Tech Tip: Secondary Air Filter

Updated: Feb 19, 2021

You may have noticed this little cylinder on top of your airbox.

This is actually an air filter for your carby. Even if you follow Suzuki's maintenance manual religiously, it's almost never mentioned.


Learn what it does and how to care for it along with suggested modifications.


DR650 carburator secondary air filter in OEM state

The issue

This air filter, often forgotten as part of regular maintenance, the original foam quickly clogs and degrades over time.

First, let's start with what it does.


If you follow the air hose, you'll see it connect directly to the upper part of the carburetor. Most DR riders call it a "secondary air filter" instead of the "main" air filter sitting inside the airbox. You might find confusing information on the web about a "crankcase breather," but this has nothing to do with this little guy.


This "secondary air filter" is a diaphragm vent filter.


NOTE: The DR650 has a breathing hose coming out of the crankcase just behind the starter motor. It leads up underneath the tank and then down to connect to the air box beside the shock. The main air filter is what prevents dirt from getting into that breather hose, nothing else.


To better understand the importance of this little air filter, we need to describe the carburetor used on this bike, the Mikuni BST-40.


We have this little air filter because the DR uses a CV (constant velocity) carburetor. These types of carburetors use a diaphragm to move the slide+needle up when accelerating.

So when you open the throttle, you are not pulling on the slide itself and lifting the needle to bring in more fuel, you are only opening a butterfly valve for air.

As more air gets in, an internal circuit creates a vacuum on one side of the diaphragm, which pulls it up.

Even with perfect carburation, there is always a slight delay between throttle opening and engine response. The popular swap over to a "pumper" type carburetor, the Mikuni TM40, mitigates this problem.


So what happens went the secondary air filter is clogged?

The hose's role allows the non-vacuumed side of the diaphragm to be at atmospheric pressure (open-air) so it can move freely while a vacuum created on the other side.


When the air filter is clogged, it dampens the diaphragm's reaction (the lift), creating an even longer delay between throttle action and engine response. Bummer, right?


And what happens went the secondary air filter is open/destroyed?

Photo thanks to Blackcap from the advriders.com forum

The deteriorated foam allows dirt to get in the carby, and into your engine; the choke circuit also breathes through this filter.


Be aware that it is the source of a few ruined engines, especially if you are riding in really dusty/sandy areas.


This hose breathes in and out based on the diaphragm's movement, so without proper filtration, it will suck in any dirt around, as shown in the picture.


If your filter has deteriorated significantly, you should pull the diaphragm/slide out and clean the carby as any grit here potentially ends up in your engine and, at a minimum, will damage your slide and slide cage.


The solution

Clean it every time you clean the main air filter, improve the OEM filter element or use a better one!

Solution#1:

Briggs&Stratton air filter $9.90 from Home Depot

You can replace the OEM foam with any other open-cell foam.

For example, you could cut a new cylinder out of an older air filter or find a cheap air filter for a lawnmower.


In any case, ALWAYS, oil the foam with air filter oil or at least with engine oil. Otherwise, all foams are useless if not appropriately oiled.

Unless you are in a pickle to quickly fix your bike with what you have, this is not the best solution.

The filtering surface is tiny using that plastic barrel, so it does clog quickly anyway. If you increase the thickness of the foam, you'll get better protection for your engine, but it will also dampen the slide action, so not great either.


Photo thanks to ER70S-2 from the advriders.com forum.

Left is a DIY foam piece from a lawnmower filter. Right is the OEM thin foam piece.


Solution#2:

So we know that the ideal modification would offer better filtration without restricting airflow. This means you need a filter with a larger filtering surface as well. The popular choice is to use a universal "breather filter" like the UNI below.


UNI UP123 air filter for DR650

UNI UP123 1/2" clip-in type

This is the model to use with the DR650. It will slide in the hose and can be secured with a zip-tie (no need to use the metal band clamp that comes with it).


As you can see, the filtering surface is much larger than OEM and done with a piece of fabric sandwiched between a metallic mesh.


Like foam, those types of filters MUST be oiled as they come dry in the package.


Installation wise, it's very straight forward but care should be given to where the filter is sitting to avoid issues with the saddle tong.


Also, if you have an open airbox, it is best to relocate the filter away from sitting right ton top of the larger opening. Even though, the surging issue is not directly related, it best to avoid possible interaction between air sucked in the airbox and air going in and out of the secondary filter.


As you can see in the pictures, the hose holder band from that top screw is not used anymore. Instead, the new filter is secured with a small zip-tie to the nearby wiring harness simply so it doesn't wiggle around and stay slightly recessed under the airbox top.


To do this, cut off about 1" of the hose at the carburetor end to allow for the filter to drop in this location. This keeps it away from the airbox opening and makes room for the saddle tong to go in the frame pocket.


This installation has proven to work very well and you can now check this filter condition easily and clean it as required.


#unifilter #airbox #dr650airbox #diyairfilter #secondaryairfilter #crankcasebreather

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