The 1983 R100RS has the BMW Pulse Air system installed. This is a passive system with no active motors or moving parts. It sucks filtered air into the exhaust manifold in the heads to help burn any gasoline and to ensure low CO emissions. That said, according to Bob Fleischer’s information on the Pulse Air system, this increases the exhaust gas temperature and likely exacerbates wear of the valves and seats and could contribute to warped heads. The original valves and seats in 1983 had problems with valve recession due to excessive wear from the metallurgy BMW used to try and compensate for unleaded fuel.
For these reasons, I’m going to update the Pulse Air system and modify the air box so the valve train stays nice and cool. I purchased a smog kit from Rubber Chicken Racing Garage that has all the parts needed to complete this upgrade, shown below.
MAX BMW has a good exploded view diagram of the Pulse Air components in their parts fiche.
Remove the Air Intake Tubes
Filtered air is sucked from the air box into the exhaust ports in the heads via the air intake tubes, one for each cylinder, where it is used to burn any unburned fuel and to reduce CO in the exhaust. The air intake tube consists two metal tubes connected with a rubber hose about mid-way along it’s length. One metal tube connects to the air box next to the carburetor inlet and the other metal tube connects to a threaded port underneath the head behind the exhaust pipe nut.
I unscrew the nut at the air box to remove that end of the air intake tube.
Then I hold onto the air valve while I remove the nut holding it to the airbox. The air valves are the two bronze color cylinders inside the air box connected together with rubber hose as shown below.
When the air valve senses a high vacuum in the carburetor venturi, it opens to allow filtered air to be sucked down the air intake manifolds on each carburetor into the exhaust ports.
After removing the air intake pipe connections on both sides, I lift out the two air valves and the rubber hose connecting them.
I remove the other end of the air intake tube from the head by unscrewing the coupling nut.
This exposes a fitting in the head that I need to remove with a 22 mm socket.
Mine unscrews very easily, but they can be stuck tight due to galling, and carbon build up in the exhaust manifold.
Read Bob Fleischer’s write-up. This fitting often won’t budge and you can damage the aluminum threads in the head if it’s really stuck. Follow Bob’s other ways of upgrading the Pulse Air system if your fitting won’t come out easily.
Here is the air intake tube showing the two metal pipes, the rubber hose joining them in the middle, and the plug that goes into the head.
Remove Oil Breather Hoses
The oil breather hoses are also inside the air box. One of mine has split so I am replacing both hoses. If yours are in good condition, you don’t have to touch them to upgrade the Pulse Air system as the oil breather is not part of Pulse Air system.
Remove Carburetor Intake Venturi Vacuum Hoses
These hoses connect from the bottom of the carburetor and go into the airbox to a tee fitting. A single hose from the tee goes to the left side air valve. The carburetor venturi vacuum opens the air valves so filtered air can be pulled by the exhaust manifold vacuum into the exhaust manifold.
I pull the hoses back inside the top engine cover. When I rebuild the engine, I’ll pull the air box and completely remove them.
Capping Off Carburetors and Vacuum Hoses
The small screws with fiber washers fit into the threads inside the vacuum port on the carburetor. They can be removed to use carburetor balancing sticks or gauges.
Capping Off Exhaust Manifold
The oil drain plug from the R1150 motor (M16x1.5, 10 mm; part# 07 11 9 902 292) makes a perfect cap for the threaded hole in the exhaust manifold. I originally snugged this up and checked it again after I rode the bike to be sure it stayed put. But after about 2,000 miles, one of them came out. So I now I use Loctite 271 to keep them installed. It’s a reasonably high temperature formulation so it ought to keep the bolt tight at the threads are in the aluminum head which is air cooled.
Capping Air Intake Tube Port in The Air Box
The timing port rubber plug fits nicely into the hole in the air box where the air intake tube was connected to it. I put a bit of silicone seal the plug to ensure it doesn’t pop out. That would let unfiltered air into the carburetor and the engine.
Install Air Filter on Top of Air Box
I install a new air filter on top of the air box. Note that the bottom of the filter has rounded and square corners which match those on the air box so it only goes one way.
The air box cover clips come off easily. One end hooks into the bracket on the air box and is easy to install by moving the top out of the way and then holding the clip it so it’s rotated toward the inside of the air box and pushing up on the clip. When it clears the bracket, rotate it towards you.
2019-11-23 Edits and typos.
2020-08-28 Changed title and link.
2021-01-01 Update regarding plug size for plugging threaded hole in heads.
Good article, great pics. I’m gonna do mine, both of ’em.
Thanks for a very nice article; great photos. This is one easy ‘fix’ that I’ve been thinking about. Keep up the good work in helping to support fellow airheads.
Thanks B M, I hope to post some more as I continue work on the R100RS project.
Thanks very much for the article, its a great help.
You’re most welcome. Thanks for coming by.
Question – Do the vacuum lines from the carbs that are stuffed in the upper engine area need to be capped off?
Thanks for all the information
They go nowhere so putting them inside the engine cover is okay, or they can be removed if you wish.
I hope this helps.
For the small screws that go into the carb vacuum port, what threading and length did you use?
Thanks again for the awesome tutorial!
The part#’s you need are:
Screw 13 11 1 259 869
Fiber Washer 13 11 1 259 870
You will find them listed on the MAX BMW parts Fiche for the 1977-1984 R100RS:
Plate: 13_0509 CARBURETOR/THROTTLE CONTROL
Any dealer can order these for you, or you can get them from MAX.
I hope that helps.
For anyone doing this to an R65, use M16x1.5 bolts for the exhaust manifold capping. The oil drain plug (M18x1.5) is too large. Speaking from experience.
Thanks again for this post, Brooks!
Thank you for adding this useful information. I appreciate it.
Thanks, Brook! You, Snowbum and Tom Cutter sure save us amateur airheads a lot of headscratching, blind alleys and breaking stuff. I’ll get Tom’s kit and desmog my R80 RT.
Thanks for the detailed write-up. This helped me to do the same for my R100GS.
On mine, I also had to plug another hole in the air-box that linked the vacuum lines via a T fitting.
Thanks again for this write up. Did this on my 83 R65LS and everything was exactly as you described. Seconding the use of M16x1.5M bolts as well.