Restoring a 1973 BMW R75/5 Motorcycle

December 29, 2006

Badda Bing!

Filed under: 1973, airhead, BMW, motorcycle, R75/5, restore — Penforhire @ 11:13 pm

Okay, so Tony Soprano’s strip club might be more fun but I’m stuck with Bing constant velocity (CV) carburetors instead. I figure they’re a couple of the most obvious parts to pry off and, um, evaluate.

So here’s the right side carb just after I popped it off and removed some of the screws on the cover. Haynes’ manual sort of sucks but Clymers’ is better. It has more pictures of the various parts to wrench on. Neither of them prevents a certain amount of head-scratching, “oh, so THAT’s how the throttle cable comes out of there!” It also took a manual impact wrench, love that hammer, to loosen the cover screws. Not a good sign. Click on the thumbnail for a larger image (that’s how I’m setting up all the images from here on).


I know, I should not be too surprised yet. Here is one of the hose clamps on the intake to this carb. Notice the lovely patina of rust. What happened to stainless steel clamps? Doesn’t that sound like a good idea?


Well, after a certain amount of effort and a small blood sacrifice later, these are the guts of the horrible little monster (see pic below). May make small children and mechanical idiots cry. There are far more parts than I can see any reason for their existence. There are three or four o-rings on parts but all of them feel hard as stone now. Oh well, I anticipated needing a “rebuild” kit with gaskets and o-rings. Hey, the choke’s gasket looks pristine. So how come the repair manual says to replace it? Probably got compressed but… don’t be thinking that much dummy, just follow instructions!


Many of these odd parts are gummed up or otherwise nasty (but not the white floats on the right side of the pic, yay!). Here’s where I cry on your shoulder about how you can’t find good carb cleaner. Once upon a time you could buy a wonderful chemical cocktail called Chemtool B12. I think they discovered it caused two-headed babies. So now-a-days I seem to be limited to stuff like Gunk brand cleaning fluid. I dipped the main carb body in that stuff for a few hours and the gooey nasty stuff mostly laughs at that, thinks it is going to the spa maybe.

Not all the parts dropped out during disassembly like they are supposed to either. I had to scrape away goo and coax various bits. Here’s my first casualty (see pic below). This is the main jet holder, if I navigated the repair manual’s drawings correctly. It did not want to unscrew regardless of the amazing force I brought to bear on it. It ignored my box-end wrench. It didn’t care that I soaked it in “Liquid Wrench.” It certainly didn’t move when I pounded it with a 10 mm socket on the end of my impact wrench. I tried icing it while heating the carb body. No go. I started to get, uh, enthusiastic with the impact wrench and that’s when I noticed the six-sided nut-shape starting to round off. Uh oh! Time to slow down and drink a beer. I got on the phone with Joe, my lifeline.


Joe figured out that one of my problems was my “12-point” wrench and socket. Some of you are slapping your foreheads at my ignorance but, excuse me, it seems that 12 point tools do not distribute loads across nuts as well as 6-point tools. Huh. I’d heard some vague thought like that before but filed it away in the mostly useless mental file. I mean, my 12 point tools got me where I am today. Nearly FUBAR. Anyway, Joe came over and rescued the bent critter from me, took it home, waved his 6-point wrench at it, and brought me back the separated parts.

I went to Sears and got a set of 6-point wrenchs today. Shhhh. Don’t tell my wife. Can you imagine trying to explain why I needed to buy something that does exactly what something else I already own does?

Stay tuned. I can’t promise any regularity to my updates. I’m on Xmas vacation this week so my progress is phenomenal now. The tortoise returns in January. I will leave you a parting scary image. The photo below shows the corrosion or crud in the engine intake AFTER the carb. Hmmm, the engine must be yummy!



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