How to fine-tune your jetting: Difference between revisions

[[Category:Intake]]

Jetting is 100% bike specific.

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.

'''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!  

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.  

Reassemble.  

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.  

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.  

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

==Conclusions==

*They may contribute to [[What is carburetor ice?|Carb icing]].

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.  

'''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.  

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.  

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

Start from tuned jet and stock needle height, and repeat the same process used for the main jet. If it runs better with more washers, when you get to about 4, go up a size on the main jets and start with stock needle height again. Continue to add/remove washers until it runs correctly.  

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

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.  

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 [[Changing the spark plugs & brands to avoid|Spark Plug page]] for information on plug color.

[[How do I install 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 [http://www.factorypro.com/tech/carbtun.html tuning guidelines]. Obviously, follow the link to high rpm engines.