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2007 YZ250F Coughing and sputtering at low rpms, advice?

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Hey guys I am new to the forum and new to 4strokes so please be patient. I have been doing my homework and research for days and I have now decided to ask my question.

I bought a 07 yz250f a week ago and rode it one time, the bike was backfiring and popping horribly I mean baaaad. So I took it in to the shop and we worked on it for two days. We took the carb out and worked through it, changed everything that needed to be changed, its got new oil new plugs new air filter new fuel ect ect. We put it back together and still nothing. Then we looked for leaks and found the problem.

Now for the problem at hand. We started the bike and still had a little pop and just wasnt running right, so we have it now running the best it has since I bought it. The settings we had to use were on the fuel/air mix screw, its betweeen 3 and 4 turns OUT! With the screw that far out Im not sure what to do, Im still getting coughing and sputtering up to about 1/4 of the throttle, as you get more into the throttle and higher rpms it runs great. When I start into corners I gear down then start to give it to it on the way out and it wants to stutter and putt till it gets the rpms up and then takes off great. I want to get that kick off the low rpms to get me going. I keep reading about O ring Mod, Leak Jets, Pilot Jets. I think some ppl are using leak jet and pilot jet interchangably. Whats the difference? WE are almost positive its running way to lean, we base this on the white spark plug I took out of it, we have it running about as rich as the bike will allow.

As far as I know there have NOT been any mods on the jets or any other part of the bike.

Im not sure if I need a lower # leak jet or what? Im also afraid If I do the O ring mod and then it runs better or worse then change somthing else Im not sure if the O ring mod would hinder the results. Should I wait till last to try this and change the proper jets first? Im just not sure what to try first.

I also read about a adjustable leak jet, I dont care to spend the money to make it run right, I just dont want to spend the money on something I dont need.

Thanks in advance for all answers

Oh yeah I am running this bike in eastern kentucky mountains, its been f ing hot and extremely humid here and I expect it to stay this way for the next few months.


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FCR-Tuning Guide

Buy a new pilot jet, cleaning rearely works.

No, right now for sure, you do not need an adjustable leak jet. Forget about any bog/AP/leak jet/oring stuff untill you have the bike starting and idling correctly. Then make sure you have the right main jet in it, then work on the needle. Then and only then, can you deal with a bog and the AP stuff.

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Thanks for your reply, I know my question was a little jumpy it was late last night. But The bike starts great and idles perfect. Only issue Im having right now is the sputtering or coughing that I have at low rpms. My fuel screw is almost out as far as it can go, its about 3-4 turns out.

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3-4 turns??? You are lucky it has not fallen out. It should never be opened more than 2.5 turns. Why is it so far open? Did you do this to attempt to compensate for some other jetting or running issue?

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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