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How to test your jet setting.

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Many of the posts here recommend exact jets, and how to correct problems. I would like to know how do you test for correct or perfect jetting. What exactly does it feel like in a lean or rich condition on both the needle and the main jets, and the transition between the two. The pilot circuit has been covered very well and is a sticky. Hopefully an explanation can be give that doesn't require a dyno or air fuel ratio gages. Also, the assumption should be that the bike runs and does not have "major" issues with any of the circuits. Please give me your "how to test for perfect jetting." Your help will be much appreciated, and I think it will help many others and is not bike specific.

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I think perfect jetting can be achieved with enough trial and error, but I think you can also drive yourself crazy in the process. I consider perfect jetting as this. The bike idles well, hits hard off the bottom, doesnt miss or sputter, pulls clean and strong throughout the entire throttle range and small changes in the weather can be corrected with the fuel screw adjustment.

For example, my 06 wr450F with the JD kit red needle 5th clip and FMF factory 4.1 has run very well from 60 degress to over 90s degrees. That is close enough to perfect for me. Last sunday was the first time this year I had to move the fuel screw from the 1.5 turns it has been set at all year. Its was extremely hot and humid and to compound things my air filter was on it's third ride. It needed cleaned, and was contributing the slightly rich condition the weather had created. I just got the yz250f jetted so it's pretty early to tell. Sunday I did have to lean out the fuel screw 1/2 turn on Sunday, but again I think the weather extreme was the root cause.

There are some good articles at thumperfaq. They are pretty informative. There are also links at the bottom of the jetting article to questions posted here at thumpertalk that cover bog, rich or lean how can you tell, want more mid range power, etc...

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

I settled on this process:

Mark your grip using tape and a marker or put dots on your grip using liquid paper. Take the slack out of the cable and mark this spot as zero. Use the split in the throttle housing as a reference. Mark 0 (idle) then full throttle, then eye ball 1/2 and then mark 1/4 and 3/4 throttle. This is a big help while jetting to see where your sputter might be according to throttle position.

Set your mainjet first. You can do this by seat of the pants or by the thumperfaq article way. The main is tested from 3/4 to full throttle. I have read if the bike runs better power wise at 3/4 throttle, or seems to have more power than at full throttle then you are probably lean. Say the bike actually feels as though it has lost power at full compared to 3/4. If the bike continues to pull after 3/4 throttle but feels "fat or heavy" or sluggish to gain rpm then its probably rich. If the bike pulls hard from 3/4 throttle to the rev limiter or to the size of your "kahunas rev limiter" then your mainjet is probably set correctly. You can also verify by doing the spark plug test mentioned in the thumperfaq article.

Then set your needle position. This is to make the bike driveable between 1/4 and 3/4 throttle. I have found if the bike sputters some then its probably rich and you need to raise your needle clip towards the top of the needle. Needle position is tested at 1/4 throttle, 1/2 throttle and 3/4 throttle. You could also be lean, so it might be better to try +/- one clip position or more from your current position and see what is better.

Then set your pilotjet and fuel screw. Pilot controls 1/8 to 1/4 throttle and the fuel screw controls idle to 1/8. As you mentioned Burned has a sticky in the jetting section of the forum for testing the pilot circuit. Burned mentioned disconnecting the TPS is there is a low end sputter at constant throttle that will not go away. See if the TPS is causing it rather than your jetting.

There are several ways to adjust the fuel screw. Some raise the idle slightly and slowly turn out the fuel screw while listening for the peak engine rpm and the snappy exhaust note sound. Other test and set for the best off idle throttle response with a smooth idle.

Once all the major jetting has been performed, you can now time your Accelerator pump linkage (remove all the slack from the accelerator pump linkage), add an oring the linkage assembly for a faster more responsive response to the throttle being opened and put a smaller leak jet in to add more fuel when the throttle is cracked.

I also posted some specific questions at this link. It made jetting a bit clearer now on isolating what circuit and how the other circuits do not really effect one another. Read Burneds' reply. http://www.thumpertalk.com/forum/showthread.php?p=3662067#post3662067

Here is what Burned wrote:

"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 up 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."

If anyone has any corrections please make corrections. I have tried to summarize what I have read and what I have tried.

I truely think you can drive yourself crazy when trying to micro tune the bike. The weather has an effect, your air cleaner cleanliness, probably even the gas to some extent. I would truely shoot for a jetting setting where the bike runs well throughout the throttle range without any sputtering or missing. I to have tried to microtune my bikes, but found myself turning wrenches more often than roosting dirt. Don't get me wrong, I am not happy and I can't stand a bike that misses or sputters at any position in the throttle range, it drives me crazy. I have eliminated those issues in both bikes, thanks to those here at thumpertalk and articles I have read, so i'm just going to ride the rest of the summer and hopefully my jetting settings will not require any more major tweaking til the end of the riding season.

Greg

Clearfield, PA

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Stiletto, incredible how to. Keep them coming. When you begin testing (lets say the main) are you riding through each gear 1st, 2nd, 3rd, feeling how 3/4 -4/4 feels, or are you staying in 3rd or 4th gear running 3/4 through 4/4 throttle then slowing down. Same with the needle. Being as specific as possible, please post other techniques.

I just installed the jd kit on my 03 yz250f 185mj, and blue 5th clip needle, as per the instructions. It runs well but, I believe it may be a little rich. It pulls strong low through mid and settles a little on top but is still pulling. It doesn't cut or sputter at all. Am I correct in my thinking it may be rich. It is also 85 degrees out (less air). Going to try a 180mj, if that is better, I may try the jd red at clip 5.

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Air/fuel evalutaion is a learned skill. Some people ride around their whole lives never realizing their bike runs like crap (or at least is giving up performance) while others can spend 2 minutes on a bike and have specific suggestions for which jetting circuits need to be changed and in which direction. Lost of experience (there's that trial and error thing again) and reading about specific test procedures have helped me temendously in honing my skills.

A few of my favorite testing "tricks" involve purposely pushing the jetting leaner or richer and then evaluating the results. The trick here is to find ways to do this in very quick back-to-back tests so small changes in performance can be easily noticed. Changing jets often isn't all that fast. Especially needle postions. So, you can use other ways to temporarily change the the air/fuel ratio. Suspect your a little lean or rich? Push your jetting one way or the other and see what happens...

--Restricting flow to the airbox is a good way to make your jetting temporarily richer. Plane old duct tape often works well here. Where ever the airbox is sucking air from--tape off part of that opening. If you're on a machine with an airbox lid of some sort than you can take that off to get more air in which pushes your jetting the other way--leaner. In some situations you can even remove your airfilter altogether to get leaner (just don't run that way for more than a pass or two).

--Chokes and enricheners are also useful tuning devices. They're great because you can usually operate them on the move. That makes for instant back-to-back evaluation. You might feel a tiny change in performance more easily because what your trying to compare to is only a hand motion away. Turning on a choke maybe 1/16 or 1/4 of the way will instantly richen the jetting. If the bike's performance seems to improve than you're jetting is too lean.

Carbs are somewhat imperfect fueling devices. Since they rely on vacuum signals to deliver fuel there are a few situations when they'll deliver a temporary lean or rich condition. Normally we don't like this. But we can use these temporary conditions as "tuning helpers" to aid in evaluating air/fuel ratios.

--When you're at WOT the main is delivering it's full fuel load. If you suddenly back off the throttle to about the 7/8ths position then the slide closes a certain amount and restricts the volume of air flow. But the carb needs a second to react to that change. For some fraction of a second the carb will meter too much fuel for the new flow of air. This creates a temporary rich condition which is useful for tuning the main jet. The basic procedure is called the Roll-Off method and goes like this:

Fine a nice wide open space. Fourth gear tends to work best for this test for the same reasons that most dyno runs are done in 4th. Throttle to wide open and wait until the engine is in the meat of it's power and really pulling hard. Quickly back out of WOT to about 7/8th throttle. You've just created a momentary fueling condition that is richer than the jet sizes would normally allow. So, what did the bike do? If you actually accelerated or seemed to gain power for a second before things "normalized" for the new throttle postion than your main jet is too lean. If you detected a stumble or miss than you're too rich. A correctly sized main jet should be fairly smooth in making that WOT to 7/8 transition. It will be temporarily rich no matter what, but if the main is correct than it shouldn't go sooo rich that a hesitation is felt. You can experience a more dramatic demonstration of this affect by getting into a high gear (4th is good but other gears work too) at a low rpm and holding WOT for a second of two. Now back off the throttle FAST to maybe 1/2 or even 1/4. On the majority of bike and carb settups you're likely to get a pretty distinct hiccup--even if the jetting is pretty much correct. This is the sort of situation you usually have to purposely force--you'd probably never experience it in real-world riding conditions unless your trying to do it on purpose.

BTW, the above situation is the opposite of the temporary LEAN condition that occurs when you suddenly whack the throttle open from idle. The engine takes a big gulp of air but it takes the carb a moment to react and begin delievering it's normal load of fuel. That's what accelerator pumps are for. They are operated mechanically (not by vacuum) so they are immune to the time it takes for vacuum delievery to take place.

NOTE: These temporary lean/rich conditions only work with carbs who's slides are mechanically operated. They won't work well (or even at all) on constant velocity (CV) carbs.

There's another main jet test that is even more accurate than the Roll-Off method--the Roll-On method. But it's harder to do. The Roll-On method is basically a timed test that measures the engines acceleration and power. You'll probably need at least one friend and some settup time to do this right. But it will get you VERY accurate results. It goes like this:

Set up a "speed trap" if you will--a longish straight with marker cones at each end. Start your main jet at something you're alomst sure is going to be too rich. Motor along in 4th gear at low rpm and just as you pass the first marker you've set up roll-on the throttle to WOT. Hold it until you pass through your other marker at the end of your "course". A friend (or friends) will be timing you. Now go one step down on your main and repeat the test. Go one step down again and repeat. It is very important that every aspect of the runs is controled and kept as idendical as possible. Pay close attention to your entry speed through the first marker, engine temps, your body postion, available traction, everything....The main jet that got you through the test in the lowest time represents the air/fuel mixture that yields the best power. Be cautious because best power jetting is right at the threashold of lean on modern liquid cooled engines. You may choose to stay one step rich of best power for lower engine temps...

A few other random thoughts:

--Don't discount sound as a means of distinguishing rich from lean. Even if there are no stumbles or misses that can be felt under acceleration that doesn't mean the jetting is correct. Sound can be an important clue. Rich sounds "wet" and "blappy" while a more correct mixture will have a sharper, more "tinny", and "drier" sound.

--Plug reading can be used for jetting. But there is so much misinformation out there about using plugs for performance tuning that you really have to know what you're doing to get meaningful answers. I'll leave that topic for another time (or post)...

Hope that helps!

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I used all the gears when testing the main. I started in first, rolled through to 3/4 then went to full and repeated for each gear. I was more looking to see if the bike continued to pull hard through the entire rpm range, but mainly how it was 1/2 throttle and above. While testing the main I was concerned about the low end of the throttle. By starting in first gear it helps to keep the rrr's up so you don't mistake a gear bog for a rich condition. The top gear or next to top gear is your best indicator. If your main is set correctly it should pull pretty hard up top and keep pulling that is the key. These bikes really come to life from mid rpm and up.

I think to the 185 mj is too rich. I currently have the 178mj in the 06 yz250F which is the stock mainjet. Mark found on his 04 yz250F the 185 MJ was also too rich. I seriously considering putting the stock pilot back in and 180mj in for this weekend to test it. James Dean had mention in another post my 178mj and 40 pilot was not to lean, but to give the 180mj another shot.

It's easy to change in the field in a matter of minutes. I have a camel pack for water (walmart camping section. Best $30 I spent in a long time), and I carry a small tool kit (assortment of 1/4 sockets, allen wrenches, straight/phillips screw driver, small flat tip screw driver for changing the pilot, spoke wrench, needle nose pliers, and 8 inch 1/4" drive extension) in a zippered bank bag, which nicely fits in the large outside zippered cargo pouch. I have a small plano "fly fishing" plastic box I carry in the lower most cargo pocket. I have the tools and the jets with me if I need to make a quick change, tighten a loose bolt, and untweak a set of twisted forks after a crash, etcc. etcc..

My settings for the 06 yz250F is as follows. Red needle 4th clip, 40 pilot (stock was a 42), 1.5 turns on FS, 178MJ (stock), 40 leakjet (stock was an 80), and timed accelerator pump with heavy oring installed on accelerator pump linkage.

Marks 04 yz250F is as follows: Stock 42 pilot, 1.25 turns on FS, 60 leakjet (stock was a 90), stock 180mj, red needle 4th clip, timed accelerator pump (as in no free play in linkage), and the heavy oring installed on the accelerator pump linkage.

I would try your stock mainjet with the red needle on the 4th clip. Then test a mainjet .002 or .003 larger than the stock mainjet. I really dont think with the red needle you will have to go leaner than your stock mainjet.

I didnt have any luck at all with the blue needle. I really think I would have needed to go leaner than stock on the mainjet to get it to work properly at this time of the year. Maybe in the spring or fall it would be ok, but im after what I call perfect jetting. Something that once set doesn't need changed before during and after each ride.

HawkGT nice info posted. I forgot to mention pulling the hot start lever in a bit or choking it too see if the condition is rich or lean.

Greg

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The basic procedure is called the Roll-Off method and goes like this:

Fine a nice wide open space. Fourth gear tends to work best for this test for the same reasons that most dyno runs are done in 4th. Throttle to wide open and wait until the engine is in the meat of it's power and really pulling hard. Quickly back out of WOT to about 7/8th throttle. You've just created a momentary fueling condition that is richer than the jet sizes would normally allow. So, what did the bike do? If you actually accelerated or seemed to gain power for a second before things "normalized" for the new throttle postion than your main jet is too lean. If you detected a stumble or miss than you're too rich. A correctly sized main jet should be fairly smooth in making that WOT to 7/8 transition. It will be temporarily rich no matter what, but if the main is correct than it shouldn't go sooo rich that a hesitation is felt. You can experience a more dramatic demonstration of this affect by getting into a high gear (4th is good but other gears work too) at a low rpm and holding WOT for a second of two. Now back off the throttle FAST to maybe 1/2 or even 1/4. On the majority of bike and carb settups you're likely to get a pretty distinct hiccup--even if the jetting is pretty much correct. This is the sort of situation you usually have to purposely force--you'd probably never experience it in real-world riding conditions unless your trying to do it on purpose.

So can this Roll Off method also be used to test the needle circuit by going from 5/8 to 3/8 throttle?

Any chance on a sticky?, or maybe a link to this from an existing sticky?

.

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Also, can somebody elaborate on the facts and myths about plug reading. Example, many people to look at the "originally" white part of the plug, and if tan to medium brown the jetting is OK. I have also heard that the jetting can only be read at the base of the white part, and that the upper part will only give indications of a plug that is too cold (black), or too hot (white). On the other hand, I have also heard that the plug means absolutely nothing!

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Also, can somebody elaborate on the facts and myths about plug reading. Example, many people to look at the "originally" white part of the plug, and if tan to medium brown the jetting is OK. I have also heard that the jetting can only be read at the base of the white part, and that the upper part will only give indications of a plug that is too cold (black), or too hot (white). On the other hand, I have also heard that the plug means absolutely nothing!

its a waste of time and a un needed source of frustration on 4 stroke.

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Eddie,

What about on a 2 stroke. What is true and what isn't true about plug reading. And, do all of the techniques mentioned here work on both engine types.

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I am new to the site and it looks like this is a great site for info. I was wondering if I could get a little help on a jetting problemI am having. I have a yfz450 04' and I am currently at 4700 ft alt, my bike continues to bogg down when trying to accelerate. Could some one point me in the correct setting. Thanks

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chepe13,

Post your question in the jetting forum after using the search feature and reading. Hopefully this thread will teach you how to diagnose, and fix your own problem.

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