K75 Air Plenum

[Westone]

This past holiday weekend I was all set to get into my K75 and take a look into the throttle bodies in search of the cause of the #3 TB to be unable to be synced with #s 1 & 2. It won't pull near enough vacuum, and two leak tests yielded negative results. I had new bushings and clamps, my Clymer's manual and was all set.

After getting everything disassembled I was about to remove the air plenum or resonating chamber on top of the TBs (not sure which is the correct term, and I have not been able to find this part in the parts fiche), when I discovered I had ordered the incorrect clamps for the bushings connecting the TBs and air plenum. Since the bike is still rideable I elected to wait until I have the correct parts and put everything back together.

Anyway I have two questions about what I saw in the air plenum when looking into it at the port where the large diameter air hose running from the air filter box connects to it, where filtered air enters the plenum. First, I thought the air plenum was just an empty plastic box providing air to the TBs and was there due to either design requirements or sound deadening purposes. Looking into the front port of the air plenum, the top of the bushing that sits on the #1 TB is visible. A white plastic tube was rising up from the bushing, and turned 90* running towards the rear of the air plenum. What is going on in the air plenum? I have not been able to locate and exploded view of it that shows the interior of it.

Second, while looking in there at the top of the bushing on the #1 TB, there was black oily residue on the white tube rising from it, like carbon deposits maybe. Since supposedly only clean filtered air runs through this area on its way to the TBs, I expected just a clean dry interior. Could this be carbon deposits that have traveled up through the TBs, or an indication of a problem? The bike has 91k miles, and I have owned it for only a few hundred miles.

A BMW mechanic had suggested that one possibility in the TB sync issue could be that the #3 TB is carboned up and needs cleaning. I had intended to visually check it from the top and proceed from there based on what it looks like.

Thanks for any advice.

[mjydrafter]

http://www.ebay.com/itm/BMW-K75S-K75RT-K75-Air-Intake-Accumulator-Box-/360207771447?pt=Motorcycles_Parts_Accessories&hash=item53de0e6737&vxp=mtr

Check out he pic down a few in the E-bay ad.  Are these the things you are seeing?  I'm not super familiar with what's in there, but it would make sense to me that there were runners in there to sort of equalize the air flow.  So the 1st cylinder isn't always stealing the nice fresh air directly out of the air path...

Do they look like the air intakes on old ship's decks?  Like an exhaust turndown...

[rbm]

A white plastic tube was rising up from the bushing, and turned 90* running towards the rear of the air plenum. What is going on in the air plenum? I have not been able to locate and exploded view of it that shows the interior of it.

What's happening in the air plenum is depicted in the photo below.

This is a view through the air inlet port showing the top if that white pipe you are seeing.  It is basically a tuned pipe with a fluted top, not unlike an organ pipe.  The fluting and length of the pipe probably governs the air intake into the throttle bodies, in the same way an exhaust pipe is tuned to draw the spent burnt fuel from the cylinder.  In any case, there are three such pipes in the air plenum of the K75, each connected to the TB inlet.

Second, while looking in there at the top of the bushing on the #1 TB, there was black oily residue on the white tube rising from it, like carbon deposits maybe.

The deposit you are seeing is possibly oil.  The crankcase breather hose transports oily fumes from the crankcase into the air plenum.  The oil condenses on the white pipes.  Over time this deposit may blacken with oxidation.  It is unlikely to be carbon deposits.  It is normal to have oily deposits on the pipes.  There are instructions on this site on how to redirect the crankcase breather hose to an externally positioned filter, eliminating the oily deposits within the air plenum.

[Westone]

Yes, that is exactly what I saw. Thank you for the info and pictures, I have learned something new about the K bike. I'm not worried about the oily deposits if it is normal accumulation from 91K+ miles of running with the crankcase vented into the air passage. It wasn't a lot, just visible on the section of pipe I could see and I was concerned it might be an indication something serious was broke.