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Discussion Starter #1
I wanted to post this as I am having some jetting issues myself (see bottom of post if you wish to help) and have been doing a lot of reading. Hopefully this will help those that are in need of a re-jet.

Originally posted by Skipper on the dirt rider forum:

It seems that during my sabbatical from the forum (AKA Skip's disappearing act), many of my jetting posts became stale and buried where people can't find them. So - here's a quick and dirty jetting FAQ to help those SOB's who can actually RIDE right now, unlike the rest of us who are frozen in for the winter :)

Pilot jet/mixture screw:

If you can start it cold with no choke, the pilot circuit is too rich.
If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

The best way to set a pilot circuit is with an RPM guage. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.

On a 2-stroke - You should be able to ride in 3rd gear, throtlle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.

Ona 4-stroke - If it stalls in corners, flames out, and is really darn tough to start, the pilot is too lean. If it feels a little dead until you wind it out, the pilot is too rich. Another test it to rev it out a little in 2nd and then let the throttle snap shut. As it's decelerating, there should be very little backfire or popping - if it pops the whole way down, the pilot circuit is lean.

Needle Clip:

On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the otherhand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.

On a 4-stroke - Riding in 3rd with the throttle barely cracked open and roll the throttle open to 1/2. The engine should pull smoothly... if it hesitates and threatens to stall, then the needle is too lean. If it feels 'dead' and won't pick up RPM quickly, then the needle is too rich. An overheating thumper that doesn't have a radiator problem typically is an indicator of a lean needle.

Main jet:

On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.

On a 4-stroke. If the engine feels like it's run into a wall and won't pull full throttle - the engine just sounds dead - then the main is too rich. If, on the other hand, it surges, the main is lean. A lean condition will also give you some 'pinging' and a pure white plug.

Advanced Topics:

I will continue to add to this FAQ as time allows. I'll start with the needle because that's the circuit that is 'in play' the most.

The needle regulates the mixture from around 1/4 - 3/4 throttle. Most people are familiar with the clip position, as it's the most common adjustment, but there's much more to the needle. The jet needle is a long rod that fits into the needle jet. On most carbs, both are replaceable with different sized components. As the throttle is opened, the jet needle is retracted from the needle jet and this creates space between the two for gas to flow through. The more you open the throttle, the more the jet needle is pulled out of the needle jet, and consequently the more gas can pass through the increasing space between them. Below I'll outline the various parts of the jet needle.

Length - The relative length of the needle is adjustable by raising or lowering the clip. If you lower the needle (by raising the clip), then the needle sits deeper in the needle jet. This leans out the mixture across the range of the needle. Conversly, if you raise the needle (by lowering the clip), then the needle is further retracted from the needle jet, and this richens the mixture across the needle's range. Needles are offered in various lenghths. If you have a needle which is still too rich, even though it's in clip position 1, then you need to order a longer needle. For example, needle 'A' in clip position 1 is the exact same relative length as needle 'B' in clip position 3. If you had needle A in your bike, and it was still rich - even though you had the clip in position 1, then you could change to needle 'B' and lean things out by going to clip position 2.

Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).

Needle taper - Needles taper from top to bottom. As with all principles regarding the needle, the taper is relative to the diameter of the hole in the needle jet. Tapers are rarely changed, but here's a condition which warrants a taper change. Let's say the jetting is perfect at 1/4 throttle, but becomes increasingly leaner as you approach 3/4 throttle. In that case, you would want a needle with a shallower taper. Conversly, if the mixture is great at 1/4 throttle, but getting richer and richer as you approach 3/4 throttle, then the needle taper needs to be steeper. In my experience, needle taper only needs to be changed when the factory mis-spec'd it to begin with. Under very rare circumstances, big modifications to the motor - such as an overbore kit - will require a change in needle taper.


Remember that jetting needs to be adjusted for every 2000' elevation change and every 15 degree temperature change. If it was jetted right this summer, it's sure to be too lean during the winter. If you rejet it now, when it's cold out, make sure to lean it out a bit in the spring.

Some other helpful websites, all worth a read:

Dirt Bike Jetting 101: http://motocrosshideout.com/dirt-bike-jetting-101/
An Introduction to Carburetor Jetting for Four Stroke Dirt Bikes: http://www.onallcylinders.com/2013/03/21/an-introduction-to-carburetor-jetting-for-four-stroke-dirt-bikes/
Jetting 101: http://www.mainjet.com.au/tech_tips.php

My Specific Problem:

I have a 2013 WR250F supermoto setup with an FMF Powercore. It sputters around 5/8 throttle in almost all gears, but I live in the DR, high elevation, temperatures, and humidity. Is there a way to fix this without re-jetting the carb? Fuel/air screw adjustment?
 

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No adjusting the Fuel/Air screw Does Not change fuel mixture past 1/4 turn of throttle
it adjusts from idle to almost 1/4 turn of throttle -


You could try -
Raising the clip on the needle One notch IF the needle has adjustments notches -
could solve your problem-

( Raising clip --Snap Ring - drops the needle into the main jet more and lessens the amount
of fuel that is allowed to pass thew the main jet making it Leaner )

If bike is to lean --do the opposite - lower clip on needle one notch - allows More fuel to pass threw main jet -

if bike runs better after changing needle clip position -move the clip One more notch in same direction -
if you have moved the clip ALL of the way in ether direction - and still runs bad -

Then its time to change main jet -

if you have to buy different size jet -get genuine Yamaha jets made for the brand of carb you have-
dont get some weird brand because there is a difference in quality -

what altitude are you at - temperature - and humidity ??

In your country is there alcohol in the gas ?

The easiest way to tell if your are rich or lean is to do a spark plug reading -
look on Youtube and see if you can find a video explaining how to do a- -- spark plug reading on 4 stroke motorcycle --

I just got the PM message you sent me - I will answer your questions on this thread instead of on PM
then other people can read this and learn also -



here is a video that explains jetting basics - I think the same guy has some other jetting videos also -
https://www.youtube.com/watch?v=82Sm_mzcW0U

 

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Discussion Starter #3
You could try -
Raising the clip on the needle One notch IF the needle has adjustments notches -
could solve your problem
I took the bike apart yesterday, only one groove on stock needle.

If you have to buy different size jet -get genuine Yamaha jets made for the brand of carb you have-
dont get some weird brand because there is a difference in quality -
Guess I broke that rule already. Ordered a JD Jet kit on Friday as I have only heard good things.

What altitude are you at - temperature - and humidity ??

In your country is there alcohol in the gas ?
Altitude- 2000 feet; Temperature- Mainly 80's and 90's

Alcohol in the gas- I live in the Dominican Republic, there's really no telling what they put in the fuel here.

The easiest way to tell if your are rich or lean is to do a spark plug reading -
I tried to do a spark plug reading yesterday and with the setup I have, to do an immediate roadside reading is impossible without taking the tank off. There is little to no room to access the spark plug and it is encased in rubber and does not have enough overhead to be pulled completely out.
 

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JD carb kits are Good - and will likely fix your problem -

2000 ft is within the JD kits range I am pretty sure -

I had a feeling that the stock needle would not be adjustable-

one way you can do a plug reading is to remove stock gas tank and rig up
a small tank that only holds a few ounces of fuel - tiny tank needs to not be mounted higher than stock tank-

then you can ride bike at 5/8 throttle where it is not running correctly and hold throttle
at that position for a while - then pull in clutch and hit Kill button and coast to stop -
then pull spark plug and do reading -- I have done it this way before -

you will need to seal off the vacuum line that goes to the fuel valve if it has one -
 

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Discussion Starter #5
I contacted JD Jetting and they told me that richening the mixture a little by moving the clip down one groove will most likely solve my problem.

He also said that this change would work with my setup as well as the temperature and altitude.

I know that I will probably do my best and someone will come along and be able to do a lot better.
 

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Just reading this has made me think about how my bike runs. I get a pretty consistent hesitation around the mid throttle range. And after reading this i'm thinking the needle may be my issue as well. I'm going to dig into mine tomorrow hopefully. I'm not sure how long the plug that was In my bike was in there but it was pretty clean and white. So I think more fuel is in order. I really didn't want to have to dyno tune the bike at the local shop since i'd like to slap a bigbore and bigger carb on my bike at some point and kind of would be a waste of money. Guess i'll be tinkering for a while after reading this. As long as I can get the hesitation out and I don't feel like its running to lean i'll be happy to ride my bike until I can upgrade it. :)
 

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Discussion Starter #7
Glad that the thread helped out a bit. From what I have heard, that hesitation means that you are running too lean and could mean that you need to go up a main jet size. First try and lower the clip on your needle to get rid of the hesitation and slightly richen the mixture.
 

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That's my hope if the needle is adjustable any way. If the needle doesn't fix it i'm in for some tinkering inside the carb :)
 

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So i'll try and add some useful information while still asking questions. I tore it down this morning. I found one think that looked like it was plugged with gunk so I cleaned that out. I'm guessing (because I don't know how a DRZ carb actually works) that it was a vacuum port that controls how the needle gets pulled up on this carb since its a butterfly carb. Ok so I figure this will help while I was in there I also pulled the needle out and looked to see how it adjusts. No clips on the drz carb just spacers. There was a thin washer on top of the clip so I moved it to the bottom to lift the needle up (more fuel) put it all back together and took it for a ride. It is definitely better but I know it could get a little better. The hesitation is almost gone seems to only show up in 3rd gear if I slowly open the throttle just about 1/4 it seems to stutter any other gear its not noticeable to me any way. So I know i'm close. The other thing I noticed on my test ride is during down shifts it backfires a lot less. Which I also know is going in the right direction since previously running my xr50 I learned lean means more backfiring usually. So where do I go from here I pulled the plug when I got back I ran it pretty good only a couple miles but I did one full pull through all the gears and some city back home and its pretty clean yet. I just changed the plug about 12 miles ago because the old one I could actually see spark coming out of the side of the plug as well as the electrode. So I snapped a picture of it and I've never really tried to read plugs so I figured i'd post it on here let you guys tell me how it looks.

 

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Discussion Starter #10
So i'll try and add some useful information while still asking questions. I tore it down this morning. I found one think that looked like it was plugged with gunk so I cleaned that out. I'm guessing (because I don't know how a DRZ carb actually works) that it was a vacuum port that controls how the needle gets pulled up on this carb since its a butterfly carb. Ok so I figure this will help while I was in there I also pulled the needle out and looked to see how it adjusts. No clips on the drz carb just spacers. There was a thin washer on top of the clip so I moved it to the bottom to lift the needle up (more fuel) put it all back together and took it for a ride. It is definitely better but I know it could get a little better. The hesitation is almost gone seems to only show up in 3rd gear if I slowly open the throttle just about 1/4 it seems to stutter any other gear its not noticeable to me any way. So I know i'm close.
Looks like we are in the same exact boat on this one. I get a stutter early in the morning in almost all gears before the bike is warmed up and loud backfiring. I also get the higher gear/rpm pause/stutter any time of day when I really open up the throttle. I talked to customer service at JD Jetting and they said that I might have to go further than just a needle clip change and go up a full jet size to richen the mixture. My jet kit will get here next week so I will probably start by changing the main jet size and starting at the leanest needle clip setting and going from there.

If I understood everything I've heard correctly, the needle clip and groove are for fine tuning until you run out of grooves on the needle, then you have to increase or decrease the main jet until you get the proper mixture. Looks like you are headed in the right direction.

As far as reading spark plugs, I have heard a lot of mixed things. Someone said with the modern four-stroke engines and fuel additives nowadays, reading spark plugs can be inaccurate and you have to do it with a brand new spark plug and keep the RPM's consistent then pull the clutch, coast to a stop, and check the spark plug immediately.
 

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I to think i'm going to order a jet kit for mine. Just so I have it incase I do need to change it. It also does appear that the kits for my DRZ come with new needles with adjustable clips versus spacers which I would rather have less things to loose in my disassembly :)
 

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You may be able to Raise a non adjustable needle tiny amounts wrapping the needle Under the clip
with Thin Wire - Making it Richer- it will depend on how your carb is made - but I can see it working --

(this has to be done carefully -remember you are messing with throttle and
stuck throttle can cause Bad things to happen )

You need to understand that the needle and Main Jet control Different sections of the Throttle Mixture-
BUT they do Overlap some what -

The Needle controls from about 1/2 throttle to a laround 3/4 open throttle-

After 3/4 open throttle it is Mostly Main Jet but needle setting Does Still have Some effect -but not much -

Plug Readings - they Used to be the Best way to adjust fuel mixture-

BUT now that gas has alcohol in it I dont know how accurate they are anymore -
I got out of MC for about 20 years and during that time alcohol was added to fuel and I
have not talked to anyone knowledgeable enough to say if plug reading is still accurate --

the spark plug in your picture is very clean --

Before alcohol was added to gas you wanted the spark plug porcelain Insulator to be Tan in color-
but like I said I have no clue if this is still true -
 

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See now I figured since my needle isn't attached to the throttle I would monkey with it. The stock drz carb has a butterfly instead of a slide. I don't plan on doing anything more until my jet kit comes tomorrow. Then I can put a needle in with adjustable clip point hopefully. Other wise I could just add tiny washers :) The needle doesn't move a whole lot but I suppose with the vacuum chamber it would move more pull. Either way I won't be riding it until after I tinker with the jet kit anyway.
 

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Discussion Starter #14 (Edited)
See now I figured since my needle isn't attached to the throttle I would monkey with it. The stock drz carb has a butterfly instead of a slide. I don't plan on doing anything more until my jet kit comes tomorrow. Then I can put a needle in with adjustable clip point hopefully. Other wise I could just add tiny washers :) The needle doesn't move a whole lot but I suppose with the vacuum chamber it would move more pull. Either way I won't be riding it until after I tinker with the jet kit anyway.
Let us know how it goes. I'm hopeful that the larger needle jet size will help your problem and take away the stutter/lag all together.
 

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Based on where its at right now and watching that youtube video above. Ithink its going to be the needle adj. And possible a pilot change. Since they overlap a little and my problems is about ? throttle. We'll see tho all I can do is tinker ;)
 

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Good news for me :) The jet kit my local shop ordered me was a FMF kit. At first I was concerned by the directions and thinking this might not work for me. But I read a little farther and low and behold they list two different jet options one for a mainly stock bike with an aftermarket filter and maybe a pipe. And stage 2 with full exhaust aftermarket filter and opened up air box. Which mine had done prior to me buying it. So I pulled the carb and followed directions replace needle with aftermarket one put it on clip 2 from the top replace factory spacers in the same order on to new needle and place back in the slide. To me the needle looked longer than the factory one and I thought was going to run lean again but what ever i'll pay attention for now ;) next replace the main jet with a 155 size jet. So I pulled the main jet which happened to be to be a 145 size jet. Put it all back together and start it up make sure I've got nothing leaking. Button the rest up change my clothes and jump on for a quick ride. YAY its fixed now!!! No more hesitation back fires a little more than I had it before but I can deal with a little backfire now and then if the bike runs good. Going back now I can only assume some one jetted the bike with an exhaust on it and then they decided to open up the air box creating more air and less fuel. Not sure but as of right now i'm crazy happy :lol:
 

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Discussion Starter #17
Good news for me :) The jet kit my local shop ordered me was a FMF kit. At first I was concerned by the directions and thinking this might not work for me. But I read a little farther and low and behold they list two different jet options one for a mainly stock bike with an aftermarket filter and maybe a pipe. And stage 2 with full exhaust aftermarket filter and opened up air box. Which mine had done prior to me buying it. So I pulled the carb and followed directions replace needle with aftermarket one put it on clip 2 from the top replace factory spacers in the same order on to new needle and place back in the slide. To me the needle looked longer than the factory one and I thought was going to run lean again but what ever i'll pay attention for now ;) next replace the main jet with a 155 size jet. So I pulled the main jet which happened to be to be a 145 size jet. Put it all back together and start it up make sure I've got nothing leaking. Button the rest up change my clothes and jump on for a quick ride. YAY its fixed now!!! No more hesitation back fires a little more than I had it before but I can deal with a little backfire now and then if the bike runs good. Going back now I can only assume some one jetted the bike with an exhaust on it and then they decided to open up the air box creating more air and less fuel. Not sure but as of right now i'm crazy happy :lol:
Glad to hear everything worked out so well. Gives me hope for the mods to my bike. I actually realized that I also ordered the GYTR AIS Removal kit, so I'll have to make those modifications before I jet as well. I might try and make a video so I can help others out. Not too many videos on the 2013 WR250F.
 

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Discussion Starter #18
Well, I have to say that following the directions on the JD Jet kit made everything extremely easy. No more hesitation or walls hit in any gear or throttle range whatsoever. The bike rides great and sounds awesome. Can't say enough good things about the JD Jet kit.
 

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Well, I have to say that following the directions on the JD Jet kit made everything extremely easy. No more hesitation or walls hit in any gear or throttle range whatsoever. The bike rides great and sounds awesome. Can't say enough good things about the JD Jet kit.
Good news, I am getting ready to order one for my 05 WR450F
 
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