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Easy question for most of you...

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Hello family! I'm new to the forum. My name is Chris and would like to start by saying hello. I have an easy question for most of you. I had just bought a 05 YZ250F. I have never wrenched or owned a thumper b4. There is a black knob on the drivers side of the bike on the lower left hand corner of the carb. What does it do? lol. If I have read correctly, its the fuel/air ratio screw? And do you use this to adjust the idle or is there another mechanism to to that? I apologize for my ignorance. I'm pretty handy with a wrench, but being able to take something apart and put it back together is useless unless you know what does what imo. thanks for any and all help in advance.

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Welcome to the forum, I see this is your first post.

Do you have the manual for the bike? If so, check it for specifics.

I can take a guess... if its black plastic, then I'd say its the idle speed adjustment.

The fuel screw is under the carb, center front, and shrouded by the float bowl.

Test by cranking the motor, then screwing it in and listen for an INCREASE in idle speed. Adjust idle speed to a steady idle between 1300 and 1500 rpm or so. Since there is not a tachometer, its a guess on your part. I adjust mine to where it doesn't pull when I let off the throttle and doesn't stall.

Usually you don't adjust the fuel screw unless its popping on decelleration, which is a lean run condition; or bog which can be rich run.

Good luck.

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Thanks for welcoming me. Much appreciated! No manual yet. I ordered one and its on its way. I literally just got the thing a couple days ago. I'd love to take it out tomorrow but I don't want to do any damage riding it while its running rough. And the popping on deceleration is exactly what I'm trying to cure. The idle seems to be a touch low also. It stalls. The thing starts dead cold first kick consistently so I'm hoping it is just a jetting/ air-fuel issue opposed to values or something. I hope the photo I attached works. The red circle indicants the screw in question.

<a href="http://s573.photobucket.com/albums/ss171/Fistfullofthrotl/?action=view&current=airfuel-1.jpg" target="_blank"><img src="http://i573.photobucket.com/albums/ss171/Fistfullofthrotl/airfuel-1.jpg" border="0" alt="Photobucket"></a>

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OK, I see it fine. That is the idle screw. For the fuel screw, look up under the front of the carb. It's in a recessed area. To get to it, you need one of those straight screwdriver bits that's about 3/4 to 1 inch long. William1 has the routine for setting the fuel screw. Before you start messing with it, you should record where it is set now before adjust it. Engage the bit, then turn in slowly in 1/4 turn increments until is seats lightly. You do not want to screw it in tight, that will damage the seat and the brass needle. Then back it back out to where it was set before. Crank it, warm it up, and follow William's instructions. If it needs a rejetting or anything, it will show up after you go thru his routine. One more thing, when adjusting the fuel screw, let the idle resettle, then gas it a couple of times to see if idle speed changes. Adjust idle speed accordingly if it changes.

I searched and found his post:

http://www.thumpertalk.com/forum/showthread.php?t=920969&highlight=set+fuel+screw

Edited by ronbuell

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As others have said yes that knob is for the idle speed adjustment. It physically raises the slide to make it idle higher. The idle mixture screw, which is on the bottom in front of the float bowl, adjust the air/fuel mixture at idle and in turn will adjust the idle speed as well.

Do yourself a huge favor and get an extended fuel screw. Pays for itself the first time you use it.

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Aluminum isn't softer than brass, but generically it will tend to break rather than bend. Get the one from JD jetting, word has it he knows a little about these things. I got the one from TT and it sort of sucked until I made a round knurled knob for adjustments. You should get one of their jetting kits while you're at it, too.

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Thanks for the suggestions fellas. I do have a question to branch off a little bit. If my valves are out of spec, does this make it much harder to get a good tune? I just got the bike and doing my homework reveals that maybe I should check my valves before I even change anything in the carb. Apparently, out of spec valves will change the jetting? Maybe it would just be better to see for myself where they are. This way i could monitor them better. A jet kit is in order any way so I'll def order one of those. I'm at bone stock for the jetting now. 182S main, 42 pilot, and 72(opposite the pilot, Idk what it is for. starter air jet?) air/fuel screw out 3 turns. 48-55 degrees F, elevation 0 ft., charleston, sc coast, bike ran pretty good about a week ago when I got it, but it was about 10-14 degrees warmer out, ...I understand I have to get all this done anyway. I'm just blind as to what would be better to start with..I'm a rookie with a 4 stroke, but a pro with a wrench. Basically would it be better to check valves first, or jet it richer to see where I stand with that?

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The jet opposite the pilot id your enricher jet. It's my understanding that that is for the choke.

At three turns out on the fuel screw, you're risking having it fall out. If you're over 2-1/2, then bump up your pilot and start the process over. In colder conditions you will need to jet richer. If you are having trouble starting in the cold, you might have luck bumping up your enricher jet, too.

Checking your valves is never a bad thing. All you'll really need is the manual and a thin set of feeler gauges (metric works best).

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First off, I'd like to thank all you guys for taking the time to leave your feedback. I think I let the excitement of riding the new machine blind me from whats really important...proper performance. LongerDeeper, I did a search on google for manuals online and found one. Thanks for ur post. My last bike was a Honda pinger. I could never find one for free when I first got it so I had to go buy one. I assumed (thats were I first went wrong) that it would be the same for all bikes. Ron and 762, Thx for letting me know about that fuel screw. Mine is such a pain in the arse to get to and that will make thing much easier when that time comes. Thanks to everyone for their opinions. I have the day off so I'm going to get started checking all clearances inside the motor and work my way outwards. I got my pad, pen and camera to thoroughly document everything. Is there a spot on the forum to post pic of projects like this? Should I start a new thread or just keep it in this one? Or maybe just keep that kind of stuff off the forum. Idk exactly what the rules are and how everyone feels about that stuff.

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OP, welcome to TT! :bonk:

Here's a great article I found that really helped me answer questions about how the fuel screw works and how to adjust it.

For new four stroke owners it's a great read.

http://www.thumperfaq.com/fuel_screw.htm

Pictures are always welcomed and appreciated here. It may help somebody else doing the same project, or help us identify what you are talking about if you run into any problems and need another TTer to answer some questions. Just post the thread in the wr/yz 250 section.

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Yeah man start a new thread with pics and progress. Ask questions along the way. You asked earlier about your valves. You should check them. If they are tight it might make it harder to start cold. If you are going to do any mods to the bike such as cams, exhaust, big bore etc etc. then your jetting may change after that. The JD jetting kit is great. Gives you precise instructions for your elev and temp.

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