Aftermarket replacements for the stock pleated paper air filter are not good choices. Obviously, if your bike has been customized and you can’t use the stock paper air filter, you do what you have to. But realize it’s a technical compromise. Or if your bike came with a foam filter from the factory, though not many vintage Honda streetbikes did, then again, you have what you have.

The general public is not aware how restrictive Honda’s intake systems are. This restrictiveness is due to the federal certification process Honda has to go through to bring a new model into the country. During this process Honda has to, among other things, ride the prototype bike past a sound meter. They therefore engineer many things into the bike to make it as quiet as possible. One of these is the air filter housing. As a result, Honda’s intake systems are so restrictive, that on a Honda CBX1000, removing the airbox requires the carburetor’s main jets to be increased ten sizes. Yes, ten. The K&N company has capitalized on this characteristic of motorcycle sound control and has been very effective at selling their product as a power-increaser. They have also promoted the K&N product as a wash-it-don’t-replace-it and “lifetime” alternative to the stock air filter, which is misleading and very unfortunate.

The K&N gauze air filter flows more air because it flows more dirt. This is confirmed by users around the globe. Whatever your preferences, you cannot escape this simple fact. And you can easily prove to yourself how wide-open the K&N is. The nearly 100-year-old industry standard procedure for testing the contaminant condition of a pleated paper air filter is to hold the filter up to a naked 60-watt incandescant light bulb. Light should get through evenly and plentifully throughout the filter. On a good paper air filter it does, a bad paper air filter blocks some of the light. And a K&N? Not only can you see the light, you can see the actual light bulb. It’s more pasta colander than filter. To further understand how much more more air the K&N air filter will pass, consider that the carb rejetting for individual K&N filters and that required for simple intake air horns (“velocity stacks”) is exactly the same on the aforementioned CBX. That is, the engine “sees” the two as identical. Thus the individual “pod” type K&N is the same as no filter. Incidentally, pleated paper air filters cannot be accurately judged by merely looking at their external cleanness. Many high output Hondas will misfire at high rpm when the filter looks good by eye. By eye is no way to tell.

And calling the K&N air filter a filter is exactly my complaint with the product. The K&N (which represent the founders"Ken" Johnson and "Norm" McDonald) company started in 1969 as a flat track racing team, run out of a Kawasaki dealership in Riverside, California. The filter should have stayed a race-day item and not have gone prime-time. Do what you want with your race bike but telling people the K&N is simply an alternative to the stock filter is deceptive on many levels. It is a rock guard. It is not a filter. And far from being a “lifetime” air filter, at some point the gauze filter becomes un-cleanable—it reaches a point where the dirt will not come out. The company used to admit this online. Of course, there is even less filtering ability if you don’t oil the filter in the first place. The trend to not oil (I was surprised anyone would do this but it apparently really is “a thing”) may have begun when folks had problems with their fuel injection MAP sensors which would fail when the filter's oil hit them. Or maybe it’s just one of those trends that nobody knows how it started. But whatever the reason, not oiling the K&N just makes things even worse for the engine. There are reports of folks who have found silt clods—simply massive amounts of atmospheric dirt—built up inside their bike’s FI throttle bodies. Look it up. It’s real. And I’m not even talking about the independent studies that have been made that rank the K&N product as the worst filtering of all.

I don't mind if folks use K&Ns. I have used them on my own machines and I have built bikes with them—always making sure the customer knows the result will be less expected long-term miles from the engine. In vintage perhaps this is less a concern. Not many ride their vintage Hondas more than a few thousand miles in a year, if that. I don’t even, any more.

Foam air filters are also a problem, they have their own issues. Many of them are actually more restrictive—not less—than the stock paper air filter. And of course the oil the foam filter requires always gravitates to the bottom, even when a special purpose racing filter oil such as Maxima FFT is used. The foam filter also eventually disintegrates, but not until the carburetor has been fouled with the oil. And the foam air filter is a lot of work to clean correctly (see my video), requiring as it does a handful of cleaning stages including solvent, then detergent, then clean water and either hot or moving air (not compressed air). And as it nears the end of its useful life, cleaning efforts actually help destroy the filter—its glued-togther seams start to separate. Foam air filters are also hugely over-represented in air filter fires on powersports vehicles. Even the foam air filters that came stock from the factory have many of these same issues.