The air box is secured to the top of the transmission. Inside are components for the crankcase rebreather system and for the Pulse Air System which is a passive system designed to reduce emissions to meet the US standards in the early to mid-1980’s.
The picture was taken when I first saw the bike and the air box cover was missing. Leaves are not part of the crankcase rebreather or pulse air system inside the air box. 🙂
I don’t show how to remove the carburetors and air box cover as mine were already off the bike with I got it. But you can see all the details from the 1983 R100RS project.
Pulse Air System Description
The Pulse Air System 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 CO emissions are converted to CO2. 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 will purchase a smog kit from Rubber Chicken Racing Garage that has all the parts needed to complete this upgrade.
Here is a short video summarizing how I remove the air box and the Pulse Air System.
VIDEO: 1983 BMW R80ST Remove Air Box & Pulse Air System
Remove Crankcase Rebreather Components
On bikes with the rectangular air box on top of the transmission, the crankcase rebreather routes gases from the crankcase to both carburetors. The earlier version used with the aluminum clam shell housing for the air cleaner routed all the vapor to the right carburetor. The rebreather hose from the rebreather valve under the top engine cover enters the air box and has a plastic tee that connects to hoses that go to the left and right air horns the carburetor intake attaches to.
The hoses are secured to the plastic tee with wire clips. I remove the clips and pull the hoses off the tee.
The other end of the rebreather hose connects to the side of the air horn and is attached to a plastic pipe that sits in the center of the air horn.
I pull the hose out of the air horn with the plastic pipe attached and then carefully remove the plastic pipe from the hose. I pull the plastic pipe out from the outside of the air horn as it has three fragile plastic legs that center the pipe inside the air horn and I don’t want to break them.
Now I remove the metal sleeve in the side of the air box that the carburetor intake pipe attaches to. It also secures the air horn in the hole on the side of the air box. Then I push the rubber air horn into the inside of the air box to remove it.
Remove Pulse Air System Components
The pulse air system has a metal pipe that exits the air box. There is a rubber section in the middle and then another metal pipe that attaches to a fitting underneath the exhaust port in the cylinder head. The pipe supplies air to the exhaust port to burn any unburned fuel and to ensure any CO is converted to CO2. I remove this pipe from the air box. It is secured with a nut on the end of the metal pipe.
You will see leaves inside the airbox that were on the bottom of the box under the components. The bike was outside without the air box cover when I got it. Needless to say, these are not part of the internal air box components. 🙂
Then I remove the retaining nut that secures the pulse air valve to the side of the airbox. I pull the rubber vacuum hose off the nipple on the top of the pulse air valve. Then I remove the wire clip and remove the air hose from the pulse air valve so I can remove the valve from the air box.
Then I remove the right side pulse air valve in the same way. The left and right pulse air valves are different and the right valve does not have the vacuum line nipple. The left valve opens using the carburetor vacuum and sends air to the right valve via the large rubber hose.
Next I finish removing the air pipe from the fitting underneath the exhaust port and the fitting from the cylinder head. The fitting in the cylinder head can be hard to remove, but mine came out easily.
Read Bob Fleischer’s write-up in the “Pulse Air Description” section above. 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.
Remove Air Box
Now all the components inside the air box are removed, I can access the air box mounting hardware. The top left bolt and top right nut inside the air box secure it to the transmission and also secure the transmission to the engine block. There are two bottom bolts on the transmission that secure it to the engine block as well. The middle bolt inside the air box only secures the box to the transmission.
Before I remove the bolts and nut, I remove the crankcase rebreather plastic tee and then push the rebreather rubber grommet back through the air box with a screwdriver.
I remove the bolts and nuts. Each has a thick flat washer. I use a wrench on the left bolt, a box end ratchet on the right nut and a socket on the center bolt.
I pull the vacuum hose off the nipple. The two hoses that go to the carburetors are connected to the nipple tee on the back of the air box. After I remove the air box off the transmission I remove the hoses from the tee, push it into the air box and then use a screw drive the push the grommet out of the air box.
Here is the crankcase rebreather rubber hose that comes from the rebreather valve underneath the top engine cover. You can see the rebreather housing that is the rectangular box on the right side of the top engine cover
You can see where the air box mounting bolts and the nut fit on the transmission.