me. He could be wrong though. Not positive... as I dont have any knowledge other than what Ive been told so take my post with a grain of salt. The thing that is a sure thing is that it does wear, Ive always been told that once it wears to the point of new cross hatching needing to be cut its time for a replate. Which is usually a little under 1/2 of a season for me, when it comes time for a new piston it gets deglazed with a scotch brite and wd-40 followed by a soap bath to clear cross hatching a little. But I ride ALOT durring the summer months.

Share this post


Link to post
Share on other sites

Give Eric Gore a try or at least a phone call. Look him up on the net. He will do it for around $250 I think. Included in that will be a disassembly of the power valve, clean and readjust, milling of the head and re-cc of the dome, as well as resurfacing the base of the cylinder and of course the re plating. It will be plated to the size of a new piston so you should send him a new one, or buy it from him(the piston). He is the big name for these engines and he really knows his shit. He will easily fix that crack too. Also in this price is a 1 year warente and free lifetime honing or deglazing. Also, send in the old piston and he will figure out what went wrong for you.

Share this post


Link to post
Share on other sites

Sounds like a great service. Its about the same as I paid last time when replating. I am a strong believer of refreshing an engine at least once every 2 years. Most of the time I do it once a year. I do between 100 and 150 hours on each bike per year. A refreshened engine runs like a new one. Real joy when riding :bonk:

Share this post


Link to post
Share on other sites
Give Eric Gore a try or at least a phone call. Look him up on the net. He will do it for around $250 I think. Included in that will be a disassembly of the power valve, clean and readjust, milling of the head and re-cc of the dome, as well as resurfacing the base of the cylinder and of course the re plating. It will be plated to the size of a new piston so you should send him a new one, or buy it from him(the piston). He is the big name for these engines and he really knows his shit. He will easily fix that crack too. Also in this price is a 1 year warente and free lifetime honing or deglazing. Also, send in the old piston and he will figure out what went wrong for you.

I would agree.

I believe Gorr uses Millenium for his replating, but it would be nice to have such a good engine builder take a look at the work and handle your parts.

Share this post


Link to post
Share on other sites

So after a few days i got some free time again and started to really inspect the cylinder,

What we thought was a crack, is a higher point in the wearing.

Jesse qt motowerx believes he can repair it, so ill Be sending it off to him soon to get that repaired and built to a 144.:bonk:

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×