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Specific jetting and carb workings Q's

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I have read alot on jetting, how the fcr carb works but I want to know more. There are a few questions that came to me while jetting the 06 yz250F.

Does the pilot circuit only effect 1/8 to 1/4 throttle, or does it effect by a small amount the whole fuel curve, or throttle range? I looked over at the ThumperFaq and read the article about jetting the FCR, and looked at the graph and it seems that it does. Is this correct?

I noticed while jetting when the main jet was on the large size, it would effect where the fuel screw needed to be, as did changing the needle position? Can a too large of main jet or overly rich needle clip setting "over richen" the fuel screw and pilot circuit range of the throttle, does the MJ and needle position have zero effect on this circuit?

Should the bike shut off when the fuel screw is turned all the way in? I know about setting the FS and whether your PJ is too large by reading burned post but should the bike shut off if the FS is turned all the way in? I read if it doesnt this could be caused by a poorly machine FS, or is this caused by a pilot jet that is too rich or a PAJ that is too small?

While reading several different posts and articles on the range a fuel screw needs to be set I have seen 3/4 to 2, 1 to 2.5, etc turns out to be okay. Many state 1.25 to 2 as the preferred settings. Is the 1.25 to 2 turns the preferred as this would allow you a wide range of fine tuning FS adjustment for changes in the weather? Is there a right or wrong # of turns as long as your bike doesn't burp, bog, fart or stumble?

Overlapping Circuits. What if you have a problem with the bike sputtering at 3/16th throttle at a steady run, would this possibly be both the pilot circuit and needle position causing the problem? If the pilot can be set correctly can you rule out the pilot circuit being the cause?

Needle clip position. I have seen most people using needle clip 3 to 5 with a 7 clip position needle. If you have to use clip position 1 or 2, would it be better to richen up the pilot circuit some and use a lower clip position, or richen the MJ one size if you are on clip position 6 or 7?

I know many have had a problem with yz250f's idle hanging or climbing when descending a hill. I have solved this by turning down the idle a small amount. I have read the hanging idle can also be caused by too lean of pilot or FS setting. When the idle screw is set to high does the idle hang from the slide being elevated in the carb too much allowing more feul to leak by the needle and be added to the fuel being delivered to the carb?

One last questions about the JD red needle for yz250F. Is the red needle leaner in the first 1/4 throttle than the stock needle?

Thanks in advance,

Greg

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The more you play w/ them the more you will learn. If you have a slight bog when you first apply the throttle you may be able to fix it by raising the needle but raising the needle may be too much and it could be fixed by turning up the mixture.

If the main is too big it can make you too rich just off idle and you would try fixing it by lowering the needle.

Not everyone has access to a dyno like I am lucky enough to have. The dyno is the easiest way to figure out jetting. I have played around on it for almost 2 years now and I am getting pretty good at it now.

If you adjust the idle circuit it should not affect your top end and vice versa. But, if you have the needle pulled up high enough that it is allowing excess fuel to enter the venturi then you will have a rich mixture at idle.

The problem w/ the idle changing going up or down a hill would be float level probably. The carb is no longer level and this will change your fuel mixture b/c you will have a higher or lower float level.

I also think that in some cases the pilot circuit could affect you at WOT. Let's just say your PJ is too big and you have your needle one notch too high. I would think that there could be a little extra fuel that has not been burned off that could cause a rich condition. Whenever I have a bike that is doing something weird like that I get the idle set like I want it then run it WOT to see what I need to do to the MJ. Once I have the main set I can work on the needle. When the needle is done I can work on fine tuning it all to get a good transition from 0-1/4-3/4-WOT.

I don't know if maybe I have caused more confusion or helped.

I was going to post a pic of a little chart I made but photobucket is being all screwy right now and I can't get it loaded.

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Does the pilot circuit only effect 1/8 to 1/4 throttle, or does it effect by a small amount the whole fuel curve, or throttle range? I looked over at the ThumperFaq and read the article about jetting the FCR, and looked at the graph and it seems that it does. Is this correct?idle and just off.no effect on the others.

I noticed while jetting when the main jet was on the large size, it would effect where the fuel screw needed to be, as did changing the needle position? Can a too large of main jet or overly rich needle clip setting "over richen" the fuel screw and pilot circuit range of the throttle, does the MJ and needle position have zero effect on this circuit? has no effect on the pilot circuit.you could take the main jet out and the pilot circuit would no know the difference.

Should the bike shut off when the fuel screw is turned all the way in? I know about setting the FS and whether your PJ is too large by reading burned post but should the bike shut off if the FS is turned all the way in? I read if it doesnt this could be caused by a poorly machine FS, or is this caused by a pilot jet that is too rich or a PAJ that is too small? it should at least be really struggleing to stay running.

While reading several different posts and articles on the range a fuel screw needs to be set I have seen 3/4 to 2, 1 to 2.5, etc turns out to be okay. Many state 1.25 to 2 as the preferred settings. Is the 1.25 to 2 turns the preferred as this would allow you a wide range of fine tuning FS adjustment for changes in the weather? Is there a right or wrong # of turns as long as your bike doesn't burp, bog, fart or stumble?any more than 2.5 turns you run the risk of the fuel screw falling out from lack of spring tension.

Overlapping Circuits. What if you have a problem with the bike sputtering at 3/16th throttle at a steady run, would this possibly be both the pilot circuit and needle position causing the problem? If the pilot can be set correctly can you rule out the pilot circuit being the cause?disconect the tps and retest.

Needle clip position. I have seen most people using needle clip 3 to 5 with a 7 clip position needle. If you have to use clip position 1 or 2, would it be better to richen up the pilot circuit some and use a lower clip position, or richen the MJ one size if you are on clip position 6 or 7? no,thats not how jetting works.using one circuit to try to correct another will just have you chasing your tail.if the needle to be at clip one or clip 7 so be it.thats why the slots are there.

I know many have had a problem with yz250f's idle hanging or climbing when descending a hill. I have solved this by turning down the idle a small amount. I have read the hanging idle can also be caused by too lean of pilot or FS setting. When the idle screw is set to high does the idle hang from the slide being elevated in the carb too much allowing more feul to leak by the needle and be added to the fuel being delivered to the carb?when the fuel is not correct on the pilot ciruit the slide needs to be artificially high.causing the hang.this can be rich or lean.

One last questions about the JD red needle for yz250F. Is the red needle leaner in the first 1/4 throttle than the stock needle?i believe leaner than stock,but jd would be the one to ask.

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Burned thanks for the post and the information.

Jetting can be satisfying, but also frustrating at times. The information you have provided makes TBS an issue much clearer.

Thanks all, off to my real job now.

Greg

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Hanging idle

Hangine idle is likly too little slack in the cables! The spool on the carb has a "V" shapped groove and with two cables, opening and closing if the slack is insufficent the cable will get pulled into the crotch of the "V" and become just sticky enough to not close nicly. You can improve it by giving the cable more slack or you can fix it by removing the closing cable! Just take it off and set it aside as a spare opening cable, life will get better!

Needle jet size

What needle is stock? My chart says Red starts out (1/4 throttle range) the same as a needle ending in "Q" (JD's taper is different, complex two angle deal but the idle thru 1/4 is determined by the shank or straight part of the needle)

The last letter on the Kehein needles run like this:

"M", then next leaner is "N", then they skip O, then "P", then "Q".

Each is one very small step leaner then the one before, so if stock where a "P" then your Red is one step leaner at 1/4.

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I have changed my mainjets from a 178 to the 180 and then to a 185. When I did the roll off method from WOT to 3/4 throttle I got the surge as hawkgt mentioned is an indication of a lean condition.

I installed the 180mj, and the surge was nearly gone. When in 5th gear, the bikes pulls hard to a certain point then the best way to describe how the bike then ran was it entered a "cruise mode" when you kept the throttle pinned. at WOT. I put the 185 mj and I had the same experience. The bike will pull hard in 5th gear to a certain point then "cruise", and never hit the rev limiter.

What causes this "cruise" condition? Is this more an exhaust related issue? Is the main still too small or is this just the nature of the bike?

Mark on his 04 experienced the same condition with a 180mj, but he did hit the rev limiter in 3rd gear, but not 5th gear as I experienced on my 06.

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Greg,

Try a #190 main jet in your '06 YZ250F if the #185 seems too lean. The YZ250F has been using bigger main jets in the past few years, having #178-182 main jets as stock.

(send an email or call if you would like us to mail you the #190MJ)

To answer another question from before, the Red marked needle is a step leaner than stock in the 0-1/4 throttle range (larger diameter).

Thanks,

James

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Thanks for the reply James. My previous post was a bit confusing. Happens when you try to post a question on your lunch break.

I ran with the 182 in tonight and it seemed pretty good.

Here is what I was trying to say in the last post and after reading what I had typed I didn't convey my message very well.

178mj had surge when I did the roll off test.

180mj had just a tiny bit of surge. If you blinked you would of missed it.

185mj it was completely gone.

The 178mj seemed a bit lean. For two reasons, One the roll off method surge showed this and second, the bike made better power at 3/4 than WOT. After 3/4 throttle the bike pulled but almost seemed to struggle to make power. It didn't miss but just didnt feel right.

The 180mj the bike pulled longer at WOT than the 178mj did, but seemed to enter a "cruise mode" once the rpms built for a moment. No miss, no stutter, just didn't pick up anymore speed and the acceleration feeling was gone.

There really wasnt much change up top with the 185mj compared to the 180mj. Still hit the "cruise mode" but not the rev limiter.

Tonight I put the 182 main in with a 40 pilot (2 turns on fuel screw) and RN#4. The bike seemed to pull strong in 5th and keep pulling. I still didn't hit the rev limiter, but I ran out of kahunas to hold the gas pinned any longer because of the condition of the road I was on and backed off the gas.

I know the pilot isn't supposed to effect the upper end of the throttle range but does it to a certain extent? I was thinking I might have gone a bit too rich 185mj with the heat of last week we experienced.

Today its 15 degrees or so cooler than the last time I tested the bike and the humidity is much lower. It's so confusing at times trying to determine if the condition is lean or rich. Since the bike ran so well today with the 182mj I was thinking I was a bit rich last week.

I just gonna ride it now with the 182mj in. It doesnt sputter, miss or hesitate now. It pulls hard down low and there is more than enough reliable power to climb the hills around here, and plenty of 5th gear power if you want to open up the throttle. I'm sure the weather is going to warm up some soon here on the east coast. I will see how the 182 mj runs when 80's and 90's degree weather returns.

I will keep you posted as to how the bike runs.

Greg

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