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98 yz250 lean surge?

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So i bought a 98 yz250 off a friend and it runs great 1/4 throttle and up. but when your coming into a corner or you let off the gas and it goes to a idle it kind of surges. the surge is about 1sec long. i am thinking that the pilot circit is lean. i let the bike idle for about 5 mins and pulled the plug but it was black. but on the other hand my air screw is turned all the way in. i tried turning it out and the surging just got worse, it surged more often than with it turned all the way in.

as far as i know the motor is stock. i just put a brand new powervalve in it because the stops on it were worn out really bad.

the only aftermarket parts the bike has to my knowlage is:

fmf fatty pipe with stock silencer,

and vforce reeds.

 

my current jetting setup is:

pilot: 45

needle: n3vf with clip in 1st slot

main:172s

i ride in elevation from 1000-2000 ft and the ocasional 3000 ft

temps here are going to be 90s-low 100's. i like to get out early before it gets hot. humidity is pretty low most of the time, 10-30% (i live in the sonoran desert)

I have not done any wide open plug checks so i am not sure on how that jetting is. i would like to get rid of the surge and then go from there. any info would help a ton.

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First off, quit letting the bike idle for 5 minutes. 2 strokes have a tendancy to load up when letting it sit and idle. Can cause a fouled plug too.

Your symptom reminds of the pipe bang phenom. I experience that with my KTM 300. Everything I've read and been told says its nothing to worry about, its just a little annoying. The best and easiest way I've found to tune the pilot circuit/air screw is the "throttle whack" method. Just tune it so off idle response when doing that test is clean, no hesitation and call it good.

If the air screw is all the way in, that's the richest it will go. If it runs the best with it there, that's saying you need to go up on your pilot jet. I'm unaware of normal pilot jet recommendations for a YZ250, but that 45 already seems pretty rich for a 250 to me.

I suggest pulling the pilot jet and clean it and its passage. If afterwards the air screw still requires being almost all the way in for it to run the best, replace the pilot with the next larger size(48). That should allow the air screw to come out to a more preferred position of between 1-3 turns out.

Right now with the screw being almost closed, its at a minimal point in the screws influence, as it also is when past 3 turns out.

Do the suggested things first and see if you can tune out the pipe bang, if thats what it is, but tune for best off idle response as suggested, with a pilot jet to allow the air screw between 1-3 turns out. If you can't get rid of the symptom, don't get hung up on it. I have just learned to adjust my throttle input and not just chop the throttle to idle.

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First off, quit letting the bike idle for 5 minutes. 2 strokes have a tendancy to load up when letting it sit and idle. Can cause a fouled plug too.

Your symptom reminds of the pipe bang phenom. I experience that with my KTM 300. Everything I've read and been told says its nothing to worry about, its just a little annoying. The best and easiest way I've found to tune the pilot circuit/air screw is the "throttle whack" method. Just tune it so off idle response when doing that test is clean, no hesitation and call it good.

If the air screw is all the way in, that's the richest it will go. If it runs the best with it there, that's saying you need to go up on your pilot jet. I'm unaware of normal pilot jet recommendations for a YZ250, but that 45 already seems pretty rich for a 250 to me.

I suggest pulling the pilot jet and clean it and its passage. If afterwards the air screw still requires being almost all the way in for it to run the best, replace the pilot with the next larger size(48). That should allow the air screw to come out to a more preferred position of between 1-3 turns out.

Right now with the screw being almost closed, its at a minimal point in the screws influence, as it also is when past 3 turns out.

Do the suggested things first and see if you can tune out the pipe bang, if thats what it is, but tune for best off idle response as suggested, with a pilot jet to allow the air screw between 1-3 turns out. If you can't get rid of the symptom, don't get hung up on it. I have just learned to adjust my throttle input and not just chop the throttle to idle.

i think it could be what you are saying. it only does that when the bike is warm. when you first start it it runs fine, and riding it its fine until it gets warm. then it starts surging. i was looking up fmf's jetting segestions and according to them they say start at:

main: 165

pilot: 48

needle: stock 3rd position with the air screw 2 turns out.

maybe whoever put the pipe on didnt jet it?

im still somewhat confused on it. i just want to be safe, i dont want to blow the top end because of a lean condition.

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Increase the pilot, your A/S should not be less than 1 turn out, if it is more than 2.5 turns out you are too rich... buy a 48 and a 50, buy a few different main sizes as well (one or two sizes richer and leaner so you have options as the weather changes). You're looking at about $25-30 for all of them. You def want to be jetted rich rather than lean, however your odds of seizing the motor because of a lean pilot are slim to none. 

 

Read this article a few times... it will clear things up.

http://www.kawasakimotorcycle.org/forum/kawasaki-motocross-offroad/17227-spankeys-jetting-guide-info-2-stroke-riders-must-have.html

Edited by BDubb106

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