Yamaha yz125 carb adjustments

[Moto man26]

Hey guys I'm having trouble making my yz125 idle ...When I don't rev it it just dies out and then I got to start it again...Does any of you guys know wht I should do as adjustments. THX!!

[vintageguy]

Try playing with the idle screw or air fuel mixture screw on the side of the carb mine is a flathead but u might have to take it apart and rejet or something else, reeds, new top end, ect. my bike had the same problem playing with the fuel mix screw fixed that, hope this helps.

[bigtree686]

you may be a little rich at the bottom. may need a leaner pilot jet. or turn your air screw in a little. or just turn up the idle a little. just a couple ideas!

[NYMXer]

The idle adjustments are easy...........

1.) adjust your throttle cable free play
2.) start bike and warm up
3.) once warm, turn idle up a little high at the adjustment and then adjust low speed pilot screw to achieve the highest possible idle w/o turning any other screw
4.) adjust the idle screw down to an acceptable level

That's it. This assumes no mechanical problems or maintenance issues

[the_privateer]

i had the same problem with my '00 yz125, when no amount of idle/air screw adjustment would remedy the problem i dropped one size on the pilot jet and that did it, other than that i'd follow what nymxer said, except, and I'm just asking, do you really want to adjust the air screw until you get the highest possible idle, it seems like you could end up too lean on the idle doing that?

[NYMXer]

Yes you want the highest idle. That is where the pilot circuit is most efficient and gives the best off idle performance.

[the_privateer]

cool dude i guess i'll re-adjust mine and see how much it benefits, I've always just adjusted it until it idled good and steady

[Aus_Rider_22]

I set my airscrew by idleing the engine up(at operating temp) pretty high with the idle screw. Its black, kinda right above the airscrew. It might be different than on your carb, but probably close to the same area. Then I turn the airscrew "in"(clockwise) until it almost stalls..then I go the other direction until it begins to idle rough. In between those "conditions" it should reach a maximum idle rpm. You should be able to determine where that is by ear. That is what they call the "sweet spot". Then I usually go about a 1/8 to a quarter turn in the richer (clockwise) direction.

Your airscrew should be set. Hope this helps.

[ossagp1]

If I might add something that isn't obvious to some: When you start setting the low speed air screw, and you are looking for high idle, sometimes it will idle at it's highest with a miss or slight miss, but will still idle higher than a richer setting. Set it at the highest idle without the miss if that is the case. Many times that miss is caused by a crank seal that is going bad, or another air leak. And sometimes for reasons you may never know, it is just something that one bike does and another doesn't.

[Faded]

Originally Posted by Aus_Rider_22

That is what they call the "sweet spot". Then I usually go about a 1/8 to a quarter turn in the richer (clockwise) direction...

I usually end up here after setting the high idle and adjusting the air screw for throttle response. On the stand there is no load and adjusting for the highest idle will get you very close, but off the stand and under load a slightly richer setting works better for me.

[ossagp1]

It kind of depends on what I am going to be riding. Funny we are talking about yz 125's. on mine I like to set it at the highest idle speed when I am going to be using it on trails. that means i will be riding it long,,and heat will build up in the slow going,,,for anyone that has a little bit of dirty running in those conditions or on long downhills, you may want to give the highest smooth idle setting a try. on other bikes that i used for mx or flattrack,,they are on the rich side since the engines are seldom really warm when the race is started. And after it has started they are seldom at low speeds.

Most do as Faded suggests for all types of riding. It varies from bike to bike,,but few set the low speed when the engine is really hot, so on one that is going to run really long and hot, I set it like NY suggests. It is worth a try.

On other bikes like my cr500 I have a setting I used for really cold starts,,then I reset at the first opportunity.

[Faded]

Originally Posted by ossagp1

It kind of depends on what I am going to be riding.

Andy, great points. A perfect example why jetting is so dependant on the rider, his terrain and the surrounding conditions. I don't do much trail riding, most of time it's on/off stuff. Jetting can be as personal as suspension set-up/preference. Your results will vary!

[ossagp1]

Yes Andrew, it is funny how two guys with seemingly identical bikes who ride the same course can require totally different jetting. (I knew two such people who were brothers and rode identical Hodakas, which were just like the ones in the Steen catalog.) Even when they seem to be running the same speed.

That is why they make the things adjustable (unless you have the "approved" rigs).