Over the Memorial Day weekend, I took on the task of rebuilding the carburetors. The right side carb had a broken float pin boss and I had to get a used R75/5 carb body 5 years ago to get it running again. The right side has not been happy and when the heads and valves were redone, the right side exhaust was in worse shape than the left which are signs of lean running on the right side. With 103,000 and 35 years on the left side carb, and no full rebuild in that time (new floats and float needles of course, but no rebuild) it was time to get them back in condition.
To keep parts together, prevent confusing the parts from the left with those from the right side and to always have a correctly assembled carb to refer to, I rebuilt them one at a time starting with the right side. Since I was going to reinstall the original 75/6 right side carb, I would be stripping all parts off the old 75/5 carb body, replacing with new parts in the rebuild kit, and reusing the other parts. The R75/6 body had been sent to the Bing Agency to repair the broken float boss. Here are the before and after pictures.
Although Bing cleaned the carb, I polished it up a bit more with aluminum cleaner, steel wool and metal polish while I waited for parts to soak in solvent.
The first step was to remove the enricher from the R75/5 body taking out the 4 machine screws. As you can see in the picture, this is the Bad part — as in dirty. But it gets worse.
Next, I took off the float bowl … and now you can see the Uglypart … a nice bunch of glop in the bottom … not a good sign of a healthy carb by any means. I’m suspicious that the float was starting to dissolve which can happen with our methanol laced fuels. Corrosion is also clearly evident on the washer underneath the main jet in the center of the carb and the gasket between the float bowl and carb body is complete toast.
I focused on the enricher rebuild first. I dissembled it, and took pictures of the parts order so I wouldn’t make a mistake. I also have the Bing Agency carb book with full exploded view diagrams which is very helpful. I’ll refer to part numbers in this diagram in the following material.
I removed the old O-ring (pretty cracked) (#26) and soaked in parts washer solvent for several hours.I cleaned up the housing and lever using a wire wheel, aluminum cleaner and then metal polish. I used some masking tape to cover the threads of the disk (#47) to make it easier to get the O-ring (#49) on and not tear it in the process. I have some steel picks, one of which has a “C” shaped end that I hooked under the O-ring so I could pull it over the threads and onto the slot.
I found details in Snowbum’son-line BMW reference material on how to ensure the enricher is assembled correctly. There are two articles as well as another on the R75/5 Bing carburetor. As is the case with his writing, read slowly and carefully and be prepared to be told the same thing three times in different places and different ways. The right side housing has the curved passage on the right.
You should confirm you have the correct disk. The inside of the shaft is stamped with “R” for right and “W” for wrong … :-), actually, it’s “L” for left.
Then the disk is inserted as shown, with the slot at the 8:00 position and the little holes at the 1:00 position.
Then I assembled the housing putting the new gasket (#50) between it and the carb body. I put a small amount of silicone grease on the O-ring and a tiny amount of antiseize on the threads of each of the screws (#51). Note there is a dimple on one side of the shaft. This should be closer to right side on the right carburator.
The handle goes on the shaft with the nut (#52d) to attach the choke cable facing you. The shaft handle goes between the two verticle pins and the shaft should be pointing to the fuel spigot on the carb.
I put a tiny bit of antiseize on the brass threads of the shaft and then tightended up the nut.
The next step was to remove the throttle linkage (#27, #28), spring (#35) and throttle linkeage bracket (#31).
Then I remove the throttle plate (#23) and throttle shaft (#24). The screws (#25) holding the throttle plate to the shaft are peened over and are hard to remove. I was not able to remove one of them. So, I ground off the end of the screw on the back side of the throttle plate with a grinding stone on the end of a Dremel tool, and drilled a pilot hole for my smallest easy out and extracted it. The carb rebuild kit comes with new screws, so no worrries.
I soaked the parts in solvent for several hours and then cleaned them. I polished the throttle linkage parts and springs with a wire wheel, steel wool, aluminium cleaner and then finished them off with metal polish. I replaced the O-ring on the throttle shaft using tape over the threads and put a bit of silicone grease on the O-ring. Then I pushed the shaft back into the throttle body. It took a couple of tries to get the throttle plate into the slot in the shaft, so be patient and don’t force it. I found assembling the throttle linkage bracket (#31) into the groove on the throttle shaft and then tightening the bracket screws to the carb body ensured the shaft would not bind. I didn’t do that the first time and it bound up as there is some laterial play in the throttle shaft. I used lock tight on the throttle plate screws to ensure they wouldn’t come loose … if they do, they go right into the engine 🙁 I also put a tiny bit of grease on the groove in the throttle shaft to keep things turning smoothly.
Next I removed the top, pulled out the slide with the jet needle and removed the metal ring (#17) holding the rubber diaphram to the top of the slide. There is a new jet needle (#3) in the rebuild kit and a new needle jet (#4). The needle vibrates and wearing the needle jet and the needle. I could see grooves in the needle.
Then, I took out all the jets from the bottom of the carb. When I reassembled the main jet, it cracked in two. It likely had a crack in it before hand. I suspect a cracked main jet didn’t help carburetion any 🙁 I have extra main jets, so no problem.
Each of the jets I removed (#7, #5, #1, #2, #3, #10) was very dirty. Here is the crud that was inside the needle jet (#3) and the atomizer (#2).
I had read on Snowbum’s posting that Berryman B-12 Chemtool was very good at removing crud and fuel varnish, but extremely nasty stuff. I bought some and wore my nitrile gloves when handling it. When I fished the parts out of the container, the nitrile started to wrinkle … the next day, the finger parts had completely dissolved … nasty indeed.
After all the internal jets were cleaned, I put new O-rings on them again using the masking tape trick and “C” shaped steel pick to pull them into the groove on the jets. I broke one O-ring, but my handy Ace hardware had a metric replacement :-). I put a tiny bit of antiseize on the jet threads as they are brass and screw into pot metal threads. I installed the new needle jet (#3) and atomizer (#2). The Bing exploded view diagram and the picture I took helped me make sure these went together in the right order. I installed the new float (#40), float needle (#4) and float hinge pin (#1 from the rebuild kit and then put in the new cork gasket (#46) into the groove in the bottom of the float bowl.
I reassembled the slide (#13) with the new diaphram (#16) from the rebuild kit putting a dab of antiseize on the screws. I added the internal spring (#22) which was not part of the original carb assemble. Clem supplied these and said adding them would improve gas mileage.
Then, I put the top (#20) back on and put a tiny amount of antiseize on the four screws (#21) that secure it to the carb body.
Now for the Good part. Here are “before” and “after” pictures showing the rebuilt right side carb and the yet to be rebuilt left side carb. Definitely night and day.
I did it all one more time on the left carb. Both were in need of attention, so I am very hopeful they will operate much better and the bike run a lot smoother.