Jump to content
Sign in to follow this  

yz125 problems

Recommended Posts

Alright, I'm really confused as to what's going on, but here goes...

I got a yz125, guy told me it had no spark, so he figured it was the coil, or something electric.

I tested the bike for spark, and it seemed to have spark to me, (plug in cap, grounded on motor) I saw blue spark jump when I kicked it over. So I ruled spark out.

I thought maybe fuel, so I sprayed a little bit of starter fluid in the motor, but still nothing.

lastly, I did a compression test on it a little while ago. It consistently had around 60PSI after 5 kicks (while holding wide open throttle.)

Is that low enough for it not to start?

The guy who sold it said it ran fine, he quit riding one day then tried a week later and it wouldn't start. I know he could be lying, but I have no reason to believe so, seeing as it wasn't running.

Thanks for any help, guys.

:-)

Share this post


Link to post
Share on other sites

I don't think it will start with 60 psi. I think the normal low is around 120 or so and 160 being the norm. I would check the reeds and put in a new spark plug even though it sparks. Let us know what you find out.

Share this post


Link to post
Share on other sites

I tried a new spark plug earlier, but continued to not start.

I'm leaning towards compression. Would be more comfortable if I knew that it for sure wouldn't start with 60-70 psi.

Any suggested top end rebuild kits?

Share this post


Link to post
Share on other sites

I persoanlly ran pro-x in my 07 yz250 and I never ever had a problem. You can go with Wiseco, Pro-x, Vertex, etc etc. But personally i had good luck with pro-x:worthy:

But with that kind of pressure you bike i doubt can run. You rings are most likely wore but i would just replace the top end just for the insurance of the bike.

And BTW my buddy had an rm125 and it had no compression at all. Even a 65 had more PSI. But he said he could only start it by bumping it but it would bog.

I would start off by puting a top end in your bike. MAybe while your in there, maybe check and clean your powervalve. Also check your reeds to see if they are sealing properly:thumbsup:

thanks man and god bless

Alex

Share this post


Link to post
Share on other sites

Thanks for the reply!

Very helpful info there :-)

I was thinking that as well.. Even if it did run, 60 psi is much too low for it to be in good shape. I'll tear into it on Tuesday and see what I can find out. I'm going to clean just about everything that I come across, replace the top end, and then go from there. Hopefully that'll be the problem.

I tried bump starting down a hill a few times earlier, but had no luck.

Share this post


Link to post
Share on other sites

Definently put a top end in it, 60PSI is wayy low.. as long as the cylinder looks good and everything and crank is in good shape throw out $110-$120 for a top end kit and you'll be good to go :bonk:

Share this post


Link to post
Share on other sites

Also if you dont have a manual i would recomend getting one. Although top ends on two strokes arent too hard but still, its always nice to have a manual by your side when your working:thumbsup:

Share this post


Link to post
Share on other sites
Alright, I'm really confused as to what's going on, but here goes...

I got a yz125, guy told me it had no spark, so he figured it was the coil, or something electric.

I tested the bike for spark, and it seemed to have spark to me, (plug in cap, grounded on motor) I saw blue spark jump when I kicked it over. So I ruled spark out.

I thought maybe fuel, so I sprayed a little bit of starter fluid in the motor, but still nothing.

lastly, I did a compression test on it a little while ago. It consistently had around 60PSI after 5 kicks (while holding wide open throttle.)

Is that low enough for it not to start?

The guy who sold it said it ran fine, he quit riding one day then tried a week later and it wouldn't start. I know he could be lying, but I have no reason to believe so, seeing as it wasn't running.

Thanks for any help, guys.

:-)

A 125 will run with 60psi compression no problem. My old bike have 85 psi when I bought it. It ran great for it's first ride, I changed top end after that.

I would suggest a new top end and extensive time measuring your cylinder and checking your plating though, because ideally you would want way more compression.

As long as the piston and cylinder are in decent shape, I think you're looking at the wrong problem. You have spark, you have compression, only thing I haven't heard you say is fuel. Start there.

Share this post


Link to post
Share on other sites

Well, now I'm confused, because some people have said 60 psi it will not start, and others have said 60 psi will start it..

I realize that 60 is weak and needs replaced regardless...

I thought I eliminated it being a fuel problem by putting a little bit of starter fluid in the cylinder then firing that. When I did that, I got nothing. No life came from it.

A day later, I tried about a teaspoon of fuel in the cylinder but still nothing. That's why I was sure compression, because I can see it has spark, but wouldn't start even by putting fuel in it.

Share this post


Link to post
Share on other sites
Well, now I'm confused, because some people have said 60 psi it will not start, and others have said 60 psi will start it..

I realize that 60 is weak and needs replaced regardless...

I thought I eliminated it being a fuel problem by putting a little bit of starter fluid in the cylinder then firing that. When I did that, I got nothing. No life came from it.

A day later, I tried about a teaspoon of fuel in the cylinder but still nothing. That's why I was sure compression, because I can see it has spark, but wouldn't start even by putting fuel in it.

With that low of compression it may not fire with the gas in the plug hole trick. But I can assure you if everything was working well, it should start with 60 psi. Not ideal though and it should be checked out and replaced, but still should 'work'.

Have you verified the fuel tank is flowing well?

That the carb receives fuel through inlet valve?

Pilot jet plugged?

Regardless you should do the top end. It could change things, but I would continue to go over your bike with a fine tooth comb while waiting for the piston and maybe replated cylinder.

Share this post


Link to post
Share on other sites

Alright, I'll go ahead and check those things out while I'm waiting on other parts.

Was hoping it'd just be the compression, but if it's fuel, it doesn't sound like it'll be a real expensive fix.

Just curious, because I've never had it done. About how much does a shop usually charge to re-plate, or bore a cylinder? I'm not familiar with any of that, and don't know what I ought to do.

oh, also, yes I did check that the fuel was flowing well from the tank.

Did not check the carb receiving fuel through inlet valve, or pilot jet. Bike is new to me, so I have no idea how to check those.

Share this post


Link to post
Share on other sites

Here you go sir http://www.powersealusa.com/

My dealer i work at sends cylinders out this place and they do a wonderful job.

Or you could always send your cylinder into Eric gorr for a 144 big bore kit:banana:

i would also try cleaning your carb out. And like i said and everyone said i would throw a top end in your bike. That will take one thing off your list to see if it will start. :bonk::thumbsup:

Share this post


Link to post
Share on other sites

60 psi is definitely far too low.

It's different for every engine, but typically anything below 100 psi means there's damage or extreme wear. My old 125's typically were in the 140-150 psi range when healthy, and 110-120 psi when well overdue for a top end. Some lower performance, lower compression engines might run with under 100 psi, but 60 psi is far, far too low for a YZ125. Not sure how one "ran great" with 85 psi, unless it was a bad gauge, or tested without opening the throttle.

Share this post


Link to post
Share on other sites
60 psi is definitely far too low.

It's different for every engine, but typically anything below 100 psi means there's damage or extreme wear. My old 125's typically were in the 140-150 psi range when healthy, and 110-120 psi when well overdue for a top end. Some lower performance, lower compression engines might run with under 100 psi, but 60 psi is far, far too low for a YZ125. Not sure how one "ran great" with 85 psi, unless it was a bad gauge, or tested without opening the throttle.

Ran great as in: ran fine for my first time on a 125. Obviously once the top end was changed performance was dramatically increased. I'm not condoning running bike's that worn out, just saying to keep searching while fixing the top end.

Share this post


Link to post
Share on other sites

60 psi isnt enough to run the absolute lowest a 125 will run on is 90 psi and even at that it must be pop started, yo hould put in new piston and ring or just a new ring, and inspect your cyclinder for worn plating and scrapage.

Share this post


Link to post
Share on other sites

Alright, well here's what's going on. I hope I can get some good advice on what to do. By the way, thanks a lot for the advice so far, it has been very helpful and is much appreciated..

I took the cylinder off today and found that the piston was very worn in certain places, also that the cylinder at some parts was noticeably rough to the touch. I have no doubt that it'll need to be bored out. I just don't know how much.

I don't know if it's ever been bored before, but ontop of the piston there's a number 53.9x (not sure what the last number was)

Just curious how much I probably ought to have it bored, etc..

Share this post


Link to post
Share on other sites
Alright, well here's what's going on. I hope I can get some good advice on what to do. By the way, thanks a lot for the advice so far, it has been very helpful and is much appreciated..

I took the cylinder off today and found that the piston was very worn in certain places, also that the cylinder at some parts was noticeably rough to the touch. I have no doubt that it'll need to be bored out. I just don't know how much.

I don't know if it's ever been bored before, but ontop of the piston there's a number 53.9x (not sure what the last number was)

Just curious how much I probably ought to have it bored, etc..

The number on the piston indicates its still the factory bore which is 54mm.

You have a few options from having your cylinder repaired to factory spec, over-boring to a big bore 144cc, or buying a new or used cylinder. It all depends on your budget.

What year 125?

Share this post


Link to post
Share on other sites

Alright great.

Sorry for my lack of knowledge, but I have to ask the question.. How can I tell if my piston has been sleeved? I've heard that if a piston has been sleeved, it cannot be bored.

Here are a couple pictures

http://i1207.photobucket.com/albums/bb462/mattr408/IMAG0008.jpg

http://i1207.photobucket.com/albums/bb462/mattr408/IMAG0009.jpg

It's a 2001

Share this post


Link to post
Share on other sites

You have it backwards... a sleeved cylinder will need to be bored and a new sleeve pressed in. A plated cylinder cannot be bored.

Looks to me like your cylinder has been sleeved though. You would be better off buying a used cylinder off ebay or similar and keeping it stock bore.

Share this post


Link to post
Share on other sites

Man this is getting more expensive every day.

I've been looking on ebay for a used cylinder, but most of what I've found has been all worn out and needs bored for like $200

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

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×