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How to jet your bike
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Posted by: zeppelin

This question pops up ALOT on this site, so I figured this should be my first "how to” article.

If you are a more experience wrench you can skip the first two paragraphs, but if you are a newbie to working on your own bike read on to learn what jetting is. When someone says jets they are referring to brass screws located in your carburetor that are used to regulate fuel flow. Most bikes have two jets, a pilot jet, the smaller jet that affects your bike at idol, and a main jet, the larger jet that is located in the center of your carb, and controls fuel delivery from 3/4 to WOT (wide open throttle).

The reason most people change out their jets is because they have added modifications to their bike, like exhaust pipes and air filters. Mods like these allow the bike to flow more air, and the motor will need more fuel to go with the air to run properly. If larger jets are not added, the motor may bog under load, sputter when you rev your motor out, or backfire when you let off of the gas. Some bikes are also affected by weather so much that it is beneficial to change jets according to temperature, humidity, and elevation. Some people also like to jet differntly according to riding conditions, if it is a mud fest, you might want to jet your bike a little richer than usual. This helps the engine run a little cooler when your raditator is clogged or your cooling fins are caked and you revving the heck out of your bike to get through a swamp.

Okay so the first you need to do to change out your jets is remove your carburetor. You will usually need to take off your plastics and gas tank to get to it . After you have your carb off of your bike, remove the screws holding on the float bowl. (The float bowl is on the bottom of the carburetor and usually held on by four screws.) After you take the bowl off you can remove the main and pilot jets to figure out what sizes you have. (The main jet is in the center and the pilot is off to the side.)

Once you get your jet sizes you can either go to your local motor cycle shop and buy the jets individually, or you can order a "jet kit" online which will come with a variety of sizes that will fit you application. If you decide to get it from you dealer, you should probably buy the next five sizes bigger in main jets, and one size bigger pilot. (I personally prefer to buy the individually because it is usually cheaper.)

To decide which jets you bike needs its best to use a trial and error method. Start with the next largest main from stock, and continue to bump the sizes until you feel some power loss at WOT, then go back down on size. Most bikes don’t need a larger pilot unless you live in California, somewhere with a very cold climate, a low elevation, or you have extensive Mods on you bike. Even then you usually dot need to go much bigger than one size larger than stock.

Diagnosing jetting is a little tricky; most people use the spark plug test. To do the spark plug test you need to have a fresh spark plug in you bike right after you change jetting and run the same plug for at least 5 hours, this will give you the best reading. Most people will tell you that your jetting is perfect if your plug is a light tan color. If you plug is wet or has soot then you are running too rich. (Too big of a main jet). If your plug is white or light gray you are too lean. (Too small of a main jet.) Of course if you want to get your jetting spot on you can get an air/fuel ratio meter to check the ratio of gas to air coming out of your exhaust. The ideal ratio is 13:1.

This is a very long post so I will make another one to continue the discussion on carb settings with air screws and needle later.

Posted by: zeppelin

Okay, your carburetor has a "needle" located on your slide. The needle adjusts the amount of fuel that is allowed to flow through the main jet at different throttle positions. To adjust your needle, remove the cap or cover on top of your carb that holds the throttle cable in. After you pull out the throttle cable and slide, you will see a needle sticking out from the bottom of the slide. When you remove it from the slide you will see a circlip in a notch on the top of the needle. To make the bike richer, you can lower the clip on the needle, which actually raises the needle in the carb. If you want to lean it out you just need to do the opposite. The needle is good for fine tuning only. You will still need to have the correct size main jet for your quad to run right.

Your carburetor will either have an air or fuel screw, fuel screws are located in the front of a carb and airscrews can actually be on either side but they are usually on the back. They (depending on which type they are) either regulate the amount of air or fuel that is allowed to mix. With a fuel screw you loosen the screw for a richer mix, or tighten it for a leaner one. It is the opposite with an airscrew, you want to tighten it for a richer mix, or loosen it for a leaner one. The air/fuel screw will allow for minor adjustments to compensate for weather or altitude changes, as well as idling and fixing the pesky backfire that you can never seem to get rid of on a modded bike.

Here is a quick tip for dealing with the air/fuel screw, grab a coat hanger and cut a piece off of it about an inch and a half, to two inches long. Take the cut section to your bench grinder and grind one end of it to make it look like the end of a screwdriver. Then bend it into an "L", you can use this to adjust your air or fuel screw with out taking your carb off of your bike. I bring mine with me on rides so that I can adjust my bike for elevation changes or if it starts backfiring on the go.

Let me know if anything was unclear, or if there is something you still aren't sure about.

EDIT: please post some comments guys, I need some feed back.

Posted by: zeppelin

I thought I would help y'all out with some spark plug pictures.
If your plug looks like this, you are too lean. Note that in this picture it looks almost new, but yours may be ashy, or slightly lighter in color than this.

Posted by: zeppelin

If your plug looks like this, you are too rich. Your plug may also be wet though.

Posted by: zeppelin

If it looks like this, your spot on. Tan is perfect, it should almost be the color of card board, like I said though, these plugs look pretty new so your plug's colors maybe be a little more extreme.

Posted by: zeppelin

I got this from another thread, but I thought it would be useful here. It is a main jet size conversion chart.

Width------Kehein # -- DynoJets # -- Mikuni #
0,0350---- 92,5--------- 92----------- 86,3
0,0360---- 95----------- 94----------- 88,1
0,0370---- 97,5--------- 96----------- 90,0
0,0380---- 100---------- 98----------- 91,9
0,0390---- 102,5------- 100---------- 93,8
0,0400---- 105--------- 102---------- 95,6
0,0410---- 107,5------- 104---------- 97,5
0,0420---- 110--------- 106---------- 99,4
0,0430---- 112,5------- 108--------- 101,3
0,0440---- 115--------- 110--------- 103,1
0,0450---- 117,5------- 112--------- 105,0
0,0460---- 120--------- 114--------- 106,9
0,0470---- 122,5------- 116--------- 108,8
0,0480---- 125--------- 118--------- 110,6
0,0490---- 127,5------- 120--------- 112,5
0,0500---- 130--------- 122--------- 114,4
0,0510---- 132,5------- 124--------- 116,3
0,0520---- 135--------- 126--------- 118,1
0,0530---- 137,5------- 128--------- 120,0
0,0540---- 140--------- 130--------- 121,9
0,0550---- 142,5------- 132--------- 123,8
0,0560---- 145--------- 134--------- 125,6
0,0570---- 147,5------- 136--------- 127,5
0,0580---- 150--------- 138--------- 129,4
0,0590---- 152,5------- 140--------- 131,3
0,0600---- 155--------- 142--------- 133,1
0,0610---- 157,5------- 144--------- 135,0
0,0620---- 160--------- 146--------- 136,9
0,0630---- 162,5------- 148--------- 138,8
0,0640---- 165--------- 150--------- 140,6
0,0650---- 167,5------- 152--------- 142,5
0,0660---- 170--------- 154--------- 144,4
0,0670---- 172,5------- 156--------- 146,3
0,0680---- 175--------- 158--------- 148,1
0,0690---- 177,5------- 160--------- 150,0
0,0700---- 180--------- 162--------- 151,9
0,0710---- 182,5------- 164--------- 153,8
0,0720---- 185--------- 166--------- 155,6
0,0730---- 187,5------- 168--------- 157,5
0,0740---- 190--------- 170--------- 159,4
0,0750---- 192,5------- 172--------- 161,3
0,0760---- 195--------- 174--------- 163,1
0,0770---- 197,5------- 176--------- 165,0
0,0780---- 200--------- 178--------- 166,9
0,0790---- 202,5------- 180--------- 168,8
0,0800---- 205--------- 182--------- 170,6
0,0810---- 207,5------- 184--------- 172,5
0,0820---- 210--------- 186--------- 174,4
0,0830---- 212,5------- 188--------- 176,3
0,0840---- 215--------- 190--------- 178,1
0,0850---- 217,5------- 192--------- 180,0
0,0860---- 220--------- 194--------- 181,9
0,0870---- 222,5------- 196--------- 183,7
0,0880---- 225--------- 198--------- 185,6
0,0890---- 227,5------- 200--------- 187,5

Posted by: Chango

What is the most turn in either air or fuel screw you would want to take out, before rejetting to another size in the pilot or slow jet ?I have that idle pop that happens after a minute or so of idle and it stalls .Valves are good so i figure it's the carb..

Posted by: ELewandowski

Nice write-up, it helped me a ton.

Posted by: zeppelin

thanks man, im glad i could help

Posted by: 04TRX400EX

quote:
Originally posted by Chango
What is the most turn in either air or fuel screw you would want to take out, before rejetting to another size in the pilot or slow jet ?I have that idle pop that happens after a minute or so of idle and it stalls .Valves are good so i figure it's the carb..

The air/fuel screw can be turned out about 2 1/2 turns before it's backed out all the way and you are as rich as you can be. If it is still lean, you need to go up to the next largest pilot. If you screw it all the way in, and your still rich, you need to go smaller.

Great post BTW zeppelin

Posted by: tcr0148

thanx for the conversion chart.......the honda shop here is always a hassle to deal with!

Posted by: zeppelin

^^^ no problem dude! gald i could help out.

Posted by: GPracer2500

Thought I'd add this about the coversion chart:

quote:
...We are often asked for a "cross reference" sheet that compares our main jets to Mikuni or Keihin. The fact is you cannot directly interchange the jets for a given size. Many things affect fuel flow through a jet of the same orifice size. The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburetor venture. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed. At a certain speed the two may flow equally, but a change in velocity changes the flow characteristics. The Dynojet main jet hole size is measured in millimeters. For example, a DJ142 has a 1.42mm hole.

Michael Cory
Research & Development
Phone: 800-992-4993 EXT. [xxx]
Michael@Dynojet

Posted by: okbeast

Great write up! I think I've got mine jetted right (main jet wise) but it still has that minor gurgle in it when I let it cruise down in a higher gear, so I'm thinking I just need some minor adjustments. Now I have an idea of how to do them.

Posted by: 400eXr1d3rZ

Is a Keihin pilot jet the same size a Dynojet pilot?

Posted by: JJ69

my 87 250x has the baffles pulled out of the exhaust.. will this affect the air/gas flow?
should i get it jetted?

Posted by: zeppelin

anything done to the intake or exhaust system will affect airflow, if it was my bike, id rejet

Posted by: byrdman37876

good forum zeppelin but i have a new 2007 trx 450r and i kinda hate to take it apart. it has that hesitation when i hit the gas hard. do u think i could lean it out a little bit and not mess with the jetting. it is all stock and i run 89 fuel in her. it does it at midrange when i go to stand the front end up.

Posted by: 400eXr1d3rZ

quote:
Originally posted by byrdman37876
good forum zeppelin but i have a new 2007 trx 450r and i kinda hate to take it apart. it has that hesitation when i hit the gas hard. do u think i could lean it out a little bit and not mess with the jetting. it is all stock and i run 89 fuel in her. it does it at midrange when i go to stand the front end up.

It comes lean from the factory to pass emission (i think). Go up about 2 sizes on the main jet.

Posted by: zeppelin

quote:
Originally posted by byrdman37876
good forum zeppelin but i have a new 2007 trx 450r and i kinda hate to take it apart. it has that hesitation when i hit the gas hard. do u think i could lean it out a little bit and not mess with the jetting. it is all stock and i run 89 fuel in her. it does it at midrange when i go to stand the front end up.

if i were you id try to play with the fuel screw first. just sppend an hour or two trying different setting with it, and if that doesnt help maybe move the clip on the needle down one. if you still have problems your going to have to mess with your jets.. let me know if you still need help though ; )

Posted by: Kaleigh

cool thread.. great info.. when I did my jets.. I went up a size til it would bog at full throttle.. then I backed down a jet size.. got some backfire so I turned my idle screw up 1/8 of a turn and its great.. Jetting is hidden power.. Alot of people dont spend enough time here..

Posted by: 450ERRRRRR!

byrdman37876, You need to be running 91-93 octane in your R. It even says so in the owners manual. It will make a difference. I'd open up the air box and replace the stock main with or about a 170 main. Thats what I did and no hesitation what so ever here.

Posted by: Grumpy Wookie

Zepplin, this was an awesome thread! I finally understand what will happen if I change the position on the needle! Thanks. Plus the conversion chart is great!

450ERRRR, you said you are running with the airbox lid off? When I re-jetted (Dyno-Jet kit) and tried to run with the lid off, it coughed and sputtered at anything over half throttle. What elevation are you at? And what brand Jets are you using? (so I can use the niffty conversion chart that I have now printed out and keep in my wallet). I put in a 128 DynoJet Main, and put the lid back on, and its running good enough for the girls I go out with, but I think I can get more out of it.

Oh, and agreed on the fuel octane suggestion for byrdman. Can you say DETONATION!

Thanks all, this has been helpful.

Posted by: casey_lamm

Rejetting a 07 400 ex.

I have added HMF slip on and Dynojet Stage 1 (k&n filters). Those will be my only 2 mod for awhile.

To start...Most people says change the pilot jet to a 42. Should I grab any other sizes...or is 42 pretty much a given.

My riding alt is 1300-1800ft

Now for the main jet...what size should I start with and what should I watch for?

After making the changes how should I ride the bike and what should I look for as indicators Im dialed in correct.

Also, the dynojet instructions have you turn the fuel screw in till it seats then back it out 3.5 turns! Does that seem very conservative to you! most people stay stock then out another 1/4-1/2 turn. What do you think...should I do it like the instuctions say and risk fouling the plug...

Your help is much appreciated. Im very new at this, but decided I would rather do it myself and learn.

Posted by: 400eXr1d3rZ

quote:
Originally posted by casey_lamm
Rejetting a 07 400 ex.

I have added HMF slip on and Dynojet Stage 1 (k&n filters). Those will be my only 2 mod for awhile.

To start...Most people says change the pilot jet to a 42. Should I grab any other sizes...or is 42 pretty much a given.

My riding alt is 1300-1800ft

Now for the main jet...what size should I start with and what should I watch for?

After making the changes how should I ride the bike and what should I look for as indicators Im dialed in correct.

Also, the dynojet instructions have you turn the fuel screw in till it seats then back it out 3.5 turns! Does that seem very conservative to you! most people stay stock then out another 1/4-1/2 turn. What do you think...should I do it like the instuctions say and risk fouling the plug...

Your help is much appreciated. Im very new at this, but decided I would rather do it myself and learn.

Try a 155/158 main jet (keihin, just convert it to dynojet using the chart above.) and grab a 40 and 42 pilot jet, turn the fuel screw fully in, then turn it 2.5 turns out from seated.

Posted by: casey_lamm

stage one instuctions are telling me to use a DJ146 which is a 160

My kit includes a dj165 dj160 dj 155 dj150 dj146 dj142 dj138 dj134.

Should I start with the DJ146 (keihin 160) and work down from there?

If it were too big, how would I know when test riding.

quote:
Originally posted by 400eXr1d3rZ
Try a 155/158 main jet (keihin, just convert it to dynojet using the chart above.) and grab a 40 and 42 pilot jet, turn the fuel screw fully in, then turn it 2.5 turns out from seated.

Posted by: honda6490

Does anyone have a link to a pilot jet size conversion chart or are they almost the same when you get down to that low of a number? Thanks

Posted by: Glocked2

I just added a Big Gun Evo X slip-on pipe to me '07 TRX450ER with a Mod-Quad power intake system (K&N filter) with a Dynojet jet kit. I replaced my stock needle w/ the one Dynojet sent and set clip at 4th position as DJ recommended. I then used the DJ Main jet adapter they send w/ kit and installed thier DJ124 Main jet. Both of these steps are easily done w/o removing the carb. I then used the supplied DJ air/feul screw tool and turned the screw out from all the way in about a turn and a half approx. I am currently running my stock airbox lid w/ no snorkel on it (just pull it off the lid). My bike runs pretty good w/ no hesitation at all at 3000 ft. I can also run the Big Gun Quiet core insert w/o any changes to carb or anything else so I can jump back and forth between quiet and loud depending on where I'm riding which is great. I do run 92oct.

The only thing that I have to try now is to put on my K&N powerlid and try. I may have to bump the main jet up to DJ128 and possibly adjust the air/fuel screw, but we will see.

Posted by: hondaman440ex

Great Post!

Posted by: leasureryan

I have a 99' blaster...and understand how to jett, somewhat, but With opening up the air flow as much as I am going to all at once, I don't want to burn my rings out by running to lean, would rather start out a little bigger, and work my way down. What do you suggest?

Okay.... I have stock jetts now. I'm going to put on a DG pipe, FMF Turbine core 2 silencer, remove air box lid, replace reeds with Vito's carbon flex reeds and a spacer, Rick's Hot Shot CDI box, K&N filter with outerware. I know that is letting alot more air flow flow happen, so how much fuel should I be pushin' in this thing?

Also, if you think that I should use something differant from this combination, or have suggestions for me, let me hear it. Tryin' to give it a little sac, and keep it on a budget.

Thanks in advance

Posted by: jzanehope

Ok now that I understand the jet issue I still have a question. I have an '07 250EX with a HMF full exhaust kit, and K&N filter. I know I need to rejet but the confusion came with the exhaust kit from HMF. They sent me a 125 jet with the kit. Stock on the 250EX is a 95 jet. Oh and all they sent me was a jet, no needle at all. Shouldn't there have been other parts than just a lone jet? Isn't this an extreme jump in jet size for my bike? I also have an "Outer Wearz" pre filter for running with the airbox lid off but the one time I tried it, it bogged too much so I put the lid back on. My elevation is 3,000 to 5,000 feet. Shouldn't I just go up to between a 100 and a 110 jet? Also last sunday I went riding all day with just adjusting the fuel screw and it had plenty of power and super throttle response but the temp gague was reading a consistent 330 degrees on average with the ambient temp about 70. Isn't this too hot? Does that mean it may be running a little lean? I pulled the spark plug to check it after riding and it looked great. mostly black with a gray electrode which would normally mean it was set just right. I really don't wanna keep riding with out rejetting if there is a chance of damage but I'd really like you all to give me your input on this since I suck when it comes to carb work.
Thanks alot

Posted by: leasureryan

it sounds like a reasmonable increase in jet size to me. Rather rich than lean. I would try the 125 with the air box lid off and outerwear on. You have to realize you are moving alot more air now, so you will need alot more fuel for the proper mixture. Try the 125, if the plug is wet, or if it boggs from to much fuel, go buy the next 2 sizes down jets from the dealership. Also, needle size only has to be changed when you are doing MAJOR upgrades. For minor upgrades like this, you will only need bigger main jet, maybe, just maybe go up one size pilot(slow) jet size too, Test and tune bro!, Start rich though.

Posted by: 97WarriorRider

hey..where is the air/fuel screw on the 400ex carb?

Posted by: Bolinracing

1) I also need to know, where is the air/fuel screw on a 400ex carb. Also, is it an air screw or is it a fuel screw. This way I know which way to turn it.

2)Same as 1. but for a 450R

3)Backfire(popping) on deceleration means to lean at LJ or pilot jet right? So if I have mile popping when I let off at the end of straights, I should go up one pilot size?

Thanks.

Posted by: tcr0148

yes, you need to change your main jet size to allow more fuel mixture. Also, make sure that you don't have an exhaust leak where the header bolts to the head, because that can cause popping also

Posted by: THORrider350

anyone know what i should do on my 07 yfz 450... i am adding afull alba exhaust... i bought a NCVS needle i am putting on, a fuel screw, and a boysen quickshot cover... and ideas???

Posted by: monkeyballs

Ok well I have a honda 400ex 2002. I have full system yosh pipe and almost no air box lid. (there is like 60 holes in it...) lol. Any who when I jetted it I took out the spark arrestor and I was wondering if it would run better with or with out the spark arrestor?