ST1100 Sub air filter replacement!!!

Somewhere I read a tip that the sub air filter should be checked every now and then. I checked my 1993 St1100A with 104,000 miles and the filter had a hole in one end and it fell apart in my hand as I took it out. The filter box is just in front of the carburetor air filter in a little black box with a hose attached to either end. This an easy thing to check when you have the fuel tank/air box plastic off (2 bolts and 2 allen head screws).

I took a new foam filter for my mower air cleaner and cut out a replacement filter with a band saw. That way I wouldn't have to wait for a factory replacement part to arrive and it may have saved me some money.

+1 on using a chunk from a lawn mower air filter.

Jerry

I've heard reports that the airflow through a foam style filter is more restricted than with the paper OEM ones, which would cause rich running. It might be worth getting a pattern part of the same construction type...

Cheers & God bless
Sam "SammyTheSnake" Penny

The new replacement OEM part looks like foam to me.

The main air filter is pleated paper but the aux filter (filters the air coming back from the crankcase) is foam. Adam C. made some from some grey packing foam one time. Looked OEM.

Is this what you have? http://www.davidsilverspares.co.uk/graphics/parts/17211MT3000_large.jpg

Looks like paper to me, but I don't have one in my hands I'll have a look at the one in my bike next time I have a reason to be in there and see what it feels like in person.

Cheers & God bless
Sam "SammyTheSnake" Penny

That photo is the primary air filter.
Original poster is refering to the sub-airfilter located just forward of the main air filter, roughly 2"x1"x0.25" in size

Sammy,

The next time you pull the shelter and air filter cover on an ST1100 take a look at where the 2 tubes connect in the front of the air filter cover.
You'll find a small rectangular box with the filter being discussed here.

Mark

I made mine from a generic green foam outer lawn mower filter. No problem. I also use this for my air compressor filter.

Ralph Sims

Do you think the "pre filter" foam that I didn't use (but saved) from my K&N would work?

Biker George said:

Do you think the "pre filter" foam that I didn't use (but saved) from my K&N would work?

Click to expand...

Yes.

Ralph Sims

Well well well, I knew the ST was a bit greedy, having two exhausts and three brakes systems and such, but I didn't realise it had two air filters

Cheers & God bless
Sam "SammyTheSnake" Penny

Made one out of a foam paint brush until the OEM replacement got in.....The OEM replacement looked a lot like it was made out of a foam paint brush....


'91 ST1100
'05 Road Star

Just checked mine, it fell apart, thanks to you guys for the idea I made one out of a lawnmower filter, a bread knife cuts it just right.


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If you roll your own...please use foam suited to petroleum atmosphere....such as from an automotive/motor vehicle air filter or lawnmower. Many foams will decay quickly when exposed to gasoline fumes.

What I pulled out of mine last winter looked like it came from the kitchen sink rather than the parts department! I replaced it with an OEM one.

I just cut a small patch out of a Terry towel and fit it in. Works great.

Okay, somebody enlighten me. I've seen that sub air filter on the parts blowup in my service manual and been wondering ever since. Getting ready to replace the big one anyhow, so I suppose I'll do this one too. Still trying to figure out why it's there....

Interesting question, I thought it had to do with the airflow to the vacuum actuated slide without introducing dust or other particles. I would guess that if it was plugged the carb would stop operating properly. Also if it disintegrates the little bits would play games with the slide operation.

Slydynbye said:

Interesting question, I thought it had to do with the airflow to the vacuum actuated slide without introducing dust or other particles. I would guess that if it was plugged the carb would stop operating properly. Also if it disintegrates the little bits would play games with the slide operation.

Click to expand...

An excellent point. I'm tempted to think I'm better off without it. My 1980 XS 11 has run up 150K without any such filter, so why does my ST need one?