missing "sub air filter" 17253-KT8-000 (not shown on regular maintenance schedule)

[aSTerix]

I was browsing the parts fiches and spotted this foam air filter (item 12 in image), it doesn't show up on the maintenance schedule. I checked and I didn't have the foam air filter fitted on either of my bikes.

I have a 2000 ST1100 AY

Anyone know what this filter does and what is the likely impact if it is missing ?

Does anyone change out these sub filters when they change the main air filter and should it be oiled or left dry ?

 

[ST Gui]

What bike is that for?

 

[dwalby]

so are you saying the filter element is missing, or the entire assembly including the housing?? I suspect you mean just the filter, its a thin strip of foam rubber that will get dirty and deteriorate over time. If you cut a piece of a foam sponge and wedge it in there you'll probably be OK. Never paid much attention to mine, I don't think its anything critical.

 

[aSTerix]

What bike is that for?

Its a year 2000 ST1100 AY Ex -police

 

[aSTerix]

so are you saying the filter element is missing, or the entire assembly including the housing?? I suspect you mean just the filter, its a thin strip of foam rubber that will get dirty and deteriorate over time. If you cut a piece of a foam sponge and wedge it in there you'll probably be OK. Never paid much attention to mine, I don't think its anything critical.

The foam air filter is missing, the plastic housing is there.

I live in an extremly dusty (sand) environment so want to do as much as possible to stop the stuff getting where it shouldn't !

 

[Roger]

On my st1100x, it fell to bits. Looks like Honda expected the filter to be washed once in a while, but there's no mention of a maintenance schedule. I simply found some foamy/filter stuff and cut it to size and shape. It keeps debris out of the carbs....................

 

[Bush]

It is important. The assembly is there to maintain atmospheric pressure on the outer side of the carb diaphragms (best I can explain it) and the little foam filter is there to keep larger particles from getting into the carbs, potentially damaging the diaphragms. When they disintegrate over time, those foam bits can also cause problems in the carbs. Best to check it and replace when necessary, while the air filter element is being changed.

 

[Erdoc48]

A good replacement is a lawnmower air filter (which is flat, like a big flat square you can cut to size- I bought one and made 2 filters out of it)- I got one from Lowes and it was green in color and had a wire mesh over the front and back of it. I cut it to size and now it resides where the deteriorated sponge filter was. Bikes work perfectly with it as well.

Also, to answer OP’s question, DON’T oil the filter

 

[ReSTored]

It is important. The assembly is there to maintain atmospheric pressure on the outer side of the carb diaphragms (best I can explain it) and the little foam filter is there to keep larger particles from getting into the carbs, potentially damaging the diaphragms. When they disintegrate over time, those foam bits can also cause problems in the carbs. Best to check it and replace when necessary, while the air filter element is being changed.

+1. Filter is a couple of bucks and not replacing it can lead to a time consuming repair. I replace mine when doing the air filter.

 

[Oldbikefixr]

These are a case of 'out of sight, out of mind'. I checked the one on my 1991 in 2015 while doing my just bought it inspection. Also bought a Honda manual and followed all the inspection steps on the schedule chart. It was hard, and little pieces would rub off the touch. Replacement was a decade or so overdue. I had a spare foam filter for my pushmower so I (per suggestions on this site at the time) sliced it into the right shape and installed it....after vacuuming out the housing and hoses. I've since rebuilt the carbs and found no residue from the filter.

 

[dwalby]

These are a case of 'out of sight, out of mind'. I checked the one on my 1991 in 2015 while doing my just bought it inspection. Also bought a Honda manual and followed all the inspection steps on the schedule chart. It was hard, and little pieces would rub off the touch. Replacement was a decade or so overdue. I had a spare foam filter for my pushmower so I (per suggestions on this site at the time) sliced it into the right shape and installed it....after vacuuming out the housing and hoses. I've since rebuilt the carbs and found no residue from the filter.

yeah, same here. At some point in the 23 years I've owned my bike I cut a piece of foam and shoved it in the plastic housing and never gave it a second thought. Can't recall how badly the original had deteriorated, I think it was still mostly there, but crumbling to the touch. I think you'd have to be extremely unlucky for the filter to get sucked into the carbs and cause problems, I can't recall anyone ever making a claim here of having problems with this.

 

[CYYJ]

...I think you'd have to be extremely unlucky for the filter to get sucked into the carbs and cause problems, I can't recall anyone ever making a claim here of having problems with this.

I recall reading a post some time ago made by a forum member who rebuilt the carbs on his ST 1100. He mentioned that he found quite a bit of debris from a perished foam sub air filter that had worked its way into the carb assembly. He didn't indicate that the debris itself was causing any problems, simply that the debris had made its way to the carb assembly.

My Swiss dealer, who has been doing maintenance on my ST 1100 for the past 18 years, has always replaced that little sub air filter with a new one at the same time they changed the main air filter. But having said that, I think that the "cut up a piece of lawn mower air filter" suggestion that @Erdoc48 and @Oldbikefixr made would be equally effective.

For the benefit of those who are not familiar with the location of this filter, the photo below shows where it is. It is easily accessed when replacing the main air filter.

Michael

Sub Air Filter Location

 

[BumbleBerry]

The above photo by CYYJ is fantastic.

Here's my input on our "little foamy friend"!

After removing the top shelter, the secondary air filter housing is visible.



The two hoses, left and right, are attached to the top box. This top box can be removed by pressing the two tabs at above and below of it. Please be gentle in pressing on these tabs, most ST1100s have aged plastics which makes them brittle.



After pressing the tabs, the small box cover can be removed. This will expose the small rectangular foam filter under it. In case of BumbleBerry, whatever was left of the foam filter!



If the foam is there, remove it (or whatever is left of it!) and clean its housing. The Honda part number for this foam is: 17253-KT8-000 and The part is called Sub-air Cleaner Filter. It cost me $3.00 in fall 2021.



Here is the comparing photo of the old and new part. The old foam deteriorated to the point that only two sides of it remained in the housing. I removed it carefully, but it still fell apart.



The new part fits in very nicely.



Finally, put the small box cover back in its place. Make sure both tabs snap in place firmly.

 

[jdmccright]

Yes this keeps the air entering the tops of the vacuum diaphragms clean. When it disintegrates, the bits can get pulled into there. So you might pop those tops off and check them.

As for a replacement filter, I've cut one from a lawn mower air filter.