how exactly does jetting work.....

i read in a forum that during the cold winter months you "fatten" up the jets meaning going up in in the spring/summer does that mean run smaller/stock jets?and what would that make it,running rich or lean?which is better?this is for a yz250f with an aftermarket pipe but has standard jetting,178 main and a 40 pilot,the bike came like this.and where i live in the spring and summer its moderate temperature but can be pretty humid at times

making the jets bigger allows more fuel, smaller jets restrict the flow. I got a 03 YZ250 2stroke and have jus rejetted my carburettor as I have fitted V-Force power reeds, wosner piston and full fmf system. You have mainly a Pilot Jet (controlled by the mixture screw) and a Main Jet, the pilot jet controls fuel flow from 0-1/4 throttle, the Needle clip position controls 1/4 to 3/4 throttle and the Main Jet controle 3/4 to full throttle. The weather will control the air temperature and density so you should jet to suit. I run a 180 Main and 45 Pilot, clip position on the needle 3 (middle).

these are my notes I put together hope they help you! :

2003 Yamaha YZ250R (5UP00) Carburettor Jetting and Tuning.

Carburettor KEIHIN PWK38S

The role of fuel is to cool the engine, and in the

case of a 2-stroke engine, to lubricate the

engine in addition to power generation. Accordingly,

if a mixture of air and fuel is too lean,

abnormal combustion will occur, and engine

seizure may result. If the mixture is too rich,

spark plugs will get wet with oil, thus making it

impossible to bring the engine into full play or if

the worst comes to the worst, the engine may


The richness of the air-fuel mixture required for

the engine will vary with atmospheric conditions

of the day and therefore, the settings of the carburetor

must be properly suited to the atmospheric

conditions (air pressure, humidity and


Finally, the rider himself must make a test-run

and check his machine for conditions (pick-up

of engine speed, road surface conditions) and

for the discoloration of the spark plug(s). After

taking these into consideration, he must select

the best possible carburetor settings.

It is advisable to make a note of settings,

atmospheric conditions, road surface condition,

lap-time, etc. so that the memorandum can be

used as a reference useful for future.

Atmospheric conditions and carburetor


The reason for the above tendency is that the

richness or leanness of a fuel mixture depends

on the density of the air (i.e. the concentration of

oxygen in it).

Test run

After warming up the engine equipped with the

standard type carburetor(s) and spark plug(s),

run two or three laps of the circuit and check the

smooth operation of the engine and discoloration

of spark plug(s).


Insulator Dry/Browm


Running Lean


Running Rich

Mark throttle at 1/8, ¼, ½, ¾ and Full Open points.

0 to 1/8 Fuel Mixture controlled by Pilot Air Screw.

Pilot air screw adjustment

The richness of air-fuel mixture with full closed to

1/8 throttle can be set by turning the pilot air


Turning in the pilot air screw will enrich the mixture

at low speeds, and turning out it will lean out

the mixture.

Standard Position 7/8 turns out

start with the pilot circuit. Turn the air screw all the way in, then turn it out 7/8 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 idling. 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 air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw 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 air screw for the fastest idle, it's time to tune the air screw 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 air screw 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 air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

Standard Pilot Jet = #52 Part No.4MX-14948-08

Bloke who had V Force reeds had #55 but changed to #45

¼ to ¾ controlled by the Needle Clip Position.

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.

Standard Position is #3 (centre groove)

½ to Full Open Fuel Mixture controlled by Main Jet.

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.

Standard Jet No. #180

Bloke who had the V-force reeds changed from #180 to #185

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, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.

Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer."

Mark the throttle housing and throttle so you know where 1/4, 1/2, 3/4 and full positions

Phil, you are close but not 100% correct.

In addition to that, Howard has a 4S. So your info is only about 15% applicable, if that much.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now