Which aftermarket air filters fit the Ninja 250?
Drop-in air filters
This type of filter is generally considered unnecessary, as any performance gains come at the cost of clean air for your engine. They pass more air but also pass larger particles. It's generally considered that there is no noticeable performance increase with drop-ins.
The simplest is the UNI drop-in air filter, which replaces the stock foam air filter in the airbox. For this kind of filter, you do not need to remove or modify the airbox or change the carburetor jetting.
The K&N drop-in does not just "drop in". It doesn't fit where the stock one did, so the original has to come out. The original is held in by a pair of plastic brackets which have to be removed. With A LOT of force. The instructions tell you that you can leave the bracketry in place. You also have to cut a useless chunk off of the lid to the airbox, which is surprisingly difficult to do. Then the thing clamps onto the two tubes in the lid with a pair of hose clamps. Have fun.
If you are willing to remove the airbox, you can use the K&N pod air filters, UNI pod air filters, or EMGO pods. The use of pod air filters slightly increases power, but it also reduces low-end driveability and reliability. Due to the fact that you will be flowing much more air without the airbox, you will need to adjust the jetting in your carburetors after you install pod filters.
Please note: If you are considering putting on pods to make working on the carbs easier, you should seriously consider simply modifying the battery box.
One thing to consider in changing to pod filters is that pods will significantly increase the amount of noise coming from your intake, compared to the stock airbox.
Ease of riding
In exchange for the small increase in power you get from pods, you get a bike that needs more work. You should not mess with your intake (or exhaust) unless you are willing to do it right, which means taking a LOT of time learning what needs to be done and how to do it. Your bike will also be a lot more sensitive to weather and elevation changes.
One of the major differences between K&N and UNI is their filtration medium. K&N uses cotton gauze, while UNI uses foam. Although no one has had an Emgo apart yet, it also uses some form of cotton. Maintenance on these filters involves cleaning and oiling them, just like the stock filter. However, both companies recommend a spray oil, instead of the motor oil that you can get by with on the stock filter.
When oiling these filters, make sure you get the proper oil for the type of filter that you have. Considering how seldom oiling needs to be done, it makes sense to use the oil made by the manufacturer of your filter. It is recommended that you clean and oil your filters at least once a year, more often if you ride under harsh conditions.
On the individual K&N's (#1822) the limiting factor is that unless you are very careful, putting the second one on knocks the first one off because the carbs are so close together.
The R-0990 single unit takes care of this problem. It has two boots sized and spaced perfectly to fit over the necks of the carb intakes. Each boot has its own hose clamp to tighten it down. It's also less expensive.
The low price of the UNI's does come with a slight penalty. The filters don't fit really well right out of the box, which hasn't kept many people from using them. There is a knob on the outside part of the carb inlet that doesn't allow UNI's to seat correctly. You either have to cut a slot in the rubber part of the pods for that knob (not easy), or file that knob out, which works better. If you don't do this, one of the pods will probably jump off. It doesn't have any place to go, so you won't lose it, but you'll hear it from your engine.
Here are photos of 4" UNI pods on an EX250. Note that they do touch each other. This is normal. The left photo is at six months; the right is after three years. These were serviced regularly. Color change is unavoidable. The standard pods now come in black to start with. These are somewhat easier to work with than the stock airbox.
Here is a video clip of the 6" dual-layer UNI's. The 6" ones do fit, but they take up a lot of space. Most riders go with the 4" model.
These have been installed on E-series (1986-87) and F-series ('88-07) bikes. The only problem is that you have to grind a bit of the chrome lip off to make them fit, as they touch each other, just like the UNI and K&N ones. Don't take off enough to break the seal. The picture makes it look like more work than it actually is.
The left photo shows them mounted on the 32mm '86-87 carbs; the right on '88-07 30mm ones. Same Emgo part # for both applications.
One of our members posted that he could not find the K&N RC-1822 in Oz. He did find # KNRC-1820 from Rocket Industries, though.