Need feed back Yz 125 ice riding.

[jasper's YZ]

Did some searching on the board and saw a lot of info but for newer YZ 125's. Here are the cliff notes of my current situation and my question. I bought the bike used off of my friend and the jetting has been spot on all summer and fall. Enter winter and decided to put ice tires on it. At this point I lower the clip on the needle and increase the main jet from a 360 main to a 370 main. Also at this point the bike is losing compression time for a upper end job. After taking the piston out I notice some where that needs to be cleaned up with a mild bore job, since it has a sleeve no big deal. Go 1mm over and put a new wiseco piston in her. I richen the main one more time to 380 just to be safe. Mind you before this the plug looked pretty good with the 370 main on the ice. Break the piston in with a 15:1 ration like the manual says and the get ready to do some hot laps. Engine making funny noise have no idea what it is (I do now pre detination, sound like stoned in the engine) as I am coming in to pull the plug she briefly seizes and then breaks loose. I shut her down immediately. Next day I put a 410 main in. Runs better no more noises but unfortunate losing power and the damage was already done. Piston got so hot on top that it concaved. New upper going in tommorrow night. My question for you guys is why the drastic leanness and is a 410 big enough to start with with the hopes that it will either be spot on or too rich needing to the back down a little.

It would seem to make since that with a new piston and increased compression with the 1mm over bore it is drawing in alot more air and I need to compensate with more fuel. I am a 2 stroke newbie and absolutley love the bike. thanks guys.

Matt

 

[todd36]

I think your problem might start with you oil to fuel mixture. 15:1 is way to rich in oil unless the bike is jetted for that ratio which i doubt it is. I alway use the same mixture ( 32:1 Yamahalube R ) so my jetting is correct. Your main jet only flows a certain volume, more oil means less gas, less oil means more gas. I hope this makes sense. I ice race also and usually I go about one jet size per 20 deg F change

 

[jasper's YZ]

I also read that last night. I was only running it that rich to break it in. Did I run it in there longer than I was supposed to. I do not know if I am going to break this piston in like again that I would think 30:1 is fine for break in. What do you guys do?

 

[whenfoxforks-ruled]

I assemble the motor with the oil that will run in it. Just run the normal ratio. It is not like its a brand new bike, its an over haul.

 

[ignition retard]

I'm thinking it was the lower temperature that did it. The difference in gas per unit volume from 15:1 to 30:1 is only like 3%. The difference in displacement is about 4% (assuming 55 bore x 54.5 stroke). The difference in air density from 80F to 25F is like 11%, and thats assuming dry air, it was probably more humid in the summer. Those number might be a little off, but I still think its the air temp.

 

[jasper's YZ]

I would think a 410 jet would be safe enough to start with then work backwards if it runs like crap (too rich). Should I go bigger? A 360 to a 410 is a big jump even though there is a big difference in temp. right?

 

[whenfoxforks-ruled]

I like it for the main, but what about the others?

 

[jasper's YZ]

The clip is on the 3rd position and the pilot jet is a 35 I believe and the air screw is where the manual says it should be. Its just weird how the bike was running fine before I rebuilt the top end. I am just going to run the regular ration I aleays have and run the 410 and check the plug often.

 

[whenfoxforks-ruled]

What are stock settings, 30 and 340? Is that the 00 carb? I believe the pilot needs to come up more. Your manual should have a jetting correction chart for that model. The needle may have to be changed also. Someone put a sleeve in it, how well did they match stock port heights? The over bored 2t's I have messed with ran better on stock jetting, they went to US chrome, then Eric Gorr. What I remember of ice racing it is pretty much wfo, not where I would like to see a steel liner.

 

[jasper's YZ]

Stock is 35 on the pilot and 350 on the main. It had a 360 in it when I got it due to teh FMF exhaust system. Yes it is a 2000 also

When it blew up I had a 380 main and 15:1 mixture. Clip position in the middle at 3. Pilot jet #35

 

[whenfoxforks-ruled]

Well, if there is a safe place to start, go with the 410 and get a 45 pilot. The 30:1 should be fine.

 

[jasper's YZ]

Thanks you sir!!!

 

[Matt Fisher]

Also consider that by running 15:1 you actually leaned out the mixture a bit compared to 30:1. The additional oil molecules take up space in the jets that the fuel would have occupied.

 

[jasper's YZ]

Some people say that does not matter. I really have to believe that it does.

Thanks Matt

 

[jasper's YZ]

:cool: Well it was spot on this weekend. What a friggin good time. Thanks for the help. Great ice conditions and the bike ran sweet. I'm tired