38mm Keihin Carb Tuning

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That's Mister Black Flag to you
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3 rides in and I am finally comfortable on the bike to ride it fast and I realized on the last ride that it wasn’t the trans and gearing changes that Yamaha made to the X that was making me think it didn’t have power. It’s fat as hell......I will be buying some jets tomorrow.

4 pics of the plug in sunlight and in the garage.

I bought the bike at 2500 ft elevation and I live at 5400 ft elevation. I ride at elevation from 4600 ft all the way to 9000 ft. That said a majority of my riding is from 5400-7500ft. Live in Northern Nevada so our climate is typically dry

Main jet looks to be a 178 and the pilot is a 50.

Bike idles fine, exceleration is okay and the top end feels fat.

The bike will not power wheelie whatsoever. I feel like she’s pretty fat, but was surprised the plug wasn’t more black.

Have been reading threads on Thumper Talk and also Spankys chart and it seems I should be close to a 45 pilot and make a 168-170 main jet.

Thoughts?

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Found this on another forum and thought it was pretty good


A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.

Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.


The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.

Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best. It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.

Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting. Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range. Warm the bike completely, and shut it off.

As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.

Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.

Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power,
 
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One of the guys on the YZ250X FB page told me about this app. Cost $5.....I downloaded it......allowed it to pull current weather and elevation numbers and bob is your fucking uncle......it spits out everything you need!

I am sure this will be a good baseline......it even switches spark plugs that should be used. I have a BR8EG and it’s saying I need a 10.


Cool ass app.....I will let you know if settings are correct!


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Master-Cylinder

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We had some Excel air density program way back in the day. Had to input air temp, barometric pressure, and humidity (used a wet bulb/dry bulb set up. HRC also had a jetting chart recommending jetting for above conditions. Get it right, it hauled ass, get a bit greedy, and it cost you a couple hundred to a thousand bucks... or more.
 

Master-Cylinder

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I have a proper spark plug light. The burn ring is way down at the bottom. Doing a WFO plug chop on a dirt bike would be challenging. I would think most dirt bikes are point and squirt, not running WFO for 30 seconds at a time. If you're your running a desert sled, I'd want it a bit fat.
The Keihin needles I have are marked with the size and taper on them, that's the way they should all be. Lectron needles are pretty simple too.
 

BilletMan

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Yep, most folks don't know how to read plugs...like MC wrote, the very deepest location of the insulator...takes a proper light...
 

WarpSpeed

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What's interesting is which circuit to adjust first. I was taught by an old tuner and had read online years back to start with the Main jet since that affects all circuit below it. Tune the main, then needle, then pilot and finally mixture screw.
 
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That app is pretty spot on, I chose to go richer on the main and the pilot at 170 and 48 just to ensure it wasn’t to lean. Bike idles fine, transitions well from closed to full throttle. Bike felt much better and will loft the front wheel but she’s still rich. Going to go ahead and go down to the 168 and 45 I bought later this week when I get more time.

Going to try and pickup some new spark plugs tomorrow for my next test run.

Thanks for the input fellas!


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whipit1K

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a 250 two stroke that "will not power wheelie whatsoever" must be right on the edge of constant , horrendous blubbering and have a huge cloud of blue smoke behind it?
 
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a 250 two stroke that "will not power wheelie whatsoever" must be right on the edge of constant , horrendous blubbering and have a huge cloud of blue smoke behind it?
Actually it wasn’t even close to blubbering
 

Terry_Schiavo

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What's interesting is which circuit to adjust first. I was taught by an old tuner and had read online years back to start with the Main jet since that affects all circuit below it. Tune the main, then needle, then pilot and finally mixture screw.
yeah theres a diagram from suzuki that has cause/effect protocol on carb tuning. Another tip... spray wd40 on intake to insure no leaks
 

r1racer

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Stites, have you looked into Lectron or other smartcarbs to address elevation changes above sea level?

Not cheap by any means but if you find yourself continually re-jetting for different elevations than it may be a worthwhile investment.....


 
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Went and rode about 25-30 miles of single track tonight with the new jetting. Bike ran considerably better, but I still feel like there should be more power. Same plug......going to try and pick up some new ones tomorrow.

Thoughts? I still think it’s fat.....


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Master-Cylinder

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I don't like the grey-ness on the plug. I'd want it a bit fatter. To read a plug, it needs to be new or cleaned before each run. A good quality borescope would allow you to look at the top of the piston and look for the proper burn. Pro Tip, you can look up into the exhaust port (after you pull the pipe) and take a peek, or at least be able to shine more light to get a good look with the scope. The borescopes that take a picture are the best for this kind on stuff.
Somewhere, I have a color chart from Yamaha showing what the top of a piston should look like, when jetted correctly.

And, maybe the thing is just a bit gutless. Did you check the power valve adjustment AND actuation? I pull the cover off, on the clutch side, rev it up a few tymes and make sure it goes open allah the way. And that the throttle valve opens allah way.
 
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I don't like the grey-ness on the plug. I'd want it a bit fatter. To read a plug, it needs to be new or cleaned before each run. A good quality borescope would allow you to look at the top of the piston and look for the proper burn. Pro Tip, you can look up into the exhaust port (after you pull the pipe) and take a peek, or at least be able to shine more light to get a good look with the scope. The borescopes that take a picture are the best for this kind on stuff.
Somewhere, I have a color chart from Yamaha showing what the top of a piston should look like, when jetted correctly.

And, maybe the thing is just a bit gutless. Did you check the power valve adjustment AND actuation? I pull the cover off, on the clutch side, rev it up a few tymes and make sure it goes open allah the way. And that the throttle valve opens allah way.
Haven't checked the power valve and I still need to check the needle setting on the carb.

What do you not like about the grey? Please explain.......I am going to buy new plugs for it today or tomorrow
 

Terry_Schiavo

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does coating the piston give you a lil more margin for error? Im curious why you feel its down on power at the elevation you ride?
 

Master-Cylinder

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Haven't checked the power valve and I still need to check the needle setting on the carb.

What do you not like about the grey? Please explain.......I am going to buy new plugs for it today or tomorrow
This grey, looks a bit lean, or detonation-ish. Look how it is drying out on the ground electrode, compared to your coal mine plug picture.
40575
 
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does coating the piston give you a lil more margin for error? Im curious why you feel its down on power at the elevation you ride?
Because my 01 YZ250 made tons more power.......this one is fat on fuel and I can feel it
 
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Okay fresh plugs and down to 45 idle jet and 168 main jet.

This was about 3 miles partially on road partially on the dirt. On throttle, off throttle and as much full throttle as I could muster considering it was getting dark.

Looks better to me but what do I know

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Master-Cylinder

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Looks a lot better than the Grey Death one from earlier. You can sort of see the mixture ring down at the bottom.
I think if you indexed the plug, it would look the same on both sides. Yeah, I have a whole load of slightly different sparkplug washers for doing that sort of nonsense.
Howzit run?
 
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Looks a lot better than the Grey Death one from earlier. You can sort of see the mixture ring down at the bottom.
I think if you indexed the plug, it would look the same on both sides. Yeah, I have a whole load of slightly different sparkplug washers for doing that sort of nonsense.
Howzit run?
Mixture ring? Please educate me.....

It runs a lot better, way more power! It will pull the front end up now but not like I want. From what I have been reading it has to do with the mods they did to the power valve.

Apparently there is a washer I can remove in the PV and get the hit back. That said it might make slow transitions hard to ride so I may or may not fuck with it
 

Master-Cylinder

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The mixture ring is at the very bottom where the porcelain meets the metal. In your first picture this tyme around, you can see it. BUT, with out looking at it closely with a plug light, can't get a good read. Should have a light tan color down there.
 

BilletMan

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Maybe you didn't understand about reading a new plug that's ONLY under full power pulls....re-read above what MC and I typed....with plug reading, you're only concerned about the main jet and full load output....THAT'S where you'll damage the engine if you're lean....everything else (needle, cut-away, pilot, air screw) is done by feel of how the engine is running....you're not going to hurt the engine in those lighter load operational regimes....SO, get the main jet correct by reading a new plug after plenty of power pulls....then you're done with plug reading....

The pics you've provided don't say a thing about main jet size...you're showing a general average of your putting around...it's ok to idle around a bit before and after the plug read, just make that brief...you're ONLY interested in the correct main jet when plug reading!
 

BilletMan

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Why are you dicking with this?...it seems like you're over-thinking it....As Le05 pointed out, there's a jetting chart in your owner's manual...follow all of their recommendations, then fine tune the light-load stuff....It's not like you're ripping thru the gears in a deep, loamy track with A-rider status...
 
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Relax brother it's just jetting.........I am trying to keep bike threads on a site I love.

If you recall the bike was jetted at sea level and was way fat.........I knew there were people here smarter than me and I posted. Once again I got some of your less than positive tone, but I have come to expect it over the years.

Sad part of it all is you are a really smart/knowledgeable dude with lots to share and sometimes you are really cool about it and others times you come across as a total dick.

I will never forget what you did for me years ago and I have always spoke highly of you Evan to everyone I come in contact with, but I also won't soon forget the many times you have been a total dick here on labs and on FB.
 

BilletMan

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Wow, so I'm a dick? Geez....did you get an owner's manual with the bike? If not, get one! Dirt bike owner's manuals are kick ass! And jetting is addressed right there on page 89......so my last two posts pissed you off?? Do tell. I thoughtfully typed that out....did I just waste a bunch of time trying to help you out?....I'm a dick?? Fuck that....
 
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Re-read my post.......it says you can be a dick. But if you are all butt hurt now and the shoe fits, fucking rock it brother.
 

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Wow, so I'm a dick? Geez....did you get an owner's manual with the bike? If not, get one! Dirt bike owner's manuals are kick ass! And jetting is addressed right there on page 89......so my last two posts pissed you off?? Do tell. I thoughtfully typed that out....did I just waste a bunch of time trying to help you out?....I'm a dick?? Fuck that....
You are hoisting yourself on your own petard...

...reel it back in a bit, he wasn't attacking you, he was giving you a gift that only a friend would bother to give.

Look for offense and you will find it so, stop looking for it.
 
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This one took about 5 seconds to find just by typing your name into FB.........you even admitted coming off like an ass.
 
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