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Quick reference YZ250 jetting chart

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Here's a quick reference jetting chart for '01-'08 YZ250's

It was made by taking Yamaha's manual recommendations and using the density altitude calculator to correct for altitude and temperature.

-If you will be racing or running in open desert conditions, richen each jet by one size and the needle by 1/2 step

-If you will be racing in open desert conditions with extended wide open running, richen each jet by two sizes and the needle one full step.

-If it is extremely dry run the richer of the range given, if it is extremely humid run the leaner of the range given

-The leaner number of a range assumes either a Pro Circuit pipe, an FMF pipe or a spark arrestor and the richer range is for stock.

If you have none of these, you may have to run the richer range of an option or go up one richer jet size or 1/2 needle step richer

The colors represent the needle step to be run, blue is stock

The single brown in the upper left is 1/2-1 step richer etc. ie. from an N3EJ-2 to an N3CW-3 or N3EW-2

Each color change is a 1/2 step on the needle ie.

from a N3EJ-2 to a N3CJ-2 would be 1/2 step leaner for a stock 2002 YZ250.

A full step leaner for a stock N3EJ-2 would be an N3EJ-1 or an N3EK-2.

A full clip position or full needle change option would have to be experimented with to see which is best ie.

from a N3EJ-2 to an N3EJ-1 vs. an N3EK-2

You may need to adjust from these for your specific situation, ie. race gas may allow leaner settings etc.,

but this should get you close for a starting point based on premium pump gas, the book and density altitude.

-------------0-2000 FT ; .......2000 – 4000 FT ; ....4000 - 6000 FT ; 6000 – 8000 FT ; 8000–10000+FT

Below 60F ; ..178-175/50 ; .......175/50-48 ; ............172/48 ; ...........170/48-45 ; ......168-165/45

60-75F ; ..... 175/50-48 ; .........175-172/48 ; ..........172-170/48 ; .....168/48-45 ; ......165/45

75-90F ; ..... 175/48 ; ..............172-170/48 ; ..........168/48-45 ; .......165/45 ; ..........162/45

Above 90F ; ..172/48 ; ..............170/48-45 ; ...........168-165/48-45 ; .165/45 ; ..........162/45-42

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If it is humid lean it out????

Still not convinced on this as it is normally boody hot as well as being humid.

When you lean the bike out it runs hotter thus creating a dangerous condition for the engine.

The weather as well riding conditions should be taken into consideration when jetting.

You do not want to be lean on a humid day ripping through the heavy sand or poking along at a snails pace. You will cook the engine.

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If it is humid lean it out????

Still not convinced on this as it is normally boody hot as well as being humid.

When you lean the bike out it runs hotter thus creating a dangerous condition for the engine.

The weather as well riding conditions should be taken into consideration when jetting.

You do not want to be lean on a humid day ripping through the heavy sand or poking along at a snails pace. You will cook the engine.

Actually what heats an engine up is an excess of the amount of air compared to fuel. Higher temps and humidity make the air thinner making you run richer which can foul a plug. Leaning out the mixture a bit for hot days will not make the engine runner hotter if youre comparing proper jetting to proper jetting

What mix ratio is that based on? Is that the 30:1 in the manual? I run 32:1 and run the local desert with an upcoming night time race series, I expect temps to be around 70s-80s at about 1000 elevation but I chose to run the 180 main.

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I would not be able to get by with jetting that lean at 4000-5000 ft. no matter which pipe I have on. This is mainly true at cold temps.

I bet I coudl running 100% race gas.

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If that's true Roland, then Yamaha's numbers are correct as long as you correct them for density altitude.

The presumption that so many have of them being one size rich isn't correct then/

I changed the chart to reflect Yamaha's book numbers corrected for density altitude.

They essentially come out one size richer than the previous chart.

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If it is humid lean it out????

Still not convinced on this as it is normally boody hot as well as being humid.

When you lean the bike out it runs hotter thus creating a dangerous condition for the engine.

The weather as well riding conditions should be taken into consideration when jetting.

You do not want to be lean on a humid day ripping through the heavy sand or poking along at a snails pace. You will cook the engine.

What jetting are you running in Thailand? I'm running 30:1 and 48/175 N3CW 2nd clip. Switching to the N3EW as still a little lean in half throttle. No spooge and runs awsome.

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I dont believe Yamaha's numbers are far off from what is required....Neither are yours. The charts go out the window just as soon as we start changing parts in our motor...which I live for! LOL

I could run your chart jetting (for my altitude) when its warm, but not at the colder ends of the scale.

The other part that's hard to jet, or chart around, is the pinging this (stock) bike does in stock form when the jetting is correct. Is the "right" jetting the pieces that will eliminate pinging, or the ones that give best performance?

There are a few pipes I could put on, most specifically fmf and zip ty, that should come with your chart attached.....Actually, I wish FMF had a poster sized copy of your chart.

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Thanks Roland

I went back and used Yamaha's book numbers with density altitude and they are about one size richer now,

see if they match your experiences more closely.

It could still go about 1/2 step richer if the absolute cold/low end of the scale is used.

I wonder how many people are using ARCO/others that are known to have more ethanol (lean) than the other big fuels like Chevron/Unocal.

I also wonder how many people's timing is set correctly and have proper squish, but that's two more threads for later.

As an example, my autocross car was tuned for best response (most timing and not rich)

and I could only run Chevron or Unocal or mix some 100/race fuel in to get it to not ping.

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Your main jet size seems pretty close for me atleast. I'm riding around 1500' and anywhere from 60-80 degrees I run a 175 main jet. It is pretty clear for me though as to which needle provides the best throttle response, but there are too many different variables here. I mix 40:1, have proper squish, vf3 reeds, dep pipe/silencer, and the N3EJ-1 made the bike respond very well. I decided I did not like being in the 1st clip position so I bought the N3CJ and run it in the 2nd clip position. This is 1/2 step richer than N3EJ-1. I also run the crap out of my bike in the sand dunes all the time. Oh ya I run the 48 pilot all the time and just adjust with the air screw.

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Thanks, yeah it should be pretty close and is a little more conservative (richer) now since starting with the actual Yamaha values.

The numbers on the chart are actually taken 500 ft. above the bottom and 5 deg. above the low temp. ie.

75-90 @ 2000-4000 is at the calculation for 80 & 2500 ft. so it's on the conservative side now, along with the text adjustments.

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What jetting are you running in Thailand? I'm running 30:1 and 48/175 N3CW 2nd clip. Switching to the N3EW as still a little lean in half throttle. No spooge and runs awsome.

I'm running 40:1 with 48PJ, N3EW #1, 185 MJ, #55 power jet with a #8 slide.

This is on a 265 with a few extras.

For a standard 250 I have run with great results 40:1 with 48PJ, N3CJ #1, 180 MJ

Where do you ride in HK, A-310, only track?

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I'm running 40:1 with 48PJ, N3EW #1, 185 MJ, #55 power jet with a #8 slide.

This is on a 265 with a few extras.

For a standard 250 I have run with great results 40:1 with 48PJ, N3CJ #1, 180 MJ

Where do you ride in HK, A-310, only track?

Yes only track in Sheung Shui, about an hour out of the city close to the Shenzhen border. I ride two tracks one small and the other is medium. The small track is hard pack clay with a lot of rocks and has one 30ft and 20ft table and one 20ft double. The medium track is hard pack with no rocks has three 20ft doubles, a 20ft and 30ft table and five singles. It's not much but has all the bits to keep some skill level up. :ride: I don't ride any woods, but met a guy who rides off road with a CRM 250. All off road is illegal, so I haven't bothered. Thinking of getting a CRM as well and joining the club?:thumbsup: Here's some shots of the track and my Ultimate Dual:bonk: , which gets me to the track.

A 1988 TDR 250:

IMG_3139.jpg

Medium track:

IMG_2823.jpg

Small track:

IMG_2545_edited-1.jpg

DSC04190_JPG.jpg

harair.jpg

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Whoa that TDR is in good nick, I rode one many years ago,once. Yours looks good.

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Ok, so why am I still so very rich (air screw 3.5 turns out; waiting on a 40.0 pilot) with a 42 pilot, stock ('06) N3EJ needle, in the #1 (top) position? I am at 1000 ft., and we have barely hit 80 degrees. It will get much hotter & more humid soon.

I just today installed my milled cyl. head. I had 1.95 mm of piston to head clearance; milled it 0.70 mm. I now have 1.25 mm 'squish.' Still no detonation with 25% VP C-12, mixed with pump swill, (now with the requisite 10% ethanol.) I have not even gotten to the main, yet.

I seem to be well outside these jetting recommendations. Why is this?

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Ok, so why am I still so very rich (air screw 3.5 turns out; waiting on a 40.0 pilot) with a 42 pilot, stock ('06) N3EJ needle, in the #1 (top) position? I am at 1000 ft., and we have barely hit 80 degrees. It will get much hotter & more humid soon.

I just today installed my milled cyl. head. I had 1.95 mm of piston to head clearance; milled it 0.70 mm. I now have 1.25 mm 'squish.' Still no detonation with 25% VP C-12, mixed with pump swill, (now with the requisite 10% ethanol.) I have not even gotten to the main, yet.

I seem to be well outside these jetting recommendations. Why is this?

Worn out rings, or a chipped reeds would be the most significant things. If you have not checked your reeds lately I would look. Also make sure your crank seals are not leaking as that can cause some funny things.

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Whoa that TDR is in good nick, I rode one many years ago,once. Yours looks good.

Thanks puts a smile on my face. Had it a year now and really enjoying it. Had to do a lot of work to it before I could ride it but it's worth it.

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Ok, so why am I still so very rich (air screw 3.5 turns out; waiting on a 40.0 pilot) with a 42 pilot, stock ('06) N3EJ needle, in the #1 (top) position? I am at 1000 ft., and we have barely hit 80 degrees. It will get much hotter & more humid soon.

I just today installed my milled cyl. head. I had 1.95 mm of piston to head clearance; milled it 0.70 mm. I now have 1.25 mm 'squish.' Still no detonation with 25% VP C-12, mixed with pump swill, (now with the requisite 10% ethanol.) I have not even gotten to the main, yet.

I seem to be well outside these jetting recommendations. Why is this?

What methods are you using to determine the pilot jet is too rich? and what's the main size?

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What methods are you using to determine the pilot jet is too rich? and what's the main size?
The fact that my air screw is virtually all the way out (out of it's effective range, anyway,) indicates to me that I still need a leaner pilot jet.

And since the pilot circuit has some impact on the entire throttle range, I usually do not move on to another circuit until I have it right. I only dropped the needle while I was waiting for my # 40 pilot jet to come in. In fact, I probably should raise the needle back up one clip position (back to the stock # 2 position,) when I go back to working on the pilot circuit.

I have not touched the stock #178 yet. I won't, until I get the pilot & needle set.

I'll check the float level and the reeds. This bike has very little time on it. I did buy it used, but it was 100% original (stock everything) and very pristine. I'd say it probably has less than 15 hours on it, including the 10 hours that I've put on it. The idle is very consistant, and the bike has always run incredibly well, even when very rich (stock jetting.) It just gets better with every change. This thing now has incredible low end. If it's running, it will pull cleanly. It reminds me of a KDX 200 on steroids. I really do not think that the crank seals are leaking.

A year ago, I was having the exact same experience, having to jet my brand-new '06 YZ125 leaner than anyone/everyone else here was. And getting great low end with it, as well.

My base elevation is 1,150 ft.; I do not think I've been over 2,000 ft.

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