- Install Air Box
- Install Carburetor Cables
- Setup Carburetors
- Install Carburetors and Carburetor Air Horns
- What Done Looks Like
I show how I remove the carburetors and airbox in this write-up.
Since I am converting this RS model to an RT, the handlebar throttle and choke cables have to be longer and I show how to change the handlebar cables in the Bowden assembly. I installed them when I installed the handlebar perches as shown here.
I had the air box powder coated. Although the carburetor rebuild was a couple years ago, I rode the bike less than 1,500 miles after the rebuild and they have been off the bike for about two years. I inspected the o-rings and found they are in good condition so no need to do another carburetor rebuild.
Since I modified the engine to boost compression from 8.2:1 to 9.5:1 and had the heads bored for dual spark plugs, I have to change the carburetor jetting. This engine has the same configuration as the engine in my 1977 R100RS that has the CFO (California, Florida, Oregon) configuration, so I use the same carburetor jetting.
When I first got the bike, I rebuilt the carburetors and “updated” the pulse-air system that fed air to the exhaust ports as shown here.
- 13 BMW 1983 R100RS Rebuild Bing 94/40 Carburetors & Karcoma Petcocks
- 13 BMW 1983 R100RS Update Pulse Air System
I replaced the crankcase breather hoses and the hose clamps as the hoses were cracked and rotted and clips were rusty. I installed the vent hose that connects to the breather valve under the top engine cover (part# 11 15 1 337 258) when I rebuilt the engine. I also replaced the rubber valve that goes in the rear of the air box.
|13 72 1 337 511||Engine Oil Breather VENT HOSE, LEFT (from 09/80)||1|
|13 72 1 337 512||Engine Oil Breather VENT HOSE, RIGHT (from 09/80)||1|
|11 15 1 337 258||VENT HOSE (from 09/80), Crankcase Vent||1|
|13 72 1 337 342||HOSE CLAMP – SLS (from 09/80)||2|
|13 72 1 337 162||RUBBER VALVE, Air Box||1|
Here is a video summary of the procedure for installing the air box, carburetor cables, setting up and adjusting the carburetors, and installing them.
VIDEO: 1983 R100RS Install Carburetors, Cables And Air Box
Install Air Box
The air box consists of the box that I had powder coated, an air filter that sits on top of the box and the cover with twin snorkels that let air into the box and a number of parts that go inside the air box.
Inside the box are air horns. Inside the air horns are a plastic pipe with centering vanes that connect to plastic hoses that attach to a large plastic tee that connects to the crankcase breather hose. The horns are secured with an aluminum sleeve the fits inside the horn compressing it against the holes in the sides of the air box.
I modified the parts inside the air box removing the plumbing that supplies air from the air box to the exhaust ports on the heads. You can see how I did that work here:
Install Rubber Fittings
There several rubber fittings. I use rubber timing window plugs to plug the unused holes for the pipes that carried air to the exhaust ports in the heads. There is a tee fitting and grommet that connect to vacuum hoses from the carburetors that actuate the valves that allow air to flow to the exhaust ports. There is a rubber valve the fits in a hole on the front of the air box. And there are four clips that secure the top of the air box to the body.
I install the timing window rubber plugs into the holes on the sides of the air box.
I install the rubber air valve in the hole at the bottom, rear of the air box. The petals are on the outside of the air box.
I install the grommet and the plastic tee into the hole on the front of the air box. The tee used to connect to vacuum lines on the carburetors to actuate the valves that allowed air to flow through the pipes to the exhaust ports. Since the tee is below the air filter, I don’t want it pulling unfiltered air into the box, so I plug the holes in the tee with silicone seal. The legs of the tee go on the outside of the air box.
Attach Air Box To Transmission
The air box is secured to the top of the transmission using the top left Allen bolt and the top right stud and nut. A third bolt secures the bottom of the air box to the top of the transmission. Each bolt has a thick flat washer.
There is hole on the front of the air box the crankcase breather hose fits into. The hose fits into one leg of the plastic tee inside the air box and the other legs of the tee attach to the plastic hoses connected to the air horns. The end of the crankcase breather hose has a grommet to secure it in the hole. I push the grommet through the hole from the front of the air box and then slide the box over the transmission stud and mount the box to the top of the transmission using the two bolts and the stud.
Since the bolts and nuts are below where the air horns mount inside the air box, I mount the box before I install the air horns and crankcase breather hoses.
Install Air Horns Inside Air Box
After I mount the air box on top of the transmission, I install the air horns and connect the crankcase vent tubes on each air horn to a leg of the tee that is connected to the crankcase vent hose.
I took some of these pictures on the work bench to show the details more clearly. But you can’t install the air horns until after you install the air box as the air horns obstruct the transmission bolt and nut.
I slide the air horn through the hole so the rib on the air horn fits in the groove inside the air box.
I slide the aluminum sleeve inside the air horn to secure it inside the hole. The I turn the air horn so the two ribs on the top are inside the ribs on the side of the air box.
The plastic pipe that attaches to the side of the air horn should be pointing toward the crankcase vent hose.
I repeat the procedure to install the other air horn in the air box.
I insert the large plastic tee into the grommet of the crankcase vent hose and slid the plastic pipes connected to the air horns onto the other two legs securing them with the wire spring clamps.
Install Air Filter And Air Box Cover
I have non-stock twin tower coils since I have dual spark plugs in the heads. It is easier to install the air box cover if I remove one of the coil brackets.
I had to bend both coil brackets up about 1/2 inch to clear the intake snorkels on the air box cover.
Before I install the air filter and air box cover, I install the clips on the sides of the air box that hold the cover and air filter on the body of the air box. You can’t install the clips after the air filter and cover are on top of the air box. One end of the clip has a tab used to press the clip into a detent on the air box cover. The other end fits around the bracket on the air box.
I roll the clip toward the center of the bike to engage it with the bracket and then rotate it outward to secure it.
The top of the air box has 90 degree corners on the rear and angled corners on the front. The air filter matches this so it only goes on one way. The air box cover also matches these corners so the intake snorkels point to the front.
I slide the air box cover on top of the filter at an angle and then align it with the top of the air filter until it slides into place. I rotate the cover retaining clips and snap them into the detents on both sides of the air box cover.
Then I install the coil bracket remembering to attach the black wire that goes between the coils on the top, inside terminal.
The red tape is on the GREEN-Blue wire that powers the coils and the RED wire that powers the optical-electronic ignition. I leave them disconnected for the time being and the red tape is the reminder they aren’t connected.
Install Carburetor Cables
The carburetor throttle and choke cables use a Bowden assembly. A single cable from the handlebar pulls on a plug inside a sleeve that also attaches to the two cables that go to the carburetors. I need to remove the short RS length handlebar cable from each Bowden assembly as I converted this bike to an RT configuration that requires longer handlebar cables since the RT handlebars are longer than the RS handlebars.
Replace Shorter RS Handlebar Cable With Longer RT Cable In Bowden Connector
To remove the handlebar cable, I screw in the adjuster all the way. I unscrew the cover with the two carburetor cables and pull out the plug. I disconnect the handlebar cable that fits in the middle of the plug by sliding the cable through the slot in the plug.
I already installed the single cable from the throttle assembly and handlebar mounted choke lever when I installed the handlebars and perches. I remove the rubber boot that fit over the handlebar cable adjuster. I install the boot by removing the lock nut and then push the boot past the end of the adjuster onto the cable.
I need all the handlebar throttle cable length I can get, so I remove the throttle cable from the throttle tube on the handlebar. There is barely enough cable length to install the handlebar cable into the center of the plug and the carburetor cables into the side slots. I have to bend the plug to get a large enough opening to push the ferrule on the end of the cable between the rear of the plug and the edge of the Bowden housing.
When I get the cables inserted into the slots of the plug, I screw on the cover.
I perform the same procedure to remove the shorter handlebar choke cable from the Bowden housing and install the longer length RT length handlebar cable and the two choke cables in the plug. The handlebar choke cable is longer than the throttle cable was so it’s easy to install all three cables in the plug.
Routing Carburetor Cables
I position the left side choke Bowden connector assembly under the starter relay and route the two carburetor choke cables through the bottom large diameter lower hole in the rubber cable management loom on the frame. Then I route the cables between the air box snorkels to each side of the engine.
I position the right side throttle Bowden connector next to the frame near the top of the voltage regulator because there isn’t any room to go under it. I route the two carburetor cables through the top large diameter hole in the rubber cable management loom and again route the cables between the air box snorkels to each side of the engine.
The BING carburetors on this bike fed an 8.2:1 compression engine and are model 94/40/113-114. But when I rebuilt the engine, I installed 9.5:1 pistons which is a significant increase in compression. When I had the heads rebuilt by Randy Long he added a second spark plug hole per cylinder so the heads are dual plugged. These changes will affect the setup of the carburetors.
From the “BING Carburetors” book published by the BING Agency, the jets on the 1977 carburetors, model 94/40/103-104, installed on the CFO engines–which is the same engine I have on my 1977 RS–are different than those installed on the 1983 engine. My 1977 RS with the CFO engine has the same 9.5:1 compression and 38 mm exhaust headers as this bike and it has dual plugs. So, I’m going to install the same jetting in the 1983 carburetors that I installed in the 1977 carburetors since the engine configurations are the same.
Here is the difference in the jetting between the 1983 RS and the 1977 RS CFO carburetors.
Specification 1983 94/40/113-114 1977 94/40/103-104
Main Jet: 160 170
Needle Jet: 341 341
Jet Needle: 2.66 2.66
Needle Position: #2 #3
Idle Jet: 45 45
Atomizer: 591 591
Throttle Valve: 856 856
Start Valves (R/L): 861/860 923/924
The difference in jetting is a larger main jet and a higher jet needle position. Both these changes make the mixture richer, both in the mid-range (needle jet position) and at wide open throttle (main jet). The difference in start valves (aka, the choke) make no difference to how the engine runs. So, I’m going to install the same larger main jets in these carburetors that I installed in the 1977 RS CFO bike, check to be sure the needle jets are 2.66, and change the jet needle position from #2 to #3.
The 1977 first year RS came with two different engine configurations. The engines both had 9.5:1 compression, but the CFO (California, Florida, Oregon) engines were modified to meet air pollution standards enacted in these states. Changes included smaller exhaust headers (38 mm vs. 40 mm) and changes in the carburetor jetting as follows.
Specifications 1977 Standard 1977 CFO
Main Jet: 170 170
Needle Jet: 2.68 2.66
Jet Needle: 341 341
Needle Position: #2 #3
Change Jet Needle Position
I remove the two Phillips head screws that secure the top of the carburetor and remove the choke return spring. I pull the top off and remove the internal spring. Then I pull out the slide with the needle jet attached to the bottom of the slide.
I have measured the length of the needle jet in the 94/40 series carburetors for each of the four needle positions. That way I can be sure the needles are in the correct position. For example, for the #2 position, the needle length is nominally 47 mm (47.3 mm) and for the #3 position, the nominal length is 45 mm (45.7 mm).
I adjust the needle from the #2 to the #3 position by pushing the needle in while twisting it until I feel it click into position. The measured length is approximately 45 mm so it’s in the #3 position.
I install the slide with the needle fitting into the hole in the center of venturi of the carburetor body and insert the spring. There is a tab on the diaphragm that mates with a slot in the edge of the carburetor top so the slide is positioned correctly in the carburetor body. Nonetheless, I look at the slide and make sure the cut-away on each side of the slide are centered in the venturi and then tighten the two Phillips screws and attach the choke return spring.
Change Main Jet
The main jet is on the end of the jet carrier. I remove it and the thin flat washer underneath it. It is marked “160” which is the stock main jet for the original 8.2:1 pistons and this model carburetor. I unscrew the jet carrier so I can check the size of the needle jet and it’s the correct “2.66” jet which stays the same.
I install the needle jet so the jet needle passes through the hole in the needle jet with the long leg of the jet going in first. The beveled face of the needle jet faces the jet carrier that I screw into the sleeve and tighten.
I install the “170” main jet into the end of the jet carrier with the flat washer between the carrier and the main jet.
Before I install the float bowl, I verify that the vent tube is open and the choke well orifice and passage from the float bowl to the well are open by pushing a small diameter wire through them and squirting a little carburetor into the well to verify it flows into the float bowl. I replace the cork float bowl gasket and then I install the bowl securing it with the wire bail.
Set Throttle Stop and Idle Screws Statically
I need to set the throttle stop screw and idle screws to initial static settings that will let me start the engine. Later I’ll dynamically just the carburetors while the motor is running to get the carburetors balanced and to ensure both throttle plates are opening at the same time.
I use a piece of paper to adjust both throttle stop screws so they are just contacting the throttle arm. I slide the paper between the face of the screw and the contact tab on the throttle arm. I adjust the stop screw until it is tight enough that I can’t easily extract the paper. This ensures both carburetor butterfly plates will be in just about the same position. Then I turn the throttle stop screw one full turn inward which should let me start the engine.
I screw in the idle mixture screw gently until it’s just seated. Then I back out the screw 1-1/4 turns which should be a bit rich but will let me start the engine and should keep it idling.
Install Carburetor Choke Cables And Adjust Choke Operation
I install the carburetor choke cables into the carburetor by putting the round cylinder on the end of the cable into the hole on the choke lever arm and then screws in the cable adjuster all the way into the top of the carburetor. I adjust the handlebar cable adjuster on the Bowden housing until the choke lever touches the bottom pin when the choke is off for both carburetors. I use the carburetor cable adjuster on each carburetor to get the choke lever to touch the top pin when the handlebar choke lever is in the full on position on both carburetors. When have the chokes on both carburetors working correctly I tighten the lock nuts on the carburetor choke adjusters.
I place the carburetors on top of the cylinders while I adjust the cables so it’s easy to work on them.
The right carburetor choke lever would not return to the off position completely unless I pushed down on the lever. It felt like something was obstructing the choke assembly from closing all the way. I took the choke apart and found a burr on the edge of the choke disk that was rubbing against the housing. I used a fine file to dress the burr so the disk was smooth. This fixed the problem.
Install Carburetor Throttle Cables And Adjust Butterfly Operation
I adjust the handlebar throttle cable adjuster on the Bowden housing so there is about an 1/8 inch of free rotation of the throttle before the carburetor cables start to move. I screw the carburetor throttle cable adjuster all the way down and insert the cylinder on the end of the cable that goes to the carburetor in the throttle plate arm the same way I did with the choke cable.
I make sure that the throttle stop screw I previously adjusted is tight against the contact surface of the throttle plate arm. I want to be sure the throttle isn’t partially open when the throttle twist grip is closed all the way.
I screw the carburetor throttle cable adjusters in and out to synchronize the movement of the throttle plate arms so each carburetor butterfly plate opens at the same time.
I slowly turn the twist grip watching one throttle plate arm until I just see it move. Then I look at the other throttle plate arm and close the twist grip to see if seems to move the same amount. This takes several trials with changes in the carburetor cable adjusters until it looks like I have both carburetor throttle plates opening at the same time. This static synchronization will be good enough for the first engine start. I’ll use my Grok carburetor balance meter to dynamically synchronize the carburetors with the engine running later on.
Install Carburetors and Carburetor Air Horns
The outlet side (end with the throttle plate visible) of the carburetor mounts on the intake port of the head into a rubber bushing secured with two large hose clamps. The intake side (end with the slide visible) mounts to the air box using a plastic air horn that has a large rubber sleeve on the air box end of the air horn and is secured with three large hose clamps; two on the air box aluminum sleeve in the air box air horn and one on the carburetor.
The left and right carburetor air horns are different lengths with different curvatures since the cylinders are offset in the engine block. I marked the carburetor end of the air horns with “L” and “R” when I removed them. But the left cylinder is closer to the air box outlet than the right cylinder, so the left air horn is the shorter of the two.
I slide the rubber bushing over the intake spigot and put two hose clamps over it, but I leave them loose for now. Then I push the carburetor into the sleeve.
The carburetor air horn has a rubber sleeve that goes over the aluminum sleeve inside the air box air horn. I push the sleeve down the carburetor air horn until it’s flush with the end of the tube. I put a hose clamp on the other end and push it over the intake side of the carburetor. I put two hose clamps on the end with the rubber sleeve.
I align the carburetor air horn with the aluminum sleeve and push the rubber sleeve on the carburetor air horn over the aluminum sleeve in the air box. I orient the rubber sleeve so it’s flush with the side of the air box and then tighten the two hose clamps. Then I tighten the hose clamp on the carburetor intake and then the two clamps on the rubber sleeve on the intake spigot in the head.
What Done Looks Like
The air box, carburetor cables, carburetors and plumbing are all assembled and attached.