I have installed the engine and the electrical harness in the frame. I also completed installing the exhaust system with new Sito mufflers. So, the next piece of work is installing the carburetors, air intake system and the choke and throttle cables.
I previously rebuilt the carburetors–you can read about that work here–,
and they have been sitting in a box waiting for installation.
This is the list of new parts I used for this work.
|32 73 1 236 613||Throttle cable||2|
|13 23 1 255 977||Choke cable, Right, Stiff||1|
|13 23 1 255 976||Choke cable, Left Stiff||1|
|13 72 1 254 654||Carburetor sleeve to cylinder head||2|
|32 73 1 230 037||Handlebar, Throttle cable boot||1|
I made a minor mistake and ordered two of the right choke cables which is the longer of the two cables. I had to shorten one to fit the left side and that modification went smoothly. Good thing I ordered the longer left and not the shorter right as my choke cable stretcher was loaned to someone and I can’t remember who has it 😉
Install Starter & Air Box Covers
I install the starter cover and the left side air box cover on the engine. I temporarily installed the gas tank so I can measure the distance for the new cross-over fuel line that goes through the air box and the distance for the line from the tank pet cock to the tee fitting.
I cut the cross-over fuel line and added the left side tee and the line that will go to the tank petcock. Then I put the new air filter in the air box.
Assemble Choke Lever
Here are the parts for the choke lever.
From the upper left are the cable nipples that thread into the choke body, the cable racks the choke cables install in, the body, the cover, the brass choke cable fittings that fit the carburetor, the wave spring that puts tension on the choke lever, the handle with teeth that engages the teeth of the cable racks, and the bolt and nut that secure the choke lever assembly to the left air box cover.
The cable racks slip into the holes on one side of the body so the teeth are oriented to the inside of the body where they engage with the gear teeth of the handle.
The other side of the cable rack is cut out to accept the choke cable ferrules.
I apply wheel bearing grease to the pivot pin in the body, the teeth on the lever and the choke racks.
I insert the cables in the two choke racks and then slide them into the body so they are even with the end of the holes in the body.
Then I install the lever on the pivot pin in the body and move the lever down. It engages the first tooth of the choke racks and pulls them into the body securing the cables inside the holes.
I twist the cables so the sheath screws into the cable nipples. And then I finger tighten the nipples into the threaded holes in the body.
I tighten the nipples into the body with an 8 mm crescent wrench until they are tight.
Now, I put some wheel bearing grease on the face of the lever and put the wave washer on top.
I put the cover on and tighten the screw all the way through the body to secure the cover.
Install Choke Lever on Left Air Box Cover
The other end of the screw takes a nut and washer that secure the assembly in the casting on the left air box.
Make sure the choke body is square in the casting and then tighten the nut from the inside of the air box cover.
I slide the left side air box cover under the metal strap and tap it tight to the right side cover. The long bolt goes through the hole in left side cover and threads into the taped hole on the right side cover. If you look in the air inlet you can see the hole and thread the bolt into it.
I tighten the long bolt to pull the two air box covers tight.
Here is the choke lever assembly installed on the left air box cover with both cables straight in front.
The choke cable goes into the brass cable adjuster that mounts in the top of the carburetor in the threaded hole. The hole is threaded but the choke adjuster is a smaller diameter and doesn’t thread into the hole. Instead two lock nuts are used to secure the adjuster to the top of the carburetor as shown below.
No adjustment of the choke cables is required after they are installed in the carburetor choke lever assembly as described later.
Install Carburetors, Bushings and Air Tubes
Before I can finish installing the other end of the choke cable in the carburetor, I have to install the carburetors, rubber bushings and air tubes between the engine intake and air box spigots.
The bushings and air tubes are secured with clamps. There are four 60 mm and six 52 mm clamps.
There is a thick solid rubber bushing that mounts on the engine air intake spigot. It looks like a piece of radiator hose. It is secured using two of the 52 mm clamps, one on the end around the engine intake spigot and the other on the end around the carburetor outlet.
The other bushing is thinner and has a ridge inside that aligns the air tube into the bushing.
The air tubes are different lengths because the left cylinder is ahead of the right on the crankshaft. And the ends of the tubes have different width bands. The end of the tube that goes on the air box spigot is the wider band at just about 5/8 inch. I insert the narrow end of the tube into the thin bushing so it is flush with the ridge inside the bushing.
The thin bushing goes on the intake side of the carburetor and is secured with two of the 60 mm clamps.
NOTE: If the air tube doesn’t line up between the carburetor and the air box, you likely inserted the wrong end of the air tube into the thin bushing.
I tighten the 52 mm band on the thick bushing around the engine intake spigot and leave the band that goes around the carburetor outlet loose. I keep the two 60 mm bands on the thin bushing loose as well. This lets me bend the carburetor and the air tube at the bushings to get the carburetor installed into the bushings and the air tube installed on the air box spigot with out creasing the end of the air tube.
I slip the carburetor part way into the thick engine intake bushing and bend the air tube so I can ease it onto the air box spigot being careful not to crease the end of the tube. Then I push the carburetor toward the engine to get it seated into the thick bushing and finally seat it into thin bushing on the air tube. Then I align the carburetor so when viewed from the top, the top of the carburetor it is not rotated toward or away from the engine, but is vertically aligned.
Install Choke Cables in Carburetors
I made a mistake and ordered two right side cables which are the long ones. So, the left side cable is too long. I used my dremel tool with the cutoff wheel and carefully cut the outer sheath being careful not to nick or cut the inner cable.
NOTE: I want to be sure the choke is off when the choke lever on the air box is horizontal. So I rotate the choke lever a small amount toward the on position before I install the solid wire in the carburetor choke lever. After I get the cable installed on the carburetor lever, I pull the choke lever on the air box back back to horizontal and with my finger, I ensure that the carburetor choke lever is all the way down in the off position.
The choke arm on the carburetor has a screw with a hole in it, a brass bushing and a nut. The solid wire goes through the hole in the screw.
I remove the lower nut from the brass choke adjuster on the top of the carburetor so I can slide the adjuster and cable through the hole in the top of the carburetor. I tighten the upper lock nut all the way to the top of the adjuster so the adjuster goes all the way done through the threaded hole in the top of the carburetor.
Pushing the inner wire through the hole in the screw is tricky. I use my right hand to hold the lower lock nut of the adjuster to keep it from falling off the end of the cable. With my right hand, I maneuver the end of the wire so it pushes the brass bushing against the lever arm while I use the fingers of my left hand to turn the screw to orient the hole vertically so I can thread the wire through the hole. Now, I push the choke lever arm all the way down and tighten the nut on the screw with a 7 mm crescent wrench. When the choke lever is down, the choke is in the “off” position.
After I get both coke cables installed on the carburetors, I pull the choke lever on the air box all the way up until its horizontal and then use a finger to ensure that the both choke levers on the carburetors are all the way down.
Install Throttle Cables
The throttle housing on the handle bar has a toothed cam gear. It engages in the teeth of the twist grip. There is an alignment mark on the twist grip and an alignment mark on the cam.
When these are aligned, the cam engages the twist grip so there is some slack in the throttle cable when the twist grip is closed and the carburetor butterflies are able to open all the way when the twist grip is rotated fully.
But, on my /5, I had to replace the throttle housing with a newer, longer housing and 89 mm top cover. The original chain wouldn’t extend all the way to the end of the housing so the cables don’t have enough slack at the carburetor adjuster to let the butterfly close all the way.
To get slack in the throttle cables, I had to rotate the cam and twist grip counterclockwise from where the alignment marks matched until the chain and block that the throttle ferrules attach is closer to the end of the housing. This provides enough slack for the carburetor butterflies to close all the way. Interesting.
I use Moly 60 to lubricate the twist grip housing and teeth, the cam teeth and the chain and the bottom of the housing.
I install the rubber boot on the cables and then install the ferrules in the top and bottom slots of the block on end of the chain. I install the cover and screw to secure the cables in the throttle housing.
I route the throttle cables next to the right headlight ear and behind the right steering head gusset and split them so they run along the left and right sides of the spine tube.
Then I install the adjuster in the carburetor and screw it all the way in. I put a drop of oil on the cable nipple and insert it into the slot on the throttle linkage.
Here is the final result with the throttle and choke cables routed to the carburetors.
Lovely presentation. Steve
Thank you for the compliment and for dropping by to read the content here.
good graphics great discussion of how to do the fitment. thanks greg
You’re welcome Greg.
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Thanks for all the details. I feel like if I had 2 hours as a mouse in your garage I would learn tons about my bike!
Great information, thanks for sharing
Thanks. I’m glad this write-up on rebuilding the carbuetors was helpful. It’s been the leading artile read on the site, so I assume rebuilding Bing carburetors is something a lot of folks want to understand.
Can you please provide the pin that has teeth for this lever? it a single cable, can you assist?
If you are asking if I can provide a part, I don’t sell parts.
If you are asking if you can buy just the wheel with teeth from BMW, you can only buy the entire choke lever. This is the part number for a 1973/5:
13 23 1 260 191 COLD START OPERATION
I hope this helps
Thanks for posting and documenting this project so well. I’ve been a German car tech mostly for MB for 35 + years but don’t know my way around this R75/5 I’m putting together. Your presentations and narratives are some of the very best I’ve seen. As I am now an automotive electrical instructor, you have not only been helpful with my project, but most inspiring professionally !
Well, thank you for the very kind words. I am an amature, so if this material was valuable to you with your professional background, I couldn’t be more pleased.
Best of success on your build and I look forward to your getting another Airhead back on the road.
Hi there. Can I ask where you found those fabulous 52mm and 60mm Norma clamps for the intake tubes and carbs?
I think they were original. I cleaned and polished them and they came out pretty nice.
You can buy these at Hucky’s:
I hope this helps.
On my october 1970 75/5, the 64.32.4 Bings have no air intake bushings ( the thin ones, in this article). Must I worry ? I already sweat every time I have to assemble the tubes: if there were a bushing I reckon it would be even worse. What say you?
As far as I know, the carburetors connect to the cylinder head and to the intake tube from the air box with these bushings. They isolate heat and vibration.
I describe how I get the carburetor to fit into the two bushings in the article. It seems to be the easiest way to do it with the least opportunity to bend the edge of the plastic air intake tubes.
Excellent post and a huge help to the start of my /5 build.
First, I want to say thank you for the great service you have done for builders like me. Your blogs are amazing and the detail is unmatched. I have just moved to Denver and purchased a couple 75/5’s. One is a SWB and is about to be fired up after a few months of building and the other is a long term project, rebuild, a LWB toaster. I have had numerous airheads and K bikes in past years but have never ventured out to a rebuild. I would like to sit down and have coffee sometime if you are interested. I think I may know someone in the BMW world you do. I am downtown all day during the week. Let me know.
I’m glad you are keeping multiple airheads alive and breathing. I’ll be at an Airhead tech day this Saturday where you can meet a lot of us in the metro area. Consider joining our Colorado Airheads Beemer Club MeetUp group. It’s free and you don’t have to be a card carrying member of the ABC organization. Click this link to see the details about the Tech Day and other activities we have planned.
I hope to see you there.
This was very informative and detailed. I had just rebuilt the carbs on my ’73 R75/5 and noticed in the manual that they state to adjust the free play on the throttle cables and choke cables the same. I couldn’t see any way to adjust the choke cables so decided to go on line to see if someone had done this. You confirmed that there is no adjustment required. Thank you very much.
I wanted to thank you again for the informative post! I was in the middle of trying to replace the choke cables on my ‘71 R75/5 and couldn’t for the life of me figure out how to get it all back together. I was honestly about to give up and just order manual pulls. Like others, I greatly appreciate the time and effort you provided with these posts saving me time and money.
That’s great. That choke assembly is cleaver, but not intuitively obvious as to what goes where. That’s one reason I took time to show the details.
How did you cut the choke cable and keep the nipple? I bought two longer cables also (for the r90s) – they should tell you when you buy them!
I first cut the outer cable to length being careful not to cut the inner cable as I describe in the write-up. Then I cut the inner cable at the end that doesn’t have the nipple as explained in the write-up.
BTW, the R90S uses Del Orto carburetors and they don’t use the same choke cable as the R75. I’ve never worked on an R90S. But, I do know Del Orto carburetors are not the same as Bing. The R75 Bing carburetor uses a solid inner wire cable while the Del Orto has a stranded one. Therefore, what I did to fix a too long Bing carburetor cable will not work for the Del Orto carburetor cable.
The fiche shows only one carburetor cable for the Del Ortos so I have to assume they were supposed to be the same length.
I hope that helps.
Thanks again for the great post. It’s a huge help for us guys who are restoring our first airhead. I am installing new choke cables and made the same mistake of buying two longer ones. I will try to cut one down as you described. One question, should the choke cables be lubed, and if so can you recommend a product? Many thanks again
Sometimes things just work out 🙂 I don’t lube the choke cables. Oils will attract grit and dirt making them harder to move over time.
Thanks again Brook. Good to know!