How to fine-tune your jetting

From Ninja250Wiki
Jump to: navigation, search

Note: For the same general information presented a different way, clicky.

Before you start

Jetting is 100% bike specific.

Each bike will need something slightly different, even if they're in the Exact same location. And we all know that the majority of us aren't really close to many others.... location REALLY means major differences in what works best.

The best solution is taking the bike to a professional with a Dyno and an EGA (electronic gas analyzer, sometimes called a "Sniffer" or "4 gas analyzer") to see what kind of exhaust the bike is putting out at the different RPMs while under appropriate load. The next best solution is to dive head and heels into the massive amount of jetting information that's been published. There are dozens of different books dedicated to the subject. You'll have to read, then apply the information learned so you can feel what lean and rich are like. There's absolutely no way to impart that knowledge in the space of the FAQ.

You may want to modify the battery box if you are going to put an aftermarket exhaust on your bike or otherwise plan on rejetting. Why? Because the physical carb adjustment is simple. It's the fine tuning process that can be time consuming and tedious. It will require you to pull the carbs AT LEAST TWICE (if you are extremely lucky), but probably more like three or four times to get everything right.

Carb adjustment

DO NOT begin tearing the bike apart until you have read this ENTIRE document. You need to understand the fine tuning process before you start pulling main jets out of a hat!

Step 1: Remove the carbs

Step 2: New main jets

Remove the float bowls (4 screws.) Unscrew the main jet: the main jet screws into a nut. Hold this with pliers and turn the jet out with a flathead screwdriver. The jet will be in there TIGHT. Make absolutely sure you are using a flat head that fits properly or you will strip the jet.

See this article and this one for pictures and general information concerning carbs.


Step 3: Raise needles

Remove the black covers on the top of the carbs. The screws have different lengths, so keep track of which one goes where. The plastic vacuum pistons are held under pressure by a spring, so be careful when removing the top.

Carefully pull the vacuum diaphragm/piston unit out. The rubber diaphragm is the most delicate part of the carb and is easily damaged.

Shake the needle out, raise it with (however many) 2.5 or 3 mm washers.

  • If you're using aftermarket needles, start with the clip in the 3rd position (middle). This is usually for a stock or mildly modified setup, such as a slip-on exhaust. You can use washers to make "half" steps, too, for super-fine tuning.


Raising the needles can actually be done without removing the carbs from the bike. Pull the tank, without pulling the upper fairing. You'll need a good-condition #2 phillips screwdriver with a shaft about 10" long. A multi-bit screwdriver won't work because the shaft is too thick. Just slide the screwdriver through the brackets to pull the screws out, pop the cover off, pull the springs, and fish out the vacuum pistons. Make your adjustments and reassemble. Make sure to drop the needle straight down, so that it goes into the collar above the emulsion tube; you can easily see if it doesn't. Put the screws and brackets back on, and you're good to go.

The big key to this is to not have sticky bolts... If they are stuck, you'll need to pull the carbs so you can put them in a bench vise and hit them with an impact driver.

Step 4: Adjust mixture screws

Note: Turning the screws IN decreases the amount of gas at idle. Turning the screws OUT increases the amount of gas at idle.

Step 5: Attach carbs to bike

Carb Tuning

You have three areas to tune: idle, midrange, and upper. You start at the main jet and work your way down to the mixture screws.

Approach the mid and upper by "feeling" the bike through the respective RPMs by rolling onto full throttle. ie... roll onto full throttle from 6,000 RPMs/ roll onto full throttle from 9,000 RPMs. Approach the idle as outlined below.

NOTE: DO NOT simply survey for jetting settings and expect to drop in parts. That will not work. You CANNOT properly jet from arbitrary settings. You MUST start from stock, addressing each area one at a time. Hence, this will take the better part of a day/couple of days if you want it done right.

Step 1: Find the proper main jet.

Note that you will have to buy 2 jets of each size, one per carburetor. Start from the stock jet and needle height, and increase the main jet size by ONE SIZE at a time, until you feel the upper too rich. Then back up to the previous size. Please note that the amount of gas passed by the main jet increases exponentially with main jet size, as the circumference of a circle increases exponentially with diameter! What that means is that the difference between jet sizes is not equal.


~Start from stock.

~#105=Way too lean. Bike won't rev past 6K RPMs.

~#108=Bike revs to redline, but sputters at high RPMs.

~#110=Bike pulls hard to redline.

~#114=Bike bogs at high RPMs.

~Go back one size to #110s.


LEAN = Stuttering. Runs better when bike is hot. White plugs.

RICH = Bogging. Runs better when bike is cold. Black plugs.

Step 2: Find the proper needle height.

Start from the tuned jet that you decided on in step 1. Use the stock needle height, and keep raising the needles by adding washers. If it runs better with more washers, then, when you get to about 4, go up a size on the main jets and start with the stock needle height again.

Even after you get the main jet correct, you will often end up with a midrange stumble at 5k-6k rpm. This has little to do with main jet size. You can take care of it by adjusting the needle height. Lower the needle until you get a mid-range stumble, then raise it by 1 washer at a time until you get rid of the stumble.


LEAN = Stuttering. Runs better when bike is hot. White plugs.

RICH = Bogging. Runs better when bike is cold. Black plugs.

Step 3: Find the proper idle mixture.

Find a short (2" or less) screwdriver or driver bit and adjust the mixture screws with the carb on the bike and the bike running. Turning the screws IN decreases the amount of gas. Turning the screws OUT increases the amount of gas.


LEAN = Blipping throttle causes the rpm to rise, hang for a moment, then drop back down.

RICH = Blipping throttle causes the rpm to rise, drop down below the idle speed setting, and the rise back up.


Ideally, you would follow these steps, examining the plugs after each step. However, in the absence of a dyno machine, it's not always possible to find a place where you can, for example, do several full throttle runs and immediately shut the bike down and examine the plugs, while doing carb adjustments on the spot!

It is advisable though, to monitor overall plug conditions after the jetting process. "Feeling" the bike will get you close, plug color can help you nail it. See the links on the Spark Plug page for information on plug color.

If you're getting a jet kit

Factory Pro is the recommended jet kit for the Ninja 250. If you get one of theirs, it only makes sense to follow their tuning guidelines. Obviously, follow the link to high rpm engines.