Hey All,
I'm currently in the process of building a 1990 BMW k75 street tracker for the 2019 Quail out here in the Bay.
I've made a lot of progress thanks to many of the members here on the forum!

I'm currently working on the air-box: 
I've cleaned up the back end of the bike and moved the battery to the bottom half of the air-box using an antigravity and a couple of custom printed fillers.
The disassembly of the air-box inspired me to explore different air filtering options. I've always loved the look of pod filters and have decided to go towards that route.

I've seen other builders use pod-filters and some sites are even making custom conversion brackets:
https://www.kickstartershop.de/en/product?info=9586&country_preselect_country=DE
https://cafe4racer.eu/en/air-filter-adpaters-bmw-k-100-k75-k1100/1775-bmw-k-serie-air-intake-stainless-90-reduction.html
These brackets attach directly onto the air sensor unit as shown in the pictures.

Questions for the forum:

1. I've read online that people have done this conversion before and have not had to re-jet, can anyone confirm?
   
2. Is air intake surface area crucial here? Or do I just need to ensure that air is funneled into the system without any blockages? Angles of the intake and number of pods are not important?
   
3. What aftermarket pipes or methods can be used to make a custom air sensor to carb pipe? (The OEM elbow pipe isn't exactly the right angle)
   

Thanks in advance!

K-bikes don't have carbs; they're fuel injected and have throttle bodies. So, you can't have an air sensor to carb pipe.

Are you thinking of deleting the plenum chamber?

The Kickstarter link shows an intake which suggests retention of the critical AFM. Of course, there is nothing in the literal way of 're-jetting' can be done there. If it's wide open performance and a dash of 'look at me' you're after, that shiny metal bit is a viable option, but if you desire ridable tractabliity of power, the original airbox canna be beat.

Fw66 wrote:I've read online that people have done this conversion before and have not had to re-jet, can anyone confirm?

I know of a K75 that has 200K miles on it with a converter similar to the Kickstarter version you posted. It seems to work satisfactorily for its owner. There are no jets to re-jet for the reasons robmack indicated. There are other classic K motos where filter pods can be fitted to each throttle body but 2V motos like yours don't easily accommodate that modification.

robmack wrote:K-bikes don't have carbs; they're fuel injected and have throttle bodies. So, you can't have an air sensor to carb pipe.

Are you thinking of deleting the plenum chamber?

Thanks for the replies, I misspoke and called the plenum chamber the carb...

I am not getting rid of the plenum chamber, just need to find a way to pipe the plenum chamber to the AFM as the stock elbow pipe doesn't give me the best angle. Any ideas?

Lastly, does the shape of or volume dimension of the pod filter bracket matter? I assume the bracket will have to have a diameter as wide as the AFM hole but other than that there shouldn't be any other considerations right?

You can trim the 90' rubber elbow that links the AFM to the plenum. I halved mine so's the AFM sits at a rakish 45' to the plenum. You might have to swap the large hose clips around, can't remember exactly, I'll check my K100 tomorrow.

There's a small cast ally flange on the intake side of the AFM which you can remove to leave a 4-bolt mounting and a square inlet hole. This gives you more possibilities viz the adaptor for the filter.

The AFM also has a couple of M6 threaded holes on the side which are unused. You can use these for a support bracket for the AFM - there's a couple of M8 tappings on the top of the crankcase nearby too.

Photos tomorrow, promise.

Not great photos, but here's the filter adaptor:

And the bracket:

And the shortened rubber elbow:

Thanks for the photos! Super helpful.
I actually think I'm going to reuse the airbox and have a custom air intake coming out of it since I'm using the bottom half as a battery holder.
I'll post pics as the parts come together.

So you're going to seal off the bottom half of the airbox so's it can house the battery, and cut a hole in the top half for the inlet - with a cone filter attached to the outside?

Clever thinking, I like that. Must be a first.

A product called U-Pol Tiger Seal is just about the best thing for permanently bonding plastics/metal/anything together in these sort of applications. I'm thinking of the cone filter stub that you might have to graft onto the top half of the airbox, with all its curves.

Thanks for the pics... am about to pick up bike without plenum but does have the AFM. Will probably weld up a version of your bracket.

Was amused to see you describe the angle of the pod attachment as "rakish" - the only other time I've seen that reference was in a Spike Milligan (possibly... Hitler: My Part in his Downfall) book - photo of his mate with his hat on a rakish angle. 

Must be a British thing! (My family were 10pound poms to Australia in 67... I was 2 and there was probably a 10pound pong in my nappy by the time we arrived)

The bike is in a sorry state but can't so no to another stray

GF

chris846 wrote:So you're going to seal off the bottom half of the airbox so's it can house the battery, and cut a hole in the top half for the inlet - with a cone filter attached to the outside?

Clever thinking, I like that. Must be a first.

A product called U-Pol Tiger Seal is just about the best thing for permanently bonding plastics/metal/anything together in these sort of applications. I'm thinking of the cone filter stub that you might have to graft onto the top half of the airbox, with all its curves.

Yea exactly! I'm actually getting a guy to 3D print the piece that goes into the top half of the airbox...hoping it'll come out super clean.
IF it does, there will be 3 pod filters going horizontally across the airbox.

Fw66 wrote:I'm actually getting a guy to 3D print the piece that goes into the top half of the airbox...hoping it'll come out super clean.
IF it does, there will be 3 pod filters going horizontally across the airbox.

How did it come out? Are you happy with the look and performance?  Can you post a picture?  Here is how others have done it recently. It's amazing how much access you gain to cables and hoses, and the fact that you can inspect and clean up that area easily.

I converted mine and it seems to works perfectly fine.Plus you gain so much access space.

That boat-anchor-of-an-airbox is responsible for trapping alot of heat on the faired bikes-RS/RT/LT.

RS Rider wrote:That boat-anchor-of-an-airbox is responsible for trapping alot of heat on the faired bikes-RS/RT/LT.

Is that good or bad?

bmwk100rs wrote:

RS Rider wrote:That boat-anchor-of-an-airbox is responsible for trapping alot of heat on the faired bikes-RS/RT/LT.

Is that good or bad?

I'd say it's bad because that trapped heat is the reason the underside of the fuel tank had reflective insulation on it. There are also scoops you can add to the top of the forks to redirect air towards the fuel tank. They replace the fork gaiters. They can be found on the used parts market-eBay for example.