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MC How-To

Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.


Rebuild BMW R75/5 Bing CV Carburetors

 

Bing constant velocity carburetors as used on BMW R75/5s and up are wonderful devices, generally trouble free and long wearing. Yet overhauls are still occasionally necessary, as we found with our subject 1973 BMW R75/5. Suffering from deteriorated jet and throttle shaft O-rings, the carbs were impossible to properly set and the only solution was a full overhaul.

For parts, we turned to the Bing Agency International. After describing our carburetors’ condition, Fay Laughridge suggested their #6 32mm CV rebuild kit, which comes complete with every gasket and O-ring needed, new throttle slide diaphragms, new slide needles and jets, new floats, float pins and float needles, new float needle seats, and new throttle plate screws.

A comprehensive Bing rebuild isn’t cheap. Our kit with optional needle seats came in at $261, and while you can source some of the bits cheaper the ease factor of picking one part number and knowing you’re getting everything you’ll need is huge. Further, as near as we can find, the Bing Agency is the only source for replacement float needle seats. The Bing Agency offers troubleshooting help and can also supply Bing manuals. BikeMaster

The Bing Agency also offers a float and bowl kit with alcohol-proof floats. The stock floats aren’t alcohol proof, but they seem to hold up reasonably well over time. Whether you want the upgrade — we went stock — is a judgment call you’ll have to make.

While this is a somewhat fiddly rebuild, it’s certainly within reach of the average mechanic. The trick, as always, is to take your time. When something doesn’t want to move, find out why before you force it. The only special tool required might be a carb synchronizer for tuning, but a good ear can do well on a twin. That said, for this project it’s essential to have a good manual on hand to help with parts identification and location, and final tuning.

 MC How-To

1. The #6 kit from the Bing Agency for 32mm constant velocity Bing carburetors includes gaskets, floats, float needles, CV diaphragms, slide needles and jets, throttle plate screws and every O-ring for both carburetors. Float needle seats are optional.

 

2. The first step is to remove the carburetors. We removed the gas tank, not strictly necessary as the carbs are so easy to access. Loosen the clamps at the intake elbow and the engine, then gently twist the carburetor free.

 

3. Once the carb is free, remove the throttle cable from the throttle plate and release the choke cable from the choke plate by loosening the holding nut.

 

4. Release the float bowl retaining bail and remove the bowl. Put the carburetor on its side and gently tap out the float retaining pin. Note: The pin has serrations on one end to lock it in place. Drive the pin out from the opposite end.

 

5. Remove the main jet with washer, needle jet assembly, idle jet and idle mixture screw, noting their locations. Also note the order of the needle jet assembly, which has three pieces: the venturi, the jet and the pre-atomizer.

 

6. Turn the carb over. Remove the four screws securing the throttle slide cover. Remove the cover and then the throttle slide complete with diaphragm. It will drop free easily.

 

7. Remove the four screws holding the choke cover and remove the cover. Remove the throttle spring and idle screw. Remove the nut securing the throttle cable lever to the throttle shaft and remove the lever. Remove the two screws holding the throttle shaft retaining plate and remove the plate.

 

8. To remove the throttle shaft you must first remove the throttle plate. The two screws securing it to the shaft are peened at the factory to prevent them loosening. The exposed threaded ends must be filed off before you try to remove the screws.

 

9. Using a tight-fitting screwdriver, loosen the screws. If they fight you, turn them back in and file the ends again. Repeat until the screws will come out, then pull the throttle plate free.

 

10. Remove the throttle shaft. Note the orientation. The screw head recess is to the outside, as shown. The throttle plate has a small dimple at 12 o’clock that also faces to the outside.

 

11. Remove the four screws and plate securing the diaphragm to the slide and remove the diaphragm. The slide needle is held by an internal spring clip and has four stops. Remove it by twisting it 90 degrees then pulling it out. If it clicks and stops, twist another 90 degrees and continue. Repeat until it pulls free. Note the number of stops to remove; you’ll want to reinstall the new needle the same number of stops. 

 

12. Soak the carb body and all parts in solvent. Rinse and, if available, give them a final clean in an ultrasonic washer, rinsing when done. Blow out all ports with compressed air. To replace the float needle seat, thread a 7mm x 0.75mm pitch tap into the seat until it stops.

 

13. Next, clamp the end of the tap firmly in a vice. With an assistant holding the carb body, place a block of wood over the jet ports and hit the wood with a hammer until the body separates from the seat. We heated the body with a heat gun, making removal easier.

 

14. Make sure the seat bore is clean. Coat the outside of the new seat with anti-seize compound. Using a wooden dowel, drive the new seat into place until it bottoms out.

 

15. Put a new O-ring on the throttle shaft. Coat the O-ring with silicone lubricant and install the shaft into the body, noting its orientation. Install the throttle shaft retainer and screws.

 

16. Install the throttle plate in the shaft making sure the dimple on the plate is facing up (toward the top of the carb) and out. Coat the retaining screws with permanent-type thread-locking compound. Screw firmly in place.

 

17. Install the throttle cable plate on the throttle shaft. Reinstall the idle screw and spring. Place a new O-ring on the choke valve shaft and coat with silicone lube. Install the choke valve with index hole on choke plate aligned with oblong intake port on choke valve body. Using a new gasket, reinstall the choke assembly to the carb body.

 

18. Install new O-rings on the idle jet, main jet, needle valve venturi and idle mixture screw. The central needle valve jet has three pieces: Install the pre-atomizer followed by the jet and finally the jet venturi. Install the idle and main jets and the idle mixture screw.

 

19. Place new float needle into seat. Install new float and pivot pin. Seat the serrated end of the pin by lightly squeezing it into the float bowl post. Set the float so it’s parallel to the carb body when it touches the needle. Install new float gasket followed by the float bowl.

 

20. Install new diaphragm on slide. The diaphragm is “clocked” to the slide, with a molded, raised rubber edge that locks into a corresponding slot in the top of the slide. The outer edge of the diaphragm (just visible above thumb at left) has a similar molded edge that slots into the carb body when the slide is dropped in place. Replace slide cover and four screws.

 

21. Adjust the idle screw until it just touches its stop, then screw it in one turn. Lightly seat idle mixture screw, then turn out 1-1/2 turn. Attach throttle and choke cables and install carbs. Finally, start the engine and fine-tune the carbs.