K&N is a brand of air filter.
Alright, recently there have been a lot of posts about K&N filters. This is what you need to know.
1. Get the outer diameter of your carb.
2. Get the dimensions of the area where the new filter will go.
3. Go to KNFilters.com and search for a filter that will fit your particular application.
Some of these filters are rather large, especially if you have a 2.25” opening or greater on your carb. In this case, one may have to use a step-down process. I know that some mopeds (Tomos A35 motors in particular) use air-boxes that have a rubber connection tube between the box and the carb.
In these cases, one can take the measurement of the INSIDE of the tube that goes into air-box. Then, search KNFilters.com for a filter that will fit inside of it. Then, clamp it using a hose-clamp or a zip-tie. Don't worry about being EXACT with this procedure, the KN filters will stretch a bit and so will the tube.
Other than that, if you’re not worried about a huge-ass filter poking out the side of your moped then you can go for the 2.25 opening ones. They’re around 4” in diameter and about 3-6 inches long. They wont fit behind any stock plastic, but they will look cool.
Here is where you find the filters:
CONE SHAPED FILTERS
ALSO: AS A REMINDER. K&N FILTERS NEED OIL IN ORDER TO FILTER AIR PROPERLY. PLEASE, USE THE OIL INCLUDED WITH THE KIT, OTHERWISE IT’LL BE LIKE USING NO FILTER AT ALL.