Yz 125 2005 need help!

I bought the bike a few days ago, it has a big bore kit and new reeds and other motor work done, I don't have a whole lot of info on the jetting because the last owner just bought it and drove it, he is kind of dumb to dirtbikes and how they work, but he said there was carb issues after his last race , I changed the plug with a stock plug right away and tore down the carb, right away I notice the pilot jet was missing, so I borrowed my cousins for a 2000 model yz, I don't know if the sizes or different or not, but after I got it in and put together it will run great for a few minutes and the die like it is out of gas, but it will crank right up afterwards, I've did everything I know to do and can't get it right, I don't know what the pilot jet size should be or if that's the probelem , I run a 32:1 ratio and also my air filter is in ruff condition, I don't know what to do so any help would be much appreciated

Found this on woodsracer.com:

I've not heard much complaint about the stock YZ jetting, for MX, but if your running 87 octane at 32:1 and not getting any ping under hard loads, then I'd say it's a little fat.

Your stock jetting should be a 40 pilot, 410 main, 6CHY5-80 needle in the third clip.

The Mikuni TMX carbs are very sensitive and somewhat tricky to keep jetted spot on, so go get yourself some jets, you'll need them.

From there I'd go to 40:1 on the mix with at least a 91 octane fuel. The BR8ES should work fine, but I agee with bikepilot, the BR8IEX are a little harder to foul and going to a 7 heat range may not be out of the question. It just depends on what your doing with the bike. If your just polking around on goat trails then it's very possible you might have too.

But, I'd start with the fuel mix and dropping the main. From what you've said a 390 main would be a good starting point, if the jetting is stock.

You got to remember that most MX bike jetting specs are developed in So. Cal. on fast MX tracks. That means a lot of high speed wide open runs on plowed dirt. That translates into a lot of loads and heat on the motor so, they have to be jetted for that. If you're goat trailing in WV you may never see those kinds of loads, even blasting down fire roads.

So, it could very easily come down to running the stock jetting with a hotter plug is the way to go for you. It just depends on what you doing with the bike and your riding level.

Edited by ozmx

Well I run it fairly hard, in in the band 90 percent of the time, the jets don't seem to be the probelem because if the way it runs while it's running, the bike runs great the stalls like its out of gas, but last night I noticed the carb was pissin out gas it has continued to do it the morning, so I think I have a float probelem , but as much as I hate to say I have some make shift premix in it for now with 87 octane , the closest place that sales good premix is a hour away and haven't ran good oil and gas in ur yet, but I plan on trying it all out today so ill let you know what happens

And also since the pilot jet was not in the carb I don't know about it but you can almost fit a stock main jet down the hole of the one that is in it, this huge jet may be required for that 164 bore kit but I'm not sure, the bike has a splotchy past, it was a pro circuit bike the frame and all was built , it definately has a lot done to the motor because I've ridden many 125s and it has a different meaning for power what little I've ride it, but I can't find out everything that is in it or really anything, but stock jets are probably out of the question, I plan in starting with a 440 main and 45 pilot and working my way down

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