- Parts List
- Cleaning and Polishing
- Rebuild Karcoma Petcocks
- Carburetor Disassembly
- Remove Varnish and Carbon
- Rebuild Kit Parts
- Assemble Carburetor
- What Done Looks Like
- Adjust the Throttle Stop Screw
Over the Christmas holiday, I restored the Bing carburetors. A restoration is a bit more work than a rebuild since the goal is to restore the finish to factory new condition as well as replace worn parts and all the O-rings and gaskets. Much of the time invested is in cleaning and polishing all the parts.
Here are the new parts I used for both the petcocks and carburetors.
|16 12 1 233 368||Karcoma, Screen||2|
|16 12 1 234 869||Karcoma, O-ring||2|
|16 12 1 238 924||Karcoma, Rubber gasket, 5-hole||2|
|16 12 1 240 060||Karcoma, Fiber gasket, Top||2|
|16 12 1 233 366||Karcoma, Fuel line connector||2|
|13 11 1 258 051||Carb Rebuild Kit (64/32/10, 9), Pair||1|
|13 11 1 254 766||Float||2|
|13 11 1 254 768||Float Hinge Pin||2|
|13 11 1 254 767||Float Needle||2|
|13 11 1 257 810||Needle Jet||2|
|13 11 1 255 840||Jet (Slide) Needle||2|
|13 11 1 254 738 SS||Throttle Shaft Screw SS||4|
|13 11 1 259 870||Vacuum port gasket||2|
|16 12 1 233 367||Cap, Petcock Karcoma||2|
|13 11 1 337 361 SS||Carb Dome Top Screws, M5x12||3|
|13 11 1 335 324||Slide Spring||2|
|13 11 336 585||Throttle Shaft||2|
|13 11 1 337 361||Carb Dome Top Screws, M5x12||8|
I’ve found two good sources for the Bing carburetor parts you need; the Bing Agency and Hucky’s. After pricing all the parts, Hucky won this time around. Hans Lowe, the owner, was offering a 10% discount for payment by check. Since I avoid sales tax, and I was going to order most of the initial parts for the project at one time, even with shipping, Hucky was less expensive.
Cleaning and Polishing
Here’s a picture of the various items used to clean and polish. In addition, a wire wheel, parts washer and parts soaker were also used. On the right, the restored carburetor body and top cover showing what you can achieve with time and elbow grease. It takes me about 5 hours to polish and reassemble one carburetor. You’re mileage may vary.
I find Autosol aluminum cleaning and polishing products work quite well. The aluminum cleaner, when used with “00” steel wool, is quite effective at removing oxidation and grunge without scratching aluminum. I also use a brass brush and brass wire wheel on the Dremel tool to polish an aluminum surface. Final application of the Autosol metal polish and/or Autosol aluminum polish and then some hand buffing makes aluminum shine.
To start, I use the parts washer to clean as much of the grunge off the outside as possible. I used KleenTec 600C water-soluble degreaser (you can find this at auto parts stores) at 5:1 dilution in the parts washer and 4:1 dilution in a 1 gallon parts soaker container. This is the first time I’ve used non-petroleum solvent and I’m happy with the result. This degreaser is far less environmentally damaging than petroleum-based degreaser.
I use a tooth brush with the parts washer to remove the top layer of crud. Then I remove the float bowl and put it and the carb into the parts soaker over night. With the tooth brush again, I get rid of most of the outside crud. The stubborn backed on varnish and carbon will be dissolved later on, so if they aren’t coming off easily, I move on.
In the picture below, the left carb is before the parts washer cleaning and overnight soaking in KleenTec and the right is after. You see some pink fluid on the right cleaned carburetor. That’s Kroil that I used to get one of the top cover screws to budge without stripping the head. Kroil is excellent stuff IMHO.
Rebuild Karcoma Petcocks
The petcocks are easy to rebuild. The black cover can be pried off with a screw driver. Underneath is a metal cover with knurled edges. A pair of pliers spins the cover off the petcock body reveling the internal parts of the petcock.
Here are the internal parts from outside (left) to inside (right).
The ring on the left goes up the petcock handle, turns 90 degrees and then fits on to the threads on the petcock body securing the internal parts and the spring against the internal rubber gasket (not shown above). Here is a link to a picture of the rubber gasket from the MAX BMW Parts Fiche for the carburetor. It goes inside the petcock body.
The rebuild parts are the internal rubber gasket that seals against the rotating petcock lever and an o-ring gasket that seals the threaded petcock nut to the gas tank.
The knurled nut that threads on the petcock body is pushed up the petcock lever and rotated 90 degrees to align the threads with the petcock body threads. The internal spring is pretty strong and I find that positioning the pet cock on the floor and leaning on the knurled nut with my arms extended with my thumbs on the edge of the nut let me compress the spring and tighten the nut a quarter of a turn or so to secure it to the petcock body. Then I use pliers to tighten it up.
I put the new black plastic covers up the petcock lever arm and carefully rotated it 90 degrees to avoid creasing the inside edge. I position it over the knurled nut and press it on with the “Reserve” lettering at the top.
The red gasket goes inside the top nut that secures the petcock to the gas tank. Note the short and long tubes shown in the picture below are INCORRECT and are REVERSED.
As shown below the two plastic tubes are positioned in the top of the petcock so the short one (reserve) is on the left and the long one is on the right side as you face the front of the petcock.
It’s best to work on one carburetor at a time as some parts are for the left or right carburetor and are not interchangable. And, should you need to look at an assembled carburetor to see how parts go together, you have one. That said, I’ve found it’s a good idea to take pictures as you go and to take one with parts organized from left to right to show the sequence of parts from the outside to the inside.
Here is an exploded view diagram from the Bing Agency which has proved invaluable.
I’ll refer to the parts using these part numbers and will put them in ( ). Note that part (22), the internal spring, was not used in the /5 series carburetor but was added in 1977 for the /7 series. I decided to add that spring to this rebuild, but it is an option. It should make throttle transitions smoother and improve gas mileage a bit as the throttle slide closes more quickly when the throttle is closed.
Here are the pictures of the Float Bowl (44), Float Assembly (40 ), and interior of the carb body showing the Throttle Slide (13).
Remove the “Innards”
This is the left carb. I remove all the jets and mixing tubes from the bottom going from left to right and take pictures as I go.
I removed the main jet from the main jet carrier and then removed the carrier.
You can turn the carb upright and the needle jet (3) and atomizer tube (2) will fall out. I put all the innards into a separate container so it’s easy to find parts later on.
Remove the Throttle Linkage, Butterfly and Shaft
Next, disassemble the throttle linkage starting with removing the Idle Adjusting Zcrew (36).
Next, remove the throttle linkage by removing the nut (30) and pulling the parts off the throttle shaft (24). You will end up with all the parts shown below.
Then, remove the bracket from the throttle shaft.
Then, remove the butterfly from the throttle shaft. The screws are peened over on the thread side. Its a good idea to use a Dremel tool and grind off the end of the screw so the peened over part doesn’t bung the threads in the Throttle Shaft.
Once the srews are removed, rotate the throttle shaft (24) so the throttle plate (23) is perpendicular to the opening. Gently pull the throttle plate out of the slot in the throttle shaft. Be careful not to jam the throttle plate into the sides of the inlet or to damage the edge of the butterfly. Take your time and go easy. I find gently rocking the throttle plate side to side on the shaft helps in getting it to come out.
Now you can pull the Throttle Shaft (24) out of the body.
Remove the Throttle Slide, Diaphragm and Jet Needle
Next, remove the four Screws (21) holding the Top Cover (20) to the carburetor body. Then, remove the four Screws (18) and Lock Washers (19) securing the Diaphragm Retaining Ring (17) on the Throttle Slide (13) so you can remove the Diaphragm (16).
Then, grab the Jet Needle (4) and twist it while pulling down on it. You will feel and hear a click as the needle moves past the retaining pin (14) embedded in the Throttle Slide. It should take 3 clicks if the Jet Needle was at the standard setting. Here’s the disassembled throttle slide. Note the broken lock washer I found when I removed the screws.
Remove the Choke
Now, it’s time to disassemble the choke mechanism.
Remove the Lock Nut (53) and Washer (54) from the shaft. Then disassemble the choke linkage.
Then, remove the four screws (51) holding the Choke Housing (48) to the body and remove the Start Valve Assembly (47) from the Choke Housing. Then remove the Gasket (50).
Remove Varnish and Carbon
As the previous photos show, despite the washing and soaking in parts cleaner solution, there is still varnish and carbon on the parts. I use Berryman ChemTool B-12 and soak the parts in it for 6 hours, or over night.
I put the sub-assembly parts in separate plastic containers as I disassemble the carburetor to make reassembly easier. I just add enough B-12 to cover them and let them soak.
THIS IS NASTY STUFF!!!! So I always wear eye protection and nitrile gloves when handling it. Even then, it softens the nitrile and can eat through it if you keep your fingers in it for too long.
The B-12 will really loosen the varnish and carbon so I can take a tooth brush to clean the parts up. I also use the wire wheel and “00” steel wool to lightly polish the parts. You want to be careful with the brass parts as they are soft, so easy does it.
Rebuild Kit Parts
Here are most of the parts I need for the rebuild. From left to right, Jet Needles (4), assorted O-Rings, brass Needle Jets (3), Vacuum Port Screw Washers (39a), Butterfly Screws (25), Float Needles (42), and the Float Pivot Pins (41).
From top to bottom, the O-rings are :
- 1st, 2nd row – Idle Screw (6) and Idle Air Mixture (9) [these are the same size]
- 3rd Row – Choke Valve (49) [the skinny one]
- 4th Row – Throttle Shaft (26) [the fat one]
- 5th Row – Main Jet Carrier (11)
Here’s the left side carburetor turned upside down. I reassembled the innards going from left to right starting with the Idle Mixture Screw (7).
Assemble Jets and Mixing Tube
The hard part is getting the new O-Ring into the groove as it’s a very tight fit. I use masking tape as shown and a 90 degree pick. (You could use a small diameter wire as well.) I gently move the pick around the O-Ring pulling it over the threads toward the groove and when it gets to the groove, remove the pick and push it into the groove with my fingers. This prevents damaging the O-Ring.
Then, I put the Spring (8) on and screw the Idle Mixture Screw (7) gently into the carburetor body until it lightly seats. Don’t get aggressive with this as the screw and seat can be damaged and are not supposed to be tight. In fact, the screw is backed out to adjust the idle.
Using a new piece of masking tape, work the other small O-Ring onto the Idle Mix Tube (5) screw it into the carburetor body. This should seat firmly, but don’t over tighten it.
Next, get the largest O-Ring onto the Main Jet Carrier. Then, layout the other parts in order from left to right, as shown, with the left most part being the Air Atomizer (2). Replace the original Needle Jet (3) with a new one. This is a good idea as the Jet Needle (4) will rattle around after 97,500 miles and can alter the diameter of the Needle Jet.
Here’s the Main Jet assembly ready to be inserted into the carburetor body. You screw the Main Jet Carrier firmly into the carburetor body and then add the Main Jet Washer and screw in the Main Jet. Don’t get aggressive with the main jet or you can damage it.
I won’t assemble the Float Needle and Float now, but work on the Choke next.
Assemble the Choke
Using masking tape and the 90 degree pick, slide the O-ring (49)into the groove on the stem of the Choke Valve. Apply a small amount of silicone grease to the O-ring and then insert the Choke Valve (47) into the Choke Body (48) being sure you see the “L” stamped on the Choke Valve since this is the “L”eft carburetor, and the large oval hole in the Choke Valve is on the same side (left) as the tunnel section of the Choke Body.
The pictures below show the left choke assembly. The orientation of the choke shaft in the next two pictures is the same for the right choke. That is, the punch mark on the outside of the shaft goes next to the tunnel section.
Turn over the Choke Body and you will see a punch mark on one side of the stem of the Choke Valve. Again, this should be adjacent to the tunnel section of the Choke Body. If you reverse this orientation, the choke won’t work.
Now, put the Gasket (50) on the carburetor body. Put some anti-seize on the Bolts (51), and mount the choke assembly to the carburetor body.
Next, put the Choke lever (52c), Washer (54) and Nut (53) onto the brass stem of the Choke valve. Be careful here since it’s very easy to cross-thread the nut onto the brass stem since it only has 1/2 threads. You don’t want to mangle the brass thread
Finally, assembly the choke cable nut, brass sleeve, washer and nut (52d) onto the Choke Lever (52c).
Assemble Throttle Shaft, Butterfly and Linkage
The throttle shaft also has a washer, and once again the masking tape is needed. Note there are two grooves in the shaft and the one farthest from the threads is where the O-ring goes.
Before inserting the shaft into the carburetor body, apply a thin coat of silicone grease to the O-ring (26). After you get the shaft part way into the hole, put the Throttle Bracket (31) onto the groove and then push the shaft all the way into the carburetor body.
Then attach it with the washers (33) and screws (32) .
Now, assemble the Idle Adjust Screw (36), Spring (37) and Bushing (38) and then screw them into the carburetor body.
Then, put the Throttle Screw Stop Lever bracket (27a) with the 90 degree tab on the Throttle Shaft (24) so the tab rests on the top of the Idle Adjust Screw (36).
The other Lever Bracket (28) mounts on the Throttle Shaft (24) and has a small hole for one end of the Throttle Return Spring (35). The other end of the return spring goes in the hole on the bottom of the Throttle Bracket (31). A Washer (29) and Nut (30) secure both Lever Brackets (27a, 28) to the Throttle Shaft (24).
Be careful not to cross-thread the Nut on the brassThrottle Shaft which is easy to do since there are only threads on two of the faces of the rectangular Throttle Shaft end.
Now rotate the Throttle Shaft so the counter sunk side of the holes is facing you.
Look at the Butterfly Plate (23) and on one side you will see a punch mark. This is the side that should face you after you insert the plate into the slot in the Throttle Shaft.
Rotate the Throttle Shaft so the countersunk side of the holes faces down. With the punch mark side of the plate facing down, carefully insert it into the slot in the Throttle Shaft. This is a very tight fit against the side of the carburetor throat and you don’t want to jam the plate or dent the sharp edges. Go slowly. As the plate enters the slot in the shaft, you can wiggle it a bit to so it slides easily into the slot.
When the plate is half way into the slot, gently rotate the throttle shaft so the countersunk side of the holes faces you again. Wiggle the plate until the holes line up. Put a drop of blue Locktite on the screw threads and insert the new mounting screws and screw them in leaving enough slack so you can still wiggle the plate a bit from side to side. The Locktite ensures the screws won’t come loose and get sucked into the engine. You can heat the plate with a heat gun to soften the Locktite if you have to rebuild the carburetor again in another 40 years 🙂
Hold the carburetor up to a light and wiggle the plate until the minimum amount of light shows around the edge of the plate, then tighten the screws. Ensure the shaft will turn freely with no binding.
Assemble The Throttle Slide and Diaphragm
The Throttle Slide (13) has a notch on the top which matches a button on the side edge of the Diaphragm (16).
Put the Diaphragm (16) on top of the Throttle Slide (13) so the button fits into the notch. Put the Diaphragm Retaining Plate (17) on top of the Diaphragm. Put a little anti-seize on the threads of the Screws (18), add the Lock Washers (19), and tighten.
Insert the new Jet Needle (4) into the hole in the bottom of the Throttle Slide (13). There is retaining clip inside the Throttle Slide. Push the Jet Needle upward as you twist it clock-wise until you hear and feel it click. Continue twisting and pushing until it has moved inward and clicked a total of three times. This is the standard setting for the Jet Needle.
Insert the assembled Throttle Slide with Jet Needle into the top of the carburetor body. There is another notch in the outside of ring at the top of the carburetor body and a matching button on the outside edge of the Diaphragm (16) so align the button with the notch.
Insert the Jet Needle (4) carefully into the Needle Jet (3) inside the carburetor body and push the Throttle Slide all the way down into the carburetor body.
Add Optional Carburetor Internal Spring
I decided to add the internal spring (22) as I did when I rebuilt the carburetor for my R75/6 Silver Ghost rebuild project. This spring was not included in the /5 or /6 carburetors and was added with the /7 series in 1977. It’s part number 13 11 1 335 324.
Here is how the spring fits in between the throttle body and the dome.
Assemble Float and Float Needle
Turn the carburetor upside down to assemble the Float (40), Hinge Pin (41) and Float Needle (42).
Insert the Float Needle (42) so the ball is facing you and the tapered end is inside the hole.
Slide the Float (40) in between the pins and slide the Hinge Pin (41) into the holes in the posts. Don’t push it all the way in yet. Instead, check to see of the tab just touches the Float Needle when the bottom of the Float is parallel with the bottom edge of the float bowl. If adjustment is needed, take the Float out and bend the tab down if the Float was too low, up if the Float was too high. As shown below, the tab is just touching the top of the Float Needle when bottom edge of the Float is parallel to the top edge of the float bowl.
Put the Top (20) onto the Throttle Slide (13) so that the threaded tabs are on the same side as the choke and throttle levers. Put a little anti-seize on the Screws (21) and tighten the Top to the carburetor body.
Finally, remove the Vacuum Port Screw (39) and the Washer (39a) and replace the Washer with the new one and reinsert the Vacuum Port Screw. Then insert the Wire Bale (45) into the holes on the sides of the carburetor body, put the new Gasket (46) into the groove in the bottom of the carburetor and fit the Float Bowl (44) on top of the Gasket and rotate the Wire Bale around the Float Bowl to secure it to the carburetor.
Now, do it again with the right carburetor and you’re done.
What Done Looks Like
Here is the final product and it looks like it was just removed from a new R75/5.
Adjust the Throttle Stop Screw
I used a piece of paper to set the throttle stop screws evenly. I find this difficult to do when the carburetor is mounted. I tighten the stop screw until I feel a bit of drag on the paper. This way, when I start to balance the carburetors after mounting them, both stop screws are even.
2018-12-30 Changed document name to include carburetor series number.
2020-08-26 Added note to assemble choke section about right side choke orientation.