MXA WRENCH TECH: 20 MOST IMPORTANT FILTER FACTS KNOWN TO MAN

January 16, 2014
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Most of the time you don’t think about it. It doesn’t strike very many people as a performance product. It’s an afterthought when it comes to hopping-up your steed. It’s one of the cheapest parts of your $6000 bike. Yet, nothing determines whether you go-or-blow more than your bikes air filter.

Have you been taking good care of it? The MXA test crew sat down to answer the 20 most crucial questions about air filters. The answers will surprise you. This is the whole story. 

QUESTION ONE: ARE ALL FILTERS THE SAME?

Yes and no. All modern motocross filters, whether stock or aftermarket, use open cell foam, but the makeup of that foam varies from manufacturer to manufacturer (especially between stock and aftermarket). Some of the basic differences that can be noticed are that the Acerbis filter uses a light foam, Uni has dual-stage foam (that is separable) and Twin-Air uses dual-stage foam that is bonded together.

QUESTION TWO: DO FILTER WEAR OUR?

Yes. The filter does not wear out from doing its job, but from the cleaning process. The harsher the cleaning solvents the faster the filter wears out. The foam gets out, dry, brittle and loses the integrity of its labyrinth-like webbing.

QUESTION THREE: WHAT IS IT BEST FILTER CLEANER?

Kerosene is probably the best filter cleaner. It is the least volatile and is not as harsh on the foam. Obviously, specially made air filter cleaners do any excellent job.

QUESTION FOUR: WHAT IS THE WORST FILTER CLEANER?

Gasoline. Gasoline breaks down the petroleum-based structure of the air filter. It ages the foam. Also bad is Snap-On or other parts cleaner solvents. They have degreasers in them that eat the filters glue.

QUESTION FIVE: WHAT IF YOU INSIST ON USING GAS?

Wash the filter immediately with soap and water to get all the gas out of the filter before it harms the foam.

QUESTION SIX: WHAT IS THE LIFESPAN OF AN AIR FILTER?

An air filter could last forever if you never cleaned it, but your motor would probably expire in short order. A well cared for filter will last 50 cleanings. After that many cleanings the glue that bonds the filter pieces together gives out.

QUESTION SEVEN: WHAT ABOUT THE RUBBER PLUG?

The rubber plug that guides the mounting bolt on many styles of air filters is not glued to the filter (even if it feels like it is). Some cleaning chemical will harden the rubber and make it stiff. As the rubber plug hardens it is prone to cracking or leaking around its edge. To avoid this, always pull the rubber plug out of the filter during cleaning. Then spray a little oil on it and insert it back in.

QUESTION EIGHT: WHATS THE BIGGEST FILTER MISTAKE?

Twisting or wringing the filter to get it dry.

QUESTION NINE: IS DIRT THE WORST ENEMY OF A FILTER?

No. Water is the worst enemy. Dirt is captured in the stickiness of a well-oiled filter. Water can impregnate the oil, disperse it and allow dirt to pass through the filter. If you wash your bike with a spray washer always re-oil your filter — even if it’s still clean.

QUESTION TEN: SHOULD YOU GREASE THE LIP?

If your filter has a thin base (a surface area made from compressed foam) it is wise to grease the interface between the filter and air box. Filters that have thick foam on their base don’t need to be greased, but it doesn’t hurt. Use waterproof grease.

QUESTION 11: SHOULD YOU CLEAN THE INSIDE OF THE AIRBOX?

Yes. Most modern air boxes have nooks and crannies where dirt can hide. Why put a clean filter in your bike only to have dirt inside the filter land on it when you go over the first jump?

QUESTION 12: HOW DO YOU CLEAN AN AIRBOX?

Factory mechanics remove them from the bike and hose them out. This may be too much work for the average backyard mechanic, but he should still use soapy water and a brush to clean the interior of the airbox. The simplest way to do this is to invest in a plastic air intake cover. These plastic covers thread onto the airbox post and seal off the opening to the carb. With the airbox intake cover in place you can slop water around without fear.

QUESTION 13: CAN YOU OVER TIGHTEN THE FILTER BOLT?

Yes. Some filter cages mushroom on the end of the post when it is over tightened. Back in 197 and 1998 Suzuki owners had to mill 3mm off the end of the cage post to stop the bolt from crushing the end. as a rule of thumb, just tighten the filter bolt finger tight.

QUESTION 14: HOW DO YOU KNOW IF THE FILTERS TIGHT?

Tightness of the bolt is easy to check, but it’s more important to make sure the filter and cage are seated against the airbox. To check this, reach down into the airbox and run your hand around the lip of the filter. If you feel any gaps, take it out and reinstall it. This is very important for owners of pre-2011 KTM’s. There cage could sometimes get misaligned at the back of the airbox.

QUESTION 15: HOW DIRTY CAN A FILTER BE?

Sometimes you can tell by looking at a filter if it needs cleaning. The two most important areas to look for are: (1) Surface area coverage. If the air filter has dirt covering every square inch of it. It needs to be cleaned. (2) Dirt depth. Even a filter that looks clean can have small particles of dirt deeply embedded into the pores of the filter. When in doubt. Clean the filter.

QUESTION 16: DO YOU HAVE TO USE SPECIAL FILTER OIL?

Yes. It is possible to use motor oil, but it’s not as good as filter oil. Motor oil is too thin and will pass through the filter and do two things: (1) It will drip out of the airbox vent onto your garage floor. This is bad because it causes a mess and because it means that your filter is no longer oiled. (2) Motor oil, because of its light viscosity, will suck into your motor when you gas it. Lots of times you see bikes that sound raspy and overly rich in the first couple laps practice. That is because the riders just oiled their filters and the oil is passing through their motors. Specially formulated filter oil has chemical and carriers that help disperse the filter oil evenly and then evaporate.

QUESTION 17: WHEN SHOULD YOU OIL YOUR FILTER?

It’s best to oil your filter the one day before the race. That way the carriers can disperse the oil and evaporate.

QUESTION 18: CAN YOU OIL A FILTER IN ADVANCE?

Yes. The MXA test crew pre-oils its filters and puts them in Zip-Loc bags. A pre-prepped air filter can last for months in a plastic bag (it shouldn’t be air tight).

QUESTION 19: IS THERE A WAY TO SAVE MONEY ON FILTER OIL?

Yes. Buy a couple bottles of filter oil and pour them into a sealed plastic tub (one big enough to put an air filter in). When the time comes to oil a filter: open up the tub; submerge the filter; squeeze the excess oil out of the filter; close the tub. If you only submerge clean filters in the tub you can use the same oil over and over again. Filter oil, sealed in a tub, will last for months.

QUESTION 20: WHAT IS THE BEST WAY TO CLEAN A FILTER?

There are ten steps to a perfectly clean filter.

    (1) Remove it from your bike. Be careful to avoid letting any dirt fall down in the motor. Once the dirty filter is out of the bike, slip a plastic airbox cover over the intake (Acerbis and Twin-Air make airbox covers).
    (2) Submerge the dirty filter in filter cleaner (commercial cleaners and kerosene are best, but use your own judgment). Swirl it around and massage it to get the dirt out of the pores of the foam.
    (3) Remove the filter from the cleaner and let it air dry. The cleaner will evaporate fairly quickly.
    (4) Wash the filter in a tub of luke warm soapy water. This second cleaning may seem redundant, but let the amount of dirt that comes out be the judge of that.
    (5) Let the filter air dry. We do know riders who put their filters in a clothes dryer (on cold air) and tumble dry them, but we don’t recommend this.
    (6) Once the filter is dry, submerge it in filter oil. Submerge it until the filter is saturated with oil. It must be 100 percent oily.
    (7) Squeeze as much of the excess oil out as possible. Squeeze firmly, but do not twist or wring. Twisting tears the web-like structure of the open-cell foam.
    (8) Dab the outer surface of the with a rag or paper towel to get any excess oil off.
    (9) Clean the airbox with a brush, rag and soapy water. Allow the airbox to dry and then remove the plastic airbox cover.
    (10) Insert the freshly oiled filter into the bike (greasing the lip is optional) or store it in a plastic baggy for later use.

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