How to jet your bike

[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.

[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.

[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.

[zeppelin]

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

[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.

[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

[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..

[ELewandowski]

Nice write-up, it helped me a ton.

[zeppelin]

thanks man, im glad i could help

[04TRX400EX]

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