Proper Dirt Bike/ATV Air Filter Maintenance

            I am only going to cover foam type filter maintenance. If there is only one thing that you are going to do to your motorcycle to take care of it, the air filter is the one thing that you should make the focal point. We rebuild countless motors due to neglected air filters. Air filters that range from rotted and gone (yes, foam rots after a few years), to race filters that more air = more dirt. Rotted filters are inexcusable, because that is a result of the air filter never being checked for years. It is your responsibility to know the condition of your air filter. Yes, we are air filter Nazi’s, but then, that is what we are paid for, to take care of your motorcycle/ATV. I personally don’t have a lot of use for the gauze or paper filters that are out there when used in an off road environment. The air box lid intake (to meet noise level emissions) is generally the restriction in the intake system, not the filter. The filters are designed so that they are capable of flowing more air than the motor requires at wide open throttle, the engineers realizing that the filter will become somewhat restricted during use and the motor would still need full air flow to run. Not to mention, that despite heated competition on the showroom floors for sales market share, the vast majority of the motorcycle/ATV manufacturers use a foam filter element. It would seem logical that if they could slap in a non foam air filter and obtain 5% more horsepower, maintain the intended life design of the vehicle, and get through the warranty period, that they wouldn’t be using foam filters anymore.

        Paper filters get wet, then the bike won’t run well the rest of the day. Gauze types don’t seem to filter well. I have simply taken far too many motors apart due to dirt related wear/failures that had a gauze type filter. Any race engine builder will tell you that when you lose valve to seat seal or ring to cylinder seal that you no longer have a motor (in terms of making race horsepower). Those are more important than the port job, valve job angles, or anything else that can be done to a motor. Few of the guys who spout the positives of these types of filters have leak down testers to verify their theories (or dyno’s for that matter). If they did, I am thinking that they would change their tunes a bit as they see the leak down fall off fast. There are no successful engine builders that don’t have a good air filter service program in their game plan. The air filter is the LIFE of the engine. Nothing is more important to low cost ownership and more hours of riding, and a well performing engine. If you want more air, open up the air box or the lid, carefully, so that water splash doesn’t get directly onto the air filter (start thinking like a vacuum cleaner). With proper jetting to match, the motor will run better with a foam filter in it when doing this, if in fact, your motor needs more air in the first place. 

        A good general test for any air filter or maintenance program is simply to make sure that the boot from the air box to the carburetor is CLEAN. Absolutely no sand or grime. Install your filter and go ride for a day/week/month. Remove the filter and rub your fingers in the bottom of the air boot. If there is debris, you can see my point, if not, congratulations! This is the ultimate test of proper air filtration and maintenance and this must be tested every time that the filter is serviced, otherwise, you will not catch the mistake when it gets made. Motors failures are EXPENSIVE and take time to REBUILD with proper machine work, parts ordering, and turn around times. 

        With regards to the air filter, the motor acts as a huge vacuum cleaner sucking as hard as it can. Pop the top off the air box sometime and light up a cigarette, throttle the motor up and see how far away it pulls from. Realize that it sucks MUCH harder with a load placed on it (you, a mud hole, and full throttle for instance). The off road environment is a mud bowl, water bowl, or dust bowl and while you are sitting up above everything struggling with breathing some dust yourself, the air box is lower than you are, in the middle of the mud, water, dust and the motor is sucking air harder than you are. It is a bad place to try to keep a motor alive. To properly service air filters, you have to learn to think like air. Where can it get into the carburetor at besides through the air filter, where are the leaks when riding in a dust cloud or through the river? Gotta do it. It will save you major money and keep you riding. 

         Remove the air filter from the box. On most of the dirt bikes, you must be very careful to not knock dirt down into the air boot while trying to drag the filter out the top of the air box. Wipe any dirt from the air boot. If you can’t get at it, or there has been leakage (dirt in the boot or rear of the carburetor), it will pay you to remove the complete air box assembly and clean the box, boot, and back of the carburetor totally. Use WD40 or solvent on a shop towel to wipe out the air box. This leaves an oil residue that dust will stick to. If it sticks to the air box, it won’t get through the filter. This also makes the air box easier to clean next time. When reinstalling the air box, be certain that the air boot is fully onto the rear of the carburetor. Look from both sides of the bike to be very certain that this boot is fully onto the carb. Sometimes these boots get hard from fuel exposure and shorten up. They must be replaced at that point. Don’t take a chance. We see them every week where the easy to see side looks fully on, but the other side is barely, or not at all on. Tighten the clamp, but don’t over tighten the clamp. Over tightening the clamp can cause the boot to slide off the rear of the carburetor. Simple stuff, but attention must be paid. 

        The air filter (talking about foam only) should be washed in solvent, or something such as a UNI filter service kit cleaner. Then the filter should be washed with water and dish washing detergent. Something like liquid Dawn, ERA, etc.. Wet it with water, soap it down and work the soap into the pores of the filter. The cleaner will remove the oil (you did oil it last time, didn’t you?) and the soap gets the dirt out of it. You will be surprised at the amount of dirt that comes out during the soap cleaning. 

         If time allows, leave it hang on the handlebar overnight to dry. If not, go ahead and oil it. Air filter oil is special. Motor oil will all run to the bottom of the filter , leaving the top dry. Dry air filters pass dirt. Air filter oil will stay tacky even on top of the filter. The idea is that dirt sticks to the filter and eventually plugs it up, instead of going through it and ruining the motor. The UNI service kit also includes oil (Since we have solvent in the shop, we use either Bel Ray or Silkolene air filter oils). Fully saturate the filter with oil and fold it over onto itself squeezing and working the oil into the filter, allowing the excess to drain off. There is no such thing as over oiling a foam air filter, provided you squeeze out all that you can. If the filter is properly oiled, it will not have streaks indicating unevenness, instead, it will be nearly entirely the same color throughout, indicating that the oil is evenly spread. If the filter shows signs of coming apart, time to replace it. Once you have squeezed out what you can (don’t twist the filter, it will tear, only squeeze), the filter can be reinstalled onto its holder (this usually looks like something that came from a dishwasher). The holder keeps it from sucking itself inside out when full of oil, or dirt. Apply liberal amounts of grease to the mouth (the area that seals to the front of the air box) of the filter. If there is a screw or pin in the rear of the filter to secure it, grease the filter where this passes through. You are trying to seal all potential leaks with grease.

            A special note regarding many older Yamaha ATV’s, the air filter foam slides onto a holder. This holder has a piece of foam glued to the front to seal to the front of the air box. This should be greased. Grease breaks the glue on this ring down over time. I have seen many people install it without the foam ring, leaving a 1/16” gap. Grease won’t stay in this gap. The suction is too high once the filter becomes wet, dirty or full of oil. This condition is the same as not having a filter at all. I have seen many attempts to glue new foam on. The glue that the factory uses is better than anything that I have seen yet. $25-35 for a new holder is far cheaper than $400-1500 for a motor rebuild. Don’t take a chance. Replace it and move on. With a good holder, grease the foam on the mouth of the holder, grease the backside of this holder where the foam air filter slides up against it. Slide this assembly into the box gently a bit back from the front of the box, then slide it forward until contact is made and gently downward into the holding ear. You don’t want to peel the glued-on-foam off when sliding it down. If you slide it forward and then down without getting it as low in the box as you can first, it will damage the glued on foam on that foam on the new holder that you just paid $25-35 for, ruining a motor in the end.