yz125 top end rebuild...

[char393]

The hour meter on my '01 yz125 is currently reading 27 hours, that's the amount of time I've put on the bike. I have no clue how much time the previous owner put on the bike, although judging from the lack of frame paint rubbed off and the general clean condition, I'd guess very little, maybe 10 hours at the most? I did do a compression test and got 150 psi, but I didn't have a baseline to go off of, so I don't really know how much stock to put in that. Besides that, based on the amount of time, I figured it needed a top end anyway. I will do a compression test once I get it back together, another when the rings are seated (just to satisfy my own curiosity) and I'll do them periodically after that.

I had to wait a little longer than I expected to for the parts to come in...I opted to go with OEM parts, except for the gaskets which I got from our very own Vintage Dave and [Only registered and activated users can see links. ], he ships very fast and the price was very reasonable for a complete set of gaskets.

So...here's my progress so far (I apologize for the quality of some of the pics, it's really difficult to get a good pic of the cylinder):





Looks like a little blowby on the ring....right Motoman?

Have yet to pull out the powervalves and clean them...


And the cylinder..again, sorry for the poor quality..









Aside from cleaning the powervalves and seeing if I can't get a local shop to run a hone through the cylinder for me real quickly, it's ready to go back together. I plan to check the reeds while I've got the bike apart and clean the carb & check the floats as well.

More pics to follow...hopefully it'll go smoothly for me. If I can't get it back together, I'm sending the motor off to the Honda shop in Americus, GA...I'm they can fix it for me...

[OLDMXRACER 393]

I highly recommend Honda of Americus Char. They have some famous customers!

[crdemon]

Looks pretty good. Do you really do a top end every 30 hours?? I dont ride my bike real hard but i try for once a year and a good year is a mimimal of 100 hrs

[char393]

First top end I've done on this bike. I probably could have let it go longer, but I'd hate to let it go too long and end up breaking someting real bad...

[crdemon]

Very true. Thats kool.

[MotoMan]

Lookin good Char....

[Faded]

Sounds like you know what you're doing, good luck! FWIW, check the head for warpage (see manual), it 'looks' suspect. Most local machine shops will run a hone through a cylinder for low ($5) or no cost, just be sure to specify hone type (aluminum oxide) and grit (240/320). Oh yea, make a note of your current jetting so the next time you crave a little more power you can play with $5 brass instead of $200 pipes/reeds/etc.

[kwmotocrosspunk]

i highly recomend Americus, they just installed my Turbo Bearings for my reed valve, only cost me $300!!! plus shipping

alrighty char! way to go and get it ready for Cycle Ranch!

[boogieonmyfinger]

Americus did a complete seat foam rebuild, they drained all the loose fluid from my gas tank, and they added a traction coupler for free. It was only 435.00 out the door. I'm taking all my bikes there.

[DRGSin393]

Originally Posted by boogieonmyfinger

Americus did a complete seat foam rebuild, they drained all the loose fluid from my gas tank, and they added a traction coupler for free. It was only 435.00 out the door. I'm taking all my bikes there.

Man, I bet Tryce at FFR couldve given you an ATM discount.

[Henry]

Originally Posted by DRGSin

Man, I bet Tryce at FFR couldve given you an ATM discount.

doesn't he give ATMers discount.

[char393]

Well...there was a screw that held the pushrod (?) for the powervalve in that I couldn't seem to get off. Daniel got home about 7:30 or so and had it off in about 5 minutes . He used the impact screwdriver...which I had tried, but I guess I didn't hit it hard enough or just don't know how to work the thing.

Once that was done, everything went pretty smoothly. Also cleaned the carb and took note of which jets are in there as well as needle clip position...so when you're ready to answer my silly questions Faded...I'm ready.

I'm happy to say the baby yz runs! Funny how relieved I felt when she lit off. YAY!

Now I just need to wait for the weekend and hope we get some riding time in.

[DRGSin393]

Nice job. just curious, what kind of compression did you get after the rebuild? Also, if you or anyone else can chime in on what the best break-in procedure after a rebuild is, that would be cool.

[char393]

I'll check the compression tomorrow, had to get the kidlets into bed.

When it comes to break-in procedures, the opinions seem to be as varied as those you'd get for what kind of oil is best....

What I've done in the past is let the motor warm up, ride at varying throttle through the gears for a half hour or so....give or take I guess. After whatever I deemed as "enough time" thenI just ride like I normally would. Of course, none of that applied to MX riding. I've heard to do 3 heat cycles, not to heat cycle it, to run a full tank of gas through it before running the bike hard...you get the picture.

I'd be curious what Faded's break-in procedure is...

[MotoMan]

Originally Posted by char393

He used the impact screwdriver

[DRGSin393]

Originally Posted by Henry

doesn't he give ATMers discount.

no...just to ATMers.

[char393]

Cool, thanks Faded.

As for the jetting...I suppose not hearing the bike run puts you at a disadvantage. I believe it was running too rich before as I had a serious spooge problem. I changed the silencer packing and that helped some. This last weekend the temps were a good 15-20 degrees cooler than I'd been riding in and there was no noticeable spooge dripping from the silencer.

Throttle response seems to be good, although I do notice it bogging out of turns once in a while...I usually contribute this to myself being a gear too high though. Anyway, the temps here vary so much, it's pretty wild. Today we're right around 60 degrees, I'm guessing fairly close to sea level in elevation and my jetting currently sits at this:

Main 460
Pilot is 32.5
Clip is in the 3rd position on the needle.

I wish I could tell you that I know my bike well enough to describe exactly what it's doing, but I don't. I get on the thing and I ride it. I don't know if you have any suggestions based on the lame pics I posted, but I'm certainly open to anything you've got. I'd like to learn how to tune it better and I'll try to be a good student.... sort of.

[yzernie]

Dang, a chick that rides AND works on her own scooter. Daniel is a lucky man.

[ossagp1]

I think anything you do short of seizing them is at least ok. I have over the years heard just about everthing. Steve is the first one that I have seen advocate breaking them in on a stand without a load it a long time. I didnt have good results with that when I used it in the 70's, but maybe metalurgy has changed enough. Ran it by a supplier of mine a few years ago who was in the bore and stroke business. He does it on a dyno which is much different. He was convinced as I was that a nonloaded breakin was more likely to create a glazed cylinder. You need some pressure to get the rings to seat. The tension isn't enough without it. Maybe it is my diesel heritage or bad luck but I got more blowby than I wanted by doing it on a stand.

With a properly prepared cylinder I am sure it is easier to get a "seat". I still like a dry cylinder wall, a few minutes to get to temp,,and then put it under load without getting it hot enough to seize. A rough MX course is just about perfect, with no sand or long uphills. FYI that is some 25 plus year old advice from Harry Klemm. He was dealing with plated cylinders at the time for a factory race team, and it has worked for me as it did the supercar builders.

[Aus_Rider_22]

Interesting thread! The photos bring back memories of doing top-ends on the ole YZ!

As far as I can tell you've got the stock mainjet, richer pilot (stock 25).
There must have been some changes between 00-01 as my YZ's stock jets were 350 and 35 respectively.

Hope this helps