13 BMW 1973 R75/5 Rebuild Bing Carburetors & Karcoma Petcocks

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.

Parts List

Here are the new parts I used for both the petcocks and carburetors.

Part # Description Qty
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.

Tools for Cleaning, Polishing Carb Body
Cleaning and Polishing Stuff

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.

Before Soak, After Soak
Left-Before Wash and Soak, Right-After
Before Soak, After Soak
Left-Before Wash and Soak, Right-After
Before Soak, After Soak
Left-Before Wash and Soak, Right-After

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.

Karcoma Petcock Parts Order

Karcoma Petcock Parts on Petcock Shaft

Here are the internal parts from outside (left) to inside (right).

Karcoma Petcock Parts Order

Karcoma Petcock Parts Order

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.

BMW R75/5 Karcoma Petcock Internal Gasket (Source: MAX BMW Parts Fiche)

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.

Assemble Petcock

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.

Petcock Washer

Petcock Washer Location (NOTE: Plastic Tubes are INCORRECT and should be Switched)

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.

Rebuilt Petcock

Rebuilt Petcock With Plastic Tubes in Correct Holes

Carburetor Disassembly

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.

Bing Carburetor Exploded View

 

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).

Before, Float Bowl
Some Float Bowl Crud, But Not Too Bad
Before, Floats
Floats Need Replacing-Discolored and rusty
Before, Throttle Slide
Before Soaking-Throttle Slide with Carbon Build-up in Intake Passage
Before, Throttle Slide
After Soaking and Tooth Brushing, Better, But Still Needs Cleaning with ChemTool

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.

Removing Idle Jet
Idle Screw Assembly (7)
Before, Idle Jet, O-Ring and Spring
Spring (8), Idle Jet (7), O-Ring (9)
Removing Idle Mix Tube
Idle Mixing Tube (5) Unscrewed
Idle Mix Tube w/ O-ring
Idle Mix Tube (5) w/ O-ring (6)

I removed the main jet from the main jet carrier and then removed the carrier.

Removing Main Jet Carrier
Main Jet Carrier (10)
Main Jet Carrier, Washer, Main Jet
Main Jet Carrier (10), O-Ring (11), Washer (12), Main Jet (1)

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).

Removing Idle Speed Screw
Removing the Idle Adjusting Screw (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.

Throttle Assembly
Throttle Spring (35), Lever (28), Throttle Screw Stop Lever (27a), Washer (29), Nut (30)

Then, remove the bracket from the throttle shaft.

Throttle bracket
Throttle Bracket (31)
Throttle Bracket
Screw (32), Washer (33), Bracket (31)

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.

Removing Throttle Plate
Removing screw (25) Holding Throttle Plate (23) to Throttle Shaft (24)

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.

Pulling Butterfly Off Throttle Shaft
Throttle Plate (23) Coming Off Throttle Shaft (24)

Now you can pull the Throttle Shaft  (24) out of the body.

Pulling Throttle Shaft Out
Removing Throttle Shaft (24)
Butterfly, Shaft w/ O-Ring
Throttle Plate (23), Throttle Shaft (24), O-ring (26)

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).

Slide & Diaphram, Cover
Throttle Slide (13) with Diaphragm (16), Diaphragm Retaining Ring (17), and Cover (20)

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.

Slide, Diaphram, Plate, Jet Needle, Screws
Throttle Slide (13), Diaphragm (16), Diaphragm Retaining Plate (17), Screws (18), Lock Washers (19) (note broken washer) 

Remove the Choke

Now, it’s time to disassemble the choke mechanism.

Left-Choke Assembly
Left Choke Assembly

Remove the Lock Nut (53) and Washer (54) from the shaft. Then disassemble the choke linkage.

Choke Bracket
Choke Cable Screw, Brass Washer, Linkage, Washer and Nut (52d).

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).

Choke, Gasket, Cover, Screws. Choke Cable Bracekt
Gasket (50), Choke Valve (47) with O-Ring (49), Choke Cover (48), Screws (51), Choke Lever (52c), Washer (54), Lock Nut (53)

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.

Chemtool B-12 Varnish & Gunge Cleaner
Berryman ChemTool B-12

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.

Soaking Parts by Category
Separate Plastic Containers for Each Sub-assembly

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.

Jets After Soaking
Innards After Soaking in B-12 and Light Wire Wheel Cleaning

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).

Rebuild Kit Left-to-Right, Jet Needles, O-Rings, Needle Jets, Throttle Plate Screws, Float Needles, Float Hinge Pin
Most of the Rebuild Parts.

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)

Assemble Carburetor

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).

Untitled
Clean Carburetor Ready for Reassembly

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.

Pulling O-Ring On
Masking Tape and 90 degree Pick To Install 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.

Final Assembly
Idle Mixture Screw Assembly Ready to Inset into Carburetor Body

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.

Mixing Tube w/ O-Ring
New O-Ring Inserted on Idle Mixing Tube.

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.

Needle Jet and Main Jet Assembly
Main Jet Assembly, Atomizer (2), New Needle Jet (3), Main Jet Carrier (10), New O-Ring (11), Main Jet Washer (12), Main jet (1)

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.

Correct Order of Parts for Main Jet Assembly
Correct Order and Orientation of Parts for the Main Jet Assembly

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.

Left Side Choke Assembly Orientation
Correct Choke Valve (47) Orientation Inside the Choke Body (48)

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.

Left Side Choke Showing Shaft Punch Mark Orientation
Front Side of Choke Body Showing Correct Orientation of Stem of the Choke Valve.

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

Choke Lever Mounted
Choke Lever (52c) and Washer (54) on the Choke Valve (49) stem

Finally, assembly the choke cable nut, brass sleeve, washer and nut (52d) onto the Choke Lever (52c).

Choke Cable Bushing, Washer and Nut
Choke Cable Screw, Brass Sleeve, Washer and Nut (52d) assembled on 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.

Throttle Shaft w/ O-Ring Mounted
Throttle Shaft (24) with New O-Ring Installed

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.

Throttle Bracket in Groove on Throttle Shaft
Throttle Shaft (24) with Throttle Bracket (31) Inserted in Groove

Then attach it with the washers (33)  and screws (32) .

Now, assemble the Idle Adjust Screw (36), Sprint (37) and Bushing (38) and then screw them into the carburetor body.

Idle Adjust Screw Assembly
Bushing (38), Spring (37), Idle Adjusting Screw (36)

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).

Throttle Adjuster Mounting
Throttle Scew Stop Lever (27a) on Throttle Shaft (24)

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).

Throttle Cable Bracket & Spring
Throttle Lever Bracket (28), Spring (35), Washer (29), Nut (30)

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.

Correct Orientation of Throttle Shaft w/ Counter Sunk Holes on Outside
Correct Orientation of Throttle Shaft (24)

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.

Butterfly Showing Punch Mark on Outside Face
Throttle Butterfly Plate (23) with Punch Mark At the Top Between the Two Holes

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.

Inserting Butterfly into Throttle Shaft
Inserting Butterfly Plate (23) into Slot in the Throttle Shaft (24)

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 🙂

Butterfly Alignment on Shaft
Butterfly Plate (23) Inserted Into the Throttle Shaft (24)

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.

Butterfly Alignment w/ Minimal Gap
Centering the Butterfly Plate (23) in the Carburetor Bore

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).

Throttle Slide Diaphram Alignment Notch
Notch in Throttle Slide (13)
Diaphram w/ Throttle Slide Alignment Tab
Button on Inside Edge of Diaphram (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.

Carb Body with Diaphram Alignment Slot
Notch in Outside Ring of Carburetor Body with Needle Jet (3) Inside Carburetor

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.

Throttle Slide w/ Diaphram
Throttle Body assembly Inserted into 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 Grey 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.

Carburetor Spring

Optional Carburetor Internal Spring

Here is how the spring fits in between the throttle body and the dome.

Carburetor Spring

Carburetor Internal Spring Mounted in Slide

Spring Installed

Spring Installed

Assemble Float and Float Needle

Turn the carburetor upside down to assemble the Float (40), Hinge Pin (41) and Float Needle (42).

New Float, Hinge Pin and Float Needle
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.

Float needle in Carb Body
Float Needle (42) Inserted in the Hole in Carburetor Body

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.

Float Height Adjusted- Level with Carb Body, Tab Just Toches Float Needle
Correct Float (40) Height with Tab Just Touching the Float Needle (43)

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.

Cleaned & Polished - Top View

Cleaned & Polished – Top View

 

Cleaned and Polished, Outside View

Cleaned and Polished, Outside View

Cleaned and Polished, Inside View

Cleaned and Polished, Inside (Next to Engine) View

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.

Using Paper To Adjust Throttle Screw

Using Paper To Adjust Throttle Screw

59 thoughts on “13 BMW 1973 R75/5 Rebuild Bing Carburetors & Karcoma Petcocks

  1. Pingback: 1973 BMW R75/5: Bing Carburetor Restoration | Motorcycles & Other Musings

  2. I’m in the same boat.
    Aren’t there those large springs that push the throttle body assembly down?
    Most diagrams show it…mine didn’t have it. But it seems to not idle down normally…so I’m wondering.

    • Hi Mike,

      I believe BMW added the internal spring between the dome top and throttle slide in 1977 on the /7 series. Like you, I figured the spring helps close the slide and needle quicker when the throttle is closed. The MAX BMW diagrams show the spring, but they often use the same diagram for multiple models, so parts shown in the diagram aren’t/weren’t always used with a particular model. That’s the case with this spring and the /5 and /6 carburetors.

      I decided to add it after I finished the rebuild as I did on my R75/6 carburetor rebuild. I’ve updated this write up to show this spring as an option.

      Thanks for visiting.

      Best.
      Brook.

  3. Hi Brook!

    I soaked a spare carb in simple green and noticed that where the top of the solution, it left a line on the carb. Im not sure if its simply a stain or somehow corrosion from the mixture? In any case there was no discoloration except for near the surface of the solution.

    Would B12 be able to clean this? If not, is this something that soda blasting or sand blasting might work?

  4. Hi Brook, you’ll agree that tightening the screws pinching the 2 solid monofilament choke wires that run to the choke lever on the side of the air filter is a ‘female dog’ of a job. What I’ve done ( on more than 1 pair ) is to turn 2 thin “top hat stepped washer” pieces with a small hole to suit the monofilament choke wire. The “stepped washer” smaller outside Ø is to fit into the carb choke lever hole, the larger Ø being about equal to the choke lever width around the hole, and the total thickness being about 2X the choke lever thickness. I then glued the “stepped washer” pieces onto the carb choke levers, bent the monofilament choke wires 90° to poke through the holes, bending inwards towards the carbs ( so that there’s a force to stop them coming out ) and the job is done. As you operate the choke, the wires rotate relative to the “stepped washer” pieces, so the wires will wear after about 100 years. But I’ve had more wire breaks where they bend and straighten at the choke lever end. I’d attach a picture if I knew how. Now its actually easier to remove and attach these wires than the later handlebar mounted choke cables, and I prefer this system, having 1 bike with each system. G’luk, Phil.

    In addition, you’ll have discovered that its also another ‘female dog’ of a job to lap in the steel point of the 4 sided float needle to seal at the bottom of the seat. I converted my R75/6 to rubber tipped needles by drilling out the Ø6mm brass seats ( drill Ø10mm ) and making Ø7.5 ID X Ø10 OD brass seats to suit the newer 3 sided rubber tipped needles. No more getting ‘pissed on’.

    On that score, if it continues to leak, you’ll find that a lot of gunk collects at the bottom of the brass pipe pressed into the carb body in the little chamber on top of the float needle seat. Take a drill that goes into the brass pipe and twirl it using your fingers to get the gunk out. You’ll be amazed !

    • Hi Phil,

      Thanks for coming by and reading this write-up. This is great information. Thanks for posting it.

      If your pictures are posted on-line (Flickr, Smugmug, etc.), you can just include the link to them.

      Best.
      Brook.

  5. Thank you for v. good carb rebuild scheme. I did this many years ago having experienced 38mpg (UK gallons). After the rebuild I was getting about 48mpg under the same conditions so a re-build is very much worthwhile (mostly due to new needle and new needle jet I suspect). Have recently had problems with fuel dripping from overfilling float bowls which was completely cured by fitting new floats. (New float needles did nothing). The bike has now done over 150,000 miles.

  6. Hi Brook
    100% thorough…amazing.
    Anyone should be able to rebuild an R75/5/6/7 Bing following your instructions.
    Maybe you should consider to assemble a “Reams” instead of “Haynes”.
    Thanks
    Klaus

  7. Hello Brook,
    I noticed that you are showing two different part numbers for the slide springs. Your parts list shows #13 11 254 929 and the picture shows #13 11 1 335 324. Which spring is the best to use for an R75/5. Thank you .

    • Hi Dan,

      My bad. The 13 11 1 335 324 is from the R80 series and that’s the part I used.
      The other part, 13 11 1 254 929 is for the R50/5 or R60/5 which aren’t constant vacuum carbs. I’ll fix the write up with the correct part number.

      Thanks for finding that mistake. I appreciate it.

      Best.
      Brook.

      • Brook,
        Thanks for your quick reply. This is a good learning experience for me. The springs aren’t the same. Now I know why which is due to the different carbs.

        Dan

  8. Sorry for the above question … should’ve read ALL the Comments first! The key decision seems to whether to install the 115 mm spring you used (13 11 1 335 324) or the slightly longer 120 mm spring (13 11 1 338 134) that Snowbum says Tom Cutter use(s/d). Looks like you can’t really go wrong with either, but I believe I’ll try the latter. Cheers.

    • Hi John,

      It’s great when the comments add additional value beyond what I documented. In fact, I was aware of the option on length, so I learned something new.

      Best.
      Brook.

  9. Brook, when you had the carbs apart, did you notice a difference between the choke valves for the left and right sides? Specifically:

    I find that one of my choke valves (aka start valves) has only three holes open … the smallest hole seems never to have been drilled. The other side has all four holes open.

    Judging from your photo above (http://i2.wp.com/farm9.staticflickr.com/8492/8332174126_ced419cd2c.jpg?resize=500%2C375), that particular LEFT valve only has three holes (but a fourth depression).

    Curious for your insights. It’s difficult to find many photos of that side of the valve! (I’m glad you snapped one.)

    • Hi John,

      Hmmm, I can’t say I noticed that little detail.

      Here is the picture I took of the RIGHT side enricher from my 1975 R75/6 which uses the same enricher as the 1973 R75/5 according to the MAX BMW Parts Fiche (Part #: 13 11 1 257 815)
      https://www.flickr.com/photos/brook-reams/4658873378/

      And here is the LEFT side enricher from my 1973 R75/5 which is the same part for 1975 R75/6 (Part #: 13 11 1 257 816)
      http://i2.wp.com/farm9.staticflickr.com/8492/8332174126_ced419cd2c.jpg?resize=500%2C375

      The smallest hole is not completely drilled through the disk in both disks. So, I believe the holes are the same on both sides (and that’s logical since each should enrich it’s own carburetor the same amount for smooth idle with enricher engaged). That said, I’m not sure I appreciate the function of the holes in the enricher.

      Bob Fleischer (aka snowbum) has a lot of information on the Bing CV carburetors for the /5 models and later:
      http://bmwmotorcycletech.info/earlybingR75CV.htm

      He does mention changes in the “starter cup” and “mixture bore in the rotary valve”. I suspect those changes are reflected in the MAX parts fiche with reference to (from 01/71) for the part numbers above.

      The cost for a new assembly is about $40.00. I’d call MAX BMW (ask for Rusty, but they are all very knowledgable) to confirm what comes with #1 in the parts fiches which is the part number I mention above, but it should include the disk and some other parts as well.

      I hope this helps…

  10. What a great write up. Now that you’ve added a spring to the Diaphragm assembly… what are your impressions? Worthwile, just the same? Smoother/better MPG? Inquiring minds want to know.

    • Hi Kendall,

      Thanks for your note. I believe the engine is smoother on acceleration. It seems to transition from idle with no hesitation where before there seemed to be a small lag. I did not see any mileage benefit from the spring.

      Best
      Brook

  11. Am rebuilding a 1977 R100S, and i have to congratulate you on your website,very helpful,the o rings came as part of a service kit for the Bings,there is no way i would have known which o ring went where,so many thanks.Ian.p.s. do you know if the needle position for this model is 2?

    • Hi Ian,

      Yes, those o-rings caused me to scratch my head as the old ones were swelled, so I figured I’d document where they go so other folks would have an easier time.

      The jet needle position varies based on year and model. In my Clymer manual, pg 378, they show the 1977 R100S using the model 94/40/103 and 104 carburetors. For this model carburetor, it’s the 2.66 needle jet, and the jet needle position is 3rd. Around about this time, BMW needed to ensure they could meet emission standards, so I think they had a variety of jetting and needle positions in use with several different model Bing carburetors. For example, for 1978, the R100S has the 94/40/107 and 108 model carburetors and on these, the needle jet is 2.64 and the jet needle position is 2nd. I think the game was to lean out the mixture as much as possible to meet the emission standards. They introduced the Pulse Air system as well to try and burn any remaining fuel as it exited the exhaust valve. In the end, meeting emissions required fuel injection and computers to control it which ushered in all the plumbing, extra sensors and wiring.

      The Clymer manual has several pages of bike year/model vs Bing carburetor model with all the standard sizes and settings. I think its a good resource for this information.

      I hope this helps.

      Best.
      Brook.

      • Hi,thanks for that Brook,my Haynes manual,only lists the needle position of the RS,which may or may not be the same,however,i have the 103,104 carbs,which have(according to the manual) 46-341 jet needle and the 2-66 needle jet,so i will go with the 2nd position,again many thanks IAN.

        • Ian,

          Cool beans. If the year is 1977 and the Bings are 103 and 104 models, those are the correct factory sizes and needle jet location.

          As always, monitor the plugs and see if your bike is running too rich or too lean, and adjust accordingly. Bing publishes a carburetor book with good information about tuning, and what changes in main jets, needle jets and jet needles do to the fuel mixture at various RPM ranges.
          http://www.bingcarburetor.com/manual.html

          If there is doubt about the needle jet size, you can remove the main jet with carburetor on the bike and then remove the needle jet. It will have the size stamped on the side.

          It’s useful if you don’t find the stock settings working well.

          Best.
          Brook Reams.

          • Thanks again,although;when i said i would go with the 2nd position i meant the third,hope i am not headiung into a Pirsig style “gumption” trap : ) IAN.

  12. Excellent write up. I just acquired a 1972 R75/5 with the Bing 64/32/9 & 10 carbs. All of the history seems to indicate that a complete rebuild is needed so I have ordered the parts and will use your write up as a guide.

    Question: On my carbs there is no vacuum port, so I have ordered the kit from the Bing Agency in order to install it during the rebuild. Did you install the ones on your carbs and if so do you have any helpful tips?

    Thanks again!

    • Hi Clayton,

      I’m pleased this material is useful. It’s great to get another airhead running smoothly again.

      These carbs had the vacuum port. I think this may have been added in later years on this carburetor. So I have no hints or tips to share about retrofitting them to the carburetors.

      Best.
      Brook.

      • With the help of a friends drill press I was able to install the vacuum take off ports with out any problems. The instructions from the Bing Agency were very helpful.

        Once the carbs were redone (following your excellent write-up) an adjustment/synch was performed and the bike now idles for the first time in nearly 10 years.

        Thank you again for helping me get my time machine up and running.

        • Hi Clayton,

          Well, cool beans. Isn’t it exhilarating when you breath life back into an old bike?

          Congratulations and safe riding.

          Best.
          Brook.

  13. Hi Brook,
    I used your wonderful guide to rebuild the carburators on my 1987 R100RT. The bike had been sitting in a friends garage since 2004. I thoroughly cleaned and flushed the tank. When I started the bike up for the first time, it sprang to life in seconds. However, it would conk out when the choke was turned off. The setting on the idle mixture screws was approximately 1 1/4 turns, which was the pre-rebuild setting. It then idles, albeit roughly, at app. 1100 rpm. However, when I attempt to give it gas, it stalls. What am I, or what did I do wrong? Any help in the matter would be greatly apreciated. By the way, I am a member of the Airheads Club.

    • Hi Aurelio,

      I’m sorry to hear you are having problems after doing a rebuild. If the bike won’t run unless the choke is on, then it’s running lean at idle. So that is a clue for what to look for.

      Two conditions can create a lean idle condition: too little fuel going into the idle circuit or too much air. The float level in the float bowl affects fuel delivery, so if one or both floats are keeping the fuel level low in the bowl, then not enough fuel gets into the idle circuit. Too much air can come from leaks around the idle air screw, leaks from the rubber flange the goes between the carburetor and the engine, from the throttle shaft o-ring leaking, and if the slide is hung up and not closing all the way. You can check for air leaks in the carburetor to engine and around the throttle shaft by spaying some carb cleaner on them. If the idle increases, you have an air leak. (Be careful with the carb cleaner as it’s very flammable and not good stuff to breath).

      You can have a small bit of grit in the idle air passages and you may not have the idle jet seated correctly. And, you may not have the jet needle adjusted so it is in the correct groove.

      Incorrect valve adjustment can cause running problems. You don’t mention if you have adjusted the valves prior to adjusting the carburetors, but if not, make sure the tappet clearance is correct on the intake and exhaust valves.

      You ought to confirm the choke assembly is correct as well. There is a left and right choke disk and if they get switched or if they are installed 180 degrees from the correct orientation, you will have problems.

      One other thing to confirm is the timing, and specifically that the advance unit is working correctly. If the timing can’t advance as the engine speed picks up, the bike won’t run well. Note, this doesn’t cause a lean condition, but you may have more than one problem and this one affects the ability of the engine to rev up.

      I suggest you write down each thing you are checking, what you are looking for, and what you found. With many problems, loosing track of what the test results are leads to confusion and frustration. So I take the time to document what I am checking.

      I hope this helps.

      Best.
      Brook.

      • Thanks Brook for getting back quickly. I will follow your suggestions, notebook in hand, this weekend. Will let you know how how I make out.
        Wish me luck. Aurelio

  14. Excellent tutorial, thank you.

    I’m about to rebuild the 64/32/11+12 Bings (very similar to yours) on my 1975 R90/6 and have a condition where idle through 3000 rpm is very rough when riding (OK when idling) – bucking bronco and very sluggish until it picks up revs – with power delivery fine north of 3000 rpm.

    Timing, points, and valve clearances are fine. I use the later independent ‘alcohol proof’ floats and measured fuel levels in the float bowls are correct. The floats do not bind on their posts.

    Vacuum is nicely balanced at idle (1100 rpm) and part open (2000 rpm) using the CarbTune vacuum gauge.

    The right carb idle mixture screw properly adjusts engine speed slow-fast-slow and is in the sweet spot. Right spark plug tip a tad sooty currently.

    The left idle mixture screw only speeds up the engine when unscrewed, never slows it down, so impossible to determine the sweet spot, though the spark plug tip is the correct grey.

    Any pointers based on the above as to the cause would be appreciated.

    Thanks again,

    Thomas

    • Hi Thomas,

      I’m not clear if the operating problems you describe are before or after you rebuilt the carburetors. If before, then dive in and do the rebuild. Most times that eliminates whatever the cause was as everything gets thoroughly cleaned, all rubber parts are replaced (including the rubber sleeve between the carburetor outlet and the engine intake), and when I do it, the wearing parts such as needle jet and jet needle are replaced. So far, any problems I had before the rebuild were gone after I completed the work.

      What you describe about the left carburetor idle adjustment indicates that the idle circuit is running very lean (too much air). When you screw the idle mixture screw out, you are enriching the idle mixture, but it never gets too rich, so the speed keeps rising. Grit maybe in the fuel passages to the idle circuit and/or there is air leaking into the idle circuit, or ???.

      The rough running up to 3000 RPM maybe a different problem. It could be with the ignition advance. In general, full advance occurs at about 3000 RPM, but if the springs are weak, or the weights can’t move freely, then rough running can result as you open the throttle. Note, this is a dynamic condition and you won’t find it just measuring point gap. You will see it with a timing light as movement from the OT to F marking on the flywheel won’t be smooth and uniform. You can also verify the advance weights are moving freely by disconnecting the ground on the battery, removing the front timing cover, reconnecting the battery ground, starting the bike and watching how the weights move as you increase engine speed.

      I hope some of this is helpful.

      Best.
      Brook.

      I hope this helps.

  15. Brook – That is very helpful, thank you.

    What I did was dismantle and clean with Berryman’s B12. However, I did not replace any of the O rings or any parts. The problems started after reassembly, so I’m thinking the disruption of the tired, old O rings was the cause. Everything ran fine before.

    I did find a spare idle jet O ring in my parts box and after replacing the old one in the left carburetor I reestablished the ability to go achieve the slow-fast-slow idle as the idle screw is turned, so your diagnosis of an air leak in that location appears correct.

    That leaves a couple of other O rings to order and replace.

    Thanks again – I’ll report back,

    Thomas

    • Thomas,

      Good news :-).

      Beware that Berrymans destroys rubber o-rings. I assume any rubber older than 10 years has lost elasticity.

      Best.
      Brook.

  16. Ah! Lesson learned.

    Indeed the old mixture O rings were anything but elastic.

    It very much looks like the idle jet O rings are the culprit as that would account for the low rpm range bogging, nothing else having changed. I’ll replace those as well as the main jet O ring. I’m not taking the tops off as that guarantees a torn diaphragm and argues with the “If it ain’t broke, don’t fix it” law.

    Thanks,

    Thomas

    • Thomas,

      But, if the diaphram tears when you remove the top, then it needs replacement 🙂

      It too is rubber, and tears in use and looses flexibility that affects throttle side movement. If o-rings are crusty, the diaphram is typically not up to snuff either.

      Just my 2 cents worth.

      Best.
      Brook.

  17. Ha! I hear you. But I just KNOW that those top retaining screws (I converted to the Allen socket type two decades ago) will be a living hell to remove, may have to be drilled out and will lead to all sorts of other problems, not to mention Garage Language, so I will pass for now! I did take the opportunity to install lighter tension throttle return springs after 25 years of fighting the originals, and they are wonderful. With a properly greased throttle gear I am now (almost!) at Japanese bike levels of throttle control smoothness ….

  18. I picked up a 1973 R75/5 LWB last year to rebuild. Thank you for the wonderful information. My rebuild is going very well thanks to this informative site.

    • Hi Kevin,

      Thanks for stopping in and looking at the write-ups. I wish you the best of success in your Rebuild.

      Best.
      Brook.

  19. I have an older model R 75/5 bike that I am having some problems with. It seems as if the engine wants to run on a single cylinder until the throttle is opened more. I need some help in getting the carbs to respond. I don’t even know which cylinder is operating at idle.

    • Hi James,

      I’m sorry to hear about the problem with your R75/5. When idling, the main carburetor isn’t able to fuel the engine. The idle circuit acts as a small low volume carburetor for this purpose. Any of the following will prevent the idle circuit from working:
      – leak past the idle air screw due to failed o-ring.
      – plugged passage in the idle fuel well in the float bowl. This is the small well the long brass tube in the carburetor body fits into.
      – plugged idle circuit passages on the engine side of the butterfly.
      – broken tip on the idle air screw.
      – air leak in the rubber bushing between the carburetor and the engine.
      – loose cover on the choke housing.
      – failed choke housing gasket.

      I don’t know when the carburetor was last rebuilt. If it is more than 5 years or so, I’d consider rebuilding both carburetors.

      I hope this helps.

      Best.
      Brook.

  20. Agree with above comment ….Outstanding guide. Real pleasure to read now off to remove and service my R75/5 1973 BINGs

      • All done 😉

        Only issue encountered was getting the Idle Mixing Tube out on one of the carbs… Carbs are now starting to look almost a tiday yours. Thanks again sure everyone gets the greatest encouragement from viewing your site Brook.

        Be on the road in 10 days

        • Hi Paul,

          I’m pleased to hear you completed the carburetor rebuild. The bikes always start easier and run smoother after I do one.

          Best.
          Brook.

  21. I’ver just pulled my R75 out of storage where it has been several years. Before going into storage it had just undergone a full restoration including the use of some NOS Bing carbs. I had ridden the bike a bit and then decided to “store” it in our living room. I dumped the fuel in the tank ran the engine to empty the float bowls and dropped the float covers to let the carbs air out and then replaced the covers. Over the years I squirted some oil in the cylinders and cranked the engine over on a regular schedule.

    So, last week I decided it was time to get it of the living room and starting riding it. I spent a day getting her all cleaned up, changed the oil, replaced the battery and filled up the tank. O far, all was good. I turned on the petcocks and fuel immediately began streaming from both carbs. I removed the float bowls and all was clean and the float springs and needles seemed to be fine. With the float bowl off, I could hold the floats up and the float needles were doing their job shutting of the fuel, but fuel is coming from somewhere else. Any suggestion – it seems to be more on the inner side of the carbs – both are acting exactly the same.

    The bowl gaskets were hard and brittle so I am guessing that the fuel is leaking from another gasket somewhere that has dried and shriveled up.

    It seems you have a lot of experience with Bing carbs so I’m hoping you can offer some advice or suggest some next steps.

    Thanks and great article, Robert

    • Hi Robert,

      Some thoughts come to mind, so in no particular order:
      1. Cracked fuel lines can leak slowly and those attach on a spigot on the inboard side of the carburetor. Sometimes this is a crack at the end of the fuel line and isn’t obvious and sometimes it seals until the line is moved a bit so it can be intermittent. You didn’t say how long the bike has been sitting, but putting in new fuel lines is a good idea even if they aren’t the cause of the leaks. Fixing a broken one by the side of the road is “challenging” as they say 🙂

      2. It’s possible for the float to touch the side of the float bowl if it is not centered on the hinge pin. And it’s possible for the hinge pin itself to be loose which can bind up the float and prevent it from pivoting smoothly up and down with the fuel level.

      3. You said “immediately” fuel leaked out. But, the carbs would take 10 – 20 seconds to fill the float bowls. So if the leak was indeed immediate, then it could also have been coming down the fuel line from the gas tank. The petcocks have seals and these sometimes shrink a bit and don’t seal. It this is the case, they typically swell up after a bit and seal. Check to see if there is any sign of gas leaking out of the petcock body.

      4. You said you could push the float up and fuel stopped flowing out of the float needle. That said, it’s possible for some grit to be inside the hole the float needle goes into. I’d pull out the float hinge pin and remove the float needle and use some carb cleaner to blow out the float needle hole and then put it all back together again.

      5. If the carb gasket is cracked, pinched or really hard, it won’t seal the float bowl and you can get a leak. Likely best to replace the gaskets if this is the case.

      6. The float level maybe incorrectly adjusted. You said the bike ran prior to parking it, so this isn’t highly likely. But if in doubt, verify the float level is correct.

      I hope this helps.

      Best.
      Brook.

  22. Dear Brook, thank you for the great info you provide on this website. I have a question. I’m rebuilding my carburetors (from a 1970 R75/5) and I have this short and wide spring that I believe should fit between the Choke Valve and the Choke Cover. Is that correct? I do not see it in your example and I cannot remember clearly where it was when i took the carbs apart. But the Choke Valve already has a spring in it so that is why I’m confused. I looked online and I’m not 100% sure that I found an answer. Thank you in advance. Daniele

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