Wierd YZ125 problem....what should I do?

Y

yamikawi21

Member
Oct 21, 2008
19
0
I bought my 97 YZ125 two years ago. When I bought it it would usually start in the first 2-5 kicks. Since I bought it the number of needed kicks has gone up steadily. Starts easy once warm but up to last month it would take 20-30 to get it running.
I took it out couple weeks ago, it ran fine. Bogged out because I was in too high of gear, fouled the plug. (this has happened on my bike several times and was never really a big deal). Got it back and put in a new plug, could not get it started after that.
The kick-starter is REALLY easy to push, I can easily pull it all the way down with one finger...this would lead me to beleive theres no compression. BUT, I did a compression check, and after five hard kicks it reached 130psi, but wouldnt go higher than 135. I thought that 125-135 was adequate compression for these bikes? I havent replaced my top end since I bought it, and have probable put about 30-40 hours on it (not much for two years).
I have good spark and I know theres fuel because the plug comes out with gas mixture on the plug, just WILL NOT START no matter what I do.
I was ready for a new top end until I found how high the compression still is...but the compression meter reading does not match up with the ease of the kick-starter, so I'm confused.

I need some suggestions...should I do the top end, what could be the problem, why wont it start...? Any advice is appreciated. Thanks.

-CUBE-
 
J

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
What brand of compression guage are you using? If you can push the kicker down with one finger and it's actually turning the motor over it's definately toasted. Time to tear it down.
 
J

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
For sure, but 130 psi? Only Pred should be able to kick it over with one finger!
 
Y

yamikawi21

Member
Oct 21, 2008
19
0
Tore her down today to get some measurements for new parts.
Found that the piston ring had a 3/8" - 1/2" gap in it...its supposed to be like...no gap...right?
Thats really the only thing that looked bad, except obviously the gaskets were worn and destroyed in removing the cylinder.

(I didnt know the "top end" was so easy to remove/work on. After I failed at my fork seal attempt I was nervous for toying with the engine itself...but its Piece of Cake!)

Thanks for your opinions everyone. Ill let you know after the new parts are in.
 
J

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
Yeah, it's pretty easy. Ring end gap should be in the neighborhood of .012".
 
D

DieselX

Member
Jul 21, 2008
8
0
Good luck on the rebuild. It's going to be a good feeling when your bike starts after only a few kicks and runs stronger than before.
 
M

mathd

Member
Oct 11, 2008
208
0
My YZ125 99 is doing kinda the same except my starter is not easy to push.

The 2 ride before this one i had hard time starting it when cold.
Today i went to take a ride and i didnt start at all....
Went back home, checked the plug it was pretty clean but cleaned it a bit more with a shirt in case.

Checked if it made spark and it did, set the plug back on.. and still didnt start...
Removed the hose that get the gaz to the carb, let some gaz poor then replugged.2-3 kick and it started just right....
In past the moto didnt idle itself on neutral even when hot, now it do idle itself...

I guess my problem is that there was some dirt in the gaz inlet?
So i must be better cleaning the carb?

Also today i dropped the moto on the side(there was some ice..) and fuel was flowing, dont know from where but it had gaz dropping from the moto.. is that normal?

Thanks
 
Last edited:
R

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
If your ring end gap is 3/8" then it didn't just wear, the ring broke!

I would be really surprised that the ring could break and not really mess up the cylinder. Did you inspect the cylinder for any scoring? I would expect there to be some, probably on the intake side.

Rod
 
Y

yamikawi21

Member
Oct 21, 2008
19
0
the 3/8" inch gap is just a MAX estimate of the space in the ring while its on the piston. When I take it off it closes up tighter. Ill try to post a picture tomorrow.
Cylinder was perfecto, no problems there, thankfully.

Anyone know where to get a good deal on a top end kit?
Im going to pay $130 through Motosport.com and it includes piston, ring, clip, top end gasket kit, and needle bearing...
any better deals out there for that stuff?
thanks.

-CUBE-
 
J

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
That's about the cheapest, unless you know someone who works in parts or something.
 
R

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
yamikawi21 said:
the 3/8" inch gap is just a MAX estimate of the space in the ring while its on the piston.
Click to expand...


To measure end gap on a ring you take it off the piston, then slide it into the cylinder. Use the piston to push it down about an inch into the cylinder so that it is even all the way around.

Now use a feeler gauge to check the end gap.

You should also make sure that the ring can slide freely in the groove on the piston. There will be a tiny pin in the ring groove (on the piston ) that locates the gap so the ring will only slide freely a tiny amount. The key thing is to make sure that the groove hasn't collapsed anywhere.

Speaking of the pin in the groove, when you are putting the engine back together it is essential that you get the ring gap over the pin when you install the cylinder. The pin should be opposite the exhaust port.

It is also essential that you install the piston the right direction so that the pin is opposite the exhaust.

Rod
 
Y

yamikawi21

Member
Oct 21, 2008
19
0
New update! Need help still.
Got her all put back together, and what do you know? it wont start. figures.

I didnt measure compression but it seems a little harder to kick than it was before, not a significant difference though. It will just bluh bluh bluh and not turn over. fresh gas, plug, all the credentials. Still nothing.

Question: Will the piston ring placement (ring gap in the right spot) effect whether or not the engine will start?
I was relatively careful when putting the piston into the cylinder to make sure it was in the right place, but I cant be 100% certain. Could this be the reason for my weak-feeling compression on the kickstarter and/or the reason why it wont start at all?
In other words, is it worth my time to take the pipe and head off to make sure this ring is in the right spot?

Also, I dont have a spark tester but I connected a spark plug to the boot and held it outside the bike. Seems like only every 2-3 kicks produces a significant spark (that I can see). Is this normal, or could I have some kind of electrical problem?

Just trying to figure out why that damn bike wont start. Any ideas? PLEASE HELP!
thanks in advance.

-CUBE-
 
Y

yamikawi21

Member
Oct 21, 2008
19
0
^^^ read above ^^^
but i wanted to add this:

after finishing the top end job i had a few leftover parts from the 'top end gasket kit'. A couple of rubber rings (some kind of oil seal it looks like), and another gasket that seems to me to be a reed gasket (rectangular in shape). If I go back through to check on the ring placement, should I check on the reeds too? I dont know anything about the reeds, but I have a manual that will help.
-Could the reeds be part of my problem, and what should I check for when I get down to it?
-Also, I have heard that the power valve might also be part of this problem....where is the power valve and how could it cause my bike to not start.
-Could the air intake and filter be part of the problem, how? the filter has been cleaned and oiled recently, but it is an old filter....should I try starting it without the filter on?


So many questions, Im hoping theres someone here that knows enough to diagnose this problem......
If theres a mechanic or someone in here that could discuss this issue further, Id appreciate an IM at
g3cubed21 <--AIM.

Thanks again.

-CUBE-
 
J

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
What did you set the ring end gap to? As long as you line the gap up with the pin and made sure any markings on the rings were facing up you should be fine. Was the cylinder scored at all?

You need a multi-meter to test the ignition. There's specs for everything in the manual.

The reeds can make the bike difficult as well. It's definately worth a look if you have a gasket in hand.

The power valve won't usually stop the engine from starting.
 
2

2strokerfun

Member
May 19, 2006
1,500
1
As far as compression goes, a new top end typically seals better after running a bit. Now I'm guessing you didn't hone your cylinder, but I've also skipped this step before and engine ran alright. Check the reeds, have patience as you eliminate things.
 
Z

Zach10122791

Member
Nov 17, 2008
15
0
hey man i believe i can help i've been racing 125b and many harescrambles for a while and i only ride 125 2smokers and i'm my own mechanic, you need to rebuild this bike before you kill your cylinder and hope that it isn't already killed when i rebuild min they have between 165-180 and you should have rebuilt yours a long time ago go and get your parts you should be able to get a new piston and top end gaskets for around $100
Hope I Helped
 
Y

yamikawi21

Member
Oct 21, 2008
19
0

Got Wiseco Piston kit, and cometic gaskets brand new just installed. The current problem is that I just got the new top end in and cant get it started.
^ Please see above statements by me. ^
Hoping for some answers to my current questions. thanks.
 
Last edited:
You must log in or register to reply here.

Similar Topics

M
I cant belive I was this dumb am I screwed?
Replies
3
Views
709
truespode
S
YZ250 NOT STARTING HELPPP
Replies
1
Views
2K
jimboboss302
M
Pw50 rebuild problem...
Replies
2
Views
3K
The Redrider
G
2003 Yz125 Nightmare
Replies
3
Views
2K
truespode
Pw50 Refuse To Start
Replies
1
Views
8K
jaguar
Cookies are required to use this site. You must accept them to continue using the site. Learn more…