Restoring a 1973 BMW R75/5 Motorcycle

September 2, 2007

Butterflies are free

Filed under: /5, 1973, airhead, BMW, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5, SWB — Penforhire @ 9:52 am

Or as Brad suggested, “Dr. Leaky, I presume?” At the end of this holiday weekend we complete our long journey. Labor Day, how appropriate? Huzzah for the Knights of Labor!

Have you ever had the experience of wrenching on something you worked on earlier and, seeing incorrect assembly, thinking you couldn’t possibly have been THAT inept? I get that a lot. Like “what blind brain-damaged one-armed monkey assembled this?” Oh, right. Me.

You may recall carb rebuilds were the very first thing I did, even before I learned how to post full-size photos here. I always understood the basic principles of how a carburetor works but the gap between basic principles and understanding specific functions of each carb part is huge.

There is still an “official” Bing agency in the US ( I need to order their inexpensive CV tuning guide since everyone recommends it but I’m far from fine tuning yet. Oh yeah, they offer carb rebuild service — $180 for one, $322 for both, not including any special restoration like sandblasting. So they’re talking about installing the $50 twin rebuild kit (which is overpriced to start with for some o-rings, gaskets, and diaphragms) for $322? What a ripoff! And, of course, you can buy a new Bing CV carb for $465. Ouch! Rocky Point Cycle sells a Mikuni carb retrofit kit for $375 (for the pair!). Many 5-owners swear by it. Funny because without even mentioning this to Brad he mentioned dirt racers also swap in Mikunis for Bings with some regularity. They give better acceleration and throttle response at the price of reduced gas mileage. But I want the stock Bing look and I’m not ready to be beaten by my carbs!

So I’m determined to fix whatever ails my old carbs. I took off the left carb to work on it properly on the bench. I even have, as a guide, Joe’s pair of assembled carbs that he says are in working order. The very first thing I see is something I was expecting. I did not install the butterfly correctly. There is a chamfered edge that allows it to seal tightly against the carburetor. I installed it backwards and left a gap to the carb walls.


This may or may not be my only problem with idle. According to the archives at 5 United it is sufficient to cause impossible tuning issues. All better now —


I continued to tear the carb completely down. Hmm, the internal choke assembly was 180-degrees reversed (compared to Joe’s). Smooth! No idea if that affects idle but it’d sure affect the enrichener. Some yellowish bits of something like plastic are in the main carb body —


I had soaked the body in carb cleaner back when but now I attacked it with spray cleaner. Wear goggles if you do this because the spray comes out of the carb in unexpected ways. Just because all current sprays are reformulated to be environmentally safer doesn’t mean they won’t harm you. The ingredients mention acetone, alcohol, toluene, and methyl ethyl ketone (MEK). Stay away from sparks or open flames too!

The diaphragm-driven slide (moves main jet needle up and down) felt reasonably smooth but I wasn’t taking any chances. I could feel where slight corrosion might hang up the larger base where it slides against the carb body. I sanded the carb and the needle base with a series of sandpaper grits — 1000, 1500, and 2000. Then I used the 2000 grit on the smaller polished slide-tube and the female tube of the carb top. When I was done the assembled slide felt more free to move.

I took some of that fine sandpaper and worked on the idle air mix screw. It has a pointly metal tip and I noticed some slight corrosion on it.

I might end up replacing the main jet needle but I cleaned it up and reused it. The archive knowledge says these are wear items that should be replaced between 30K and 50K miles. The symptom is worse gas mileage. I should get somewhere north of 40 MPG and possibly over 50 MPG! There are four needle adjust positions in the slide. If it sticks out more, or “drops,” it stays more in the way of gas flow and leans the mixture. The book position is the 3rd notch or 38.5 mm sticking out from the slide base (best way to check since the notches are hidden).

If you ride for days at high altitude, some say > 5,000 feet, you want to drop the needle one notch (2 mm) because thinner air richens the mixture. Even though air volume is constant in a CV carb the oxygen content drops. I think I could do this in less than 15 minutes now (you only need take the tops off the carbs).

I had to replace a few more o-rings I damaged. They just don’t stand up well to removal and installation stress. I think I figured a way to improve that situation with a light coating of synthetic grease on them prior to installation.

On the right side carb, a miracle, the butterfly was correctly installed. However the choke guts were 180-degrees wrong and the main jet needle was off. I’m glad I stumbled across that 38.5 mm dimension because if you are fumble-fingered, like me, the hidden-notch system for setting the needle lacks surety. But whip out a caliper and you’ll know beyond any doubt which 2 mm increment you’re snapped into. I moved the float position slightly (to eyeball-level engagement with the carb body, like the book says). I didn’t spot anything else wrong on the right side but I polished up the slide areas.

Of course I had to wrestle with those intake elbows again.


I did not know the left and right are different. Fear not, it is truly impossible to mount them backwards. I know because I tried. The angle is significantly different because the two cylinders are offset an inch or two even though the air filter connections are dead even. My right side elbow fits reasonably well but the left side elbow seats on the air filter connection at a little too much angle, as if the engine or air box is not sitting in exactly the right spot in the frame. When I was done I sprayed every junction with carb cleaner while idling and did not detect any change in RPM.

Snowbum is adamant that you have to warm the bike up with a real ride, say ten miles, before any carb sync will be accurate. Nevertheless we have to start somewhere. I warmed it up in the garage with a powerful fan running air across the cylinders. Hey, it seemed to start easier and the chokes do something now! I first played by ear, adjusting the idle air mix for maximum RPM and getting the throttle stops to idle near 1000 RPM. Gee, starts even easier! Then I whipped out my Morgan Carbtune again and this time it worked. Note it has to be flipped upside down , as printed in their instructions, to read the low vacuum (they say around 8 cm Hg) of a BMW boxer engine. I was pretty close to max idle mix vacuum on each cylinder by ear. After a couple of adjustments I then balanced the left and right throttle stop screws for equal vacuum while idling near 1000 RPM. Those weren’t quite as close.

From all my reading it seems best to idle a little high rather than low because the oil pump doesn’t generate enough pressure at or below 800 RPM. It might be my imagination but I could swear the mirrors don’t shake as much now that I got an initial sync. The throttle cables are roughly synced, same 2 mm slack in the cable, measured by tugging on the cable fitting right above the carb.

Shall we? Let’s.

I rode it around my block once more to make sure everything was still traffic worthy. Yep. Clutch engagement still needs coarse adjustment. Front brake is grabbing better though the rear is still too soft (pedal engages a bit low but any higher and the brake drags). Off to the gas station for a full tank of petrol. Feels a little spooky moving around in traffic. I’m being gentle and avoiding high revs so I feel a little crippled and defensive. 3rd gear runs smoothly. I ride a larger loop on my way home, barely hit 4th gear just to check it, and when I stop I notice the idle has risen to 2000 RPM. I’ve gone a total of five miles and I pull in to the garage to immediately check carb sync again. It did shift a little, now that it is properly warm. The right carb throttle stop is a bit sensitive. A quarter-turn near proper idle makes a 500+ RPM difference.

I’m a little concerned about the left side idle air mixture screw. The maximum vacuum point is not as sharply defined as it is on the right carb. Makes me think I have a leak, maybe around the needle’s o-ring? I’ll have to check it again later. But it does have an effect if I screw it too far in or out (loses more RPM than actual vacuum change).

Let me say a few things about riding this machine. After only that short jaunt it stands in stark contrast to my ’05 Yamaha FJR. I guess modern motorcycles snuck up on me. I’ve had seven different bikes over the years, each usually newer and more powerful than the last (excluding a short-lived move to a vintage Honda CX500 turbo). On the /5 there is a tremendous sense of riding something mechanical. There are all sorts of noises and vibrations. The control responses are ponderous. My FJR’s silky smooth controls, engine, suspension, and wind protection make the /5 seem agricultural by comparison. That sense is probably what the Harley community talks about and desires. I’ll admit it has a certain charm but I can see how my FJR makes the ride easier, much more of a magic carpet than a tractor. I hope Joe can wait for me at turns if we take the /5! I’d say the /5 will be easier for commute duty but even there the FJR’s ABS brakes can be a life-saver. I can see how I need to flip a mental switch to ride the /5 safely, increasing my space cushion and dealing with slower reaction speed.

I know how the rider is such a big part of the equation. And I know I’m a mediocre rider (took MSF Advanced Rider training but zero track time). But if you put Wayne Rainey or Mike Hailwood on my /5 (before Wayne crashed out or Mike died in a traffic accident, wise guy) and put me on my FJR I say I can dust them around the Isle of Man or old Nurburgring.

Okay so I’m staring at the beast in the garage and I notice a small drip of oil. Something I noticed last time I parked it after going around the block on it.


Time to investigate. As near as I can tell, this is driveshaft oil dripping from the rubber boot at the tranny. Here’s a view from under the bike.


You can maybe see the oil weeping under the rear hose clamp. Some genius, guess who, set that clamp such that its screw cannot be accessed while the battery is in place. After pulling the battery and loosening to rotate it for future access, I tightened it as much as possible (clamp ends touch) and we’ll see if the oiling diminishes. Swingarm oil level is hard to measure accurately. You just stick a post or screw driver in there and look at the depth of oil on it, give or take a mm.

I think I have another oil leak, one tiny drip after running, weeping at one of the studs at the top of the tranny (right at one nut).


This is only a drop or two a day when run and there is not even a hint of the oil level dropping in the tranny. That nut is as tight as can be and I never pulled the rear cover off the tranny. It might just be the same drive shaft oil leak flying around. We’ll just keep an eye on this one.

Hmm, now that I think I’ve got some more stars in alignment how about a longer ride? Let’s go down to my closest dealer, Irv Seaver BMW. We’re in the middle of a heat wave so I put my mesh gear on and off we go. I’m taking surface streets to vary my RPM and keep it under 4K. First thing I notice is my idle RPM is way up again after ten miles or so. Grrr. Second thing I notice is my clutch engagement is shifting a little toward the end of lever travel! I end up taking it real easy at the lights because the clutch spun up twice when I tried to accelerate hard from a stop (that sure gets your attention in traffic!). The hand lever adjustment is a fine tuning, not coarse enough. I was sweating that one because while I have the BMW toolkit with me I’m not dead sure I had the necessary 10 & 14 mm wrenches to make a clutch adjustment (later confirmed they’re in there). Front brake is working great but the rear is still only a suggestion of a brake.

I’m glad I’m wearing full boots because I feel annoying heat at both ankles from the engine (while ambient is nearing 100 degrees). I’m thinking this will be more fun in cooler weather. Total heat is slightly less than my FJR but it moved from my crotch to my ankles, an extra improvement!

By the time I get to Irv’s my right hand is telling me a Throttle Rocker is a great idea so I buy one. I dislike the new velcro style (Cramp Buster has the molded one-piece patent) but it works. I roll around on the ground to do a blind adjustment of the throttle stops to lower idle a bit. On the way home I do a little highway time. Los Angeles area traffic is thick enough to guarantee varied RPM even on a highway. I get up to 75 MPH indicated and then back down for a short cruise at 65 MPH (just under 4K RPM in 4th gear). I can feel the engine has plenty more to give, I’m nowhere near full throttle, and I believe this bike will do the ton. Just not with me at the helm! Carburation between 2K and 4K RPM is great. Quite responsive. The front end feels fine, not a hint of wobble at any speed or change in speed so far. The front suspension & tire combo doesn’t like rain grooves in the direction of travel, dancing much more than I like, but nothing hazardous. Chalk it up to feeling the road nicely.

When I pull into the garage I’ve gone 35 miles or so. And I’m dripping a new fluid! I was careful to examine the ground when I stopped at Irv’s and nothing was leaking there. I’m pretty sure it is fork oil from the left leg based on its low viscosity along with where I’m getting fluid. I think it is squirting out under fork compression but I haven’t seen it in action yet. Nothing dripping while it is on the centerstand. It got on the left side of the front fender, a little on the left sidewall of the front tire (scary!), and a light spray on the ground where I brake to make a 3-point turnaround in the driveway to back into the garage. The top and bottom of both forks seem dry so I’m thinking it must be the shaft seal under the gaitor. That’s a new seal (see photos in a much earlier post). I hope it just popped out or ?

While the bike is still hot I immediately hook up the Carbtune and re-balance the carbs. The right side idle is more sensitive than the left but I get everything looking right and near 1K idle. The air mix screws needed no changes, only the throttle stops. When the idle was racing I double-checked the throttle cables for slack and they are fine. I let the bike, and me, cool down before I start wrenching elsewhere.

I lift the right fork boot and, what-do-you-know, some fork oil pours out and I see this.


Is my mechanic’s intuition working or what? From here it looks like part of the seal was torn off and the built-in compression spring is tweaked in a few spots. You know, just an hour ago I was at Irv Seaver BMW. *Sigh*. I call to make sure they have this seal in stock & to set it aside for me, jump in the car, and drive. Hmm, temperature is a few degrees over 100 now. All this for a $5.75 ($6.20 with tax) part! I joke with the parts guy that I’d pay more if he put it in for me. He says he’d charge a whole lot more for it.

After I drain the oil, remove the front wheel, remove the fork brace, and disconnect the fender I remove the lower fork leg.


Hmm, it didn’t exactly eat the seal. It folded over at that point. Strange. I check the upper fork leg and it feels smooth. I’m thinking maybe the bike knows I close to the end of major wrenching and doesn’t like it.

Here’s the oil I recovered.


That looks like virgin oil. No darkening or bits of anything in it. I recovered 230 ml so that means the seal blew out 50 ml. I reuse the recovered oil and load 280 total back through the top of the fork. Did you know that oil fill cap is hard to access while the handlebar is mounted?

I want to check all my work and try another ride but it is so hot I’m wilted. I sit around in my air conditioning for a few hours, contributing to the possibility of rolling blackouts because our power companies can’t plan for shit but they sure know how to rake in the money. And I eventually suck it up enough to go for another ride in the oven. Hey the full-choke start worked this time but I had to immediately (started to die) go to half-choke for a few seconds and then hold throttle against no choke (engine sputtering again) for a minute to warm it up. I go about seven miles this time. As I pull in the driveway I don’t see any fluids leaking. That’s a first. Hoo-rah! Clutch adjustment is just right. Throttle springs are still too strong, gotta give the throttle a he-man twist. Can’t wait to finish break-in because the engine is just coming alive at 4K RPM. Idle RPM after warm-up is still pissing me off because it is high. I attack the throttle stop screws again but they sure are sensitive.

At this point I estimate I have somewhere around 375 hours of labor in this project, which started in December 2006 and ran to now, the end of August 2007. I did not keep as faithful tracking of my time as of the money. I never worked on it less than 10 hours per week (every weekend, some nights) and often exceeded 16 hours. A more competent wrench could probably cut that time in half but he’d charge way more than me. It was funny how I had all the patience in the world for eight months but when I caught a whiff of the end I fell into a frenzied effort for about a month to get ‘er done. I was ready to bust if I couldn’t get on it and ride.

So what else is left to do?

The oil pressure lamp still never lights. I need to double-check but I think the switch is grounding properly with the motor off so it is in the harness or the bulb. If I go into the headlight bucket to look I should also add a hose-clamp strap around Stan’s ignition key cylinder to be 100% sure it will never rotate.

I still need to replace the horn. I’m thinking to wait until I have to take the tank off for something else. Not only would horn access be easier but I have a hankering to go for a much larger horn like the Stebel Nautilus Compact (134 dB! 18+ Amp draw!) and I’ll have to run a new relay direct to the battery, triggered by the OEM horn switch.

I want to increase the wire gage of the accessory power line I added. Right now it has a 3 Amp rating, enough for my Battery Tender to charge but not really enough for a Gerbings electric liner & gloves.

I have to finish break-in (say 900 miles), change all fluids, and perform another valve adjust.

There are a raft of other items like new shiny yellow steering head reflectors, Hepco & Becker saddle bags, and some Rok Strap bungee cords. But I don’t consider those actual fix-it tasks.

I need to find the perfect location to take some high quality photographs. I have it in my mind to ride to the San Joaquin Valley National Cemetery in Gustine CA where my dad is buried but that’s a seriously long ride, about seven hours each way, and I have to be uber-confident in the Blue Baron first. Here are a couple of documentary images. The link below each image will open a big JPEG file ready to run through a typical ink jet printer at 10 inches wide (rotate them or print in Landscape mode). They are around 1 MB in size so if you’re on a slow connection you have been warned. You may use these images in any non-commercial application as long as you attribute them to this blog.





You guys are welcome to stick around but I expect to taper off my postings unless something blows up in the next week or two. I will leave this blog as a /5 resource for as long as WordPress will host it. Even with anti-spam software I have to constantly weed junk-comments here so the weeds may linger longer.

Now that I’m feeling my oats I’m considering a go at restoring my father-in-law’s early 1960’s Jaguar E-Type (XK-E), a series one and possibly one of the earliest “flat floors.” I need to dig it out to evaluate it. He bought it new but it sat in his garage for decades after a crash. I’m sort of doubting I’ll have the energy to run a similar blog on it and the economics make it a multi-year proposition… or “foolish” to summarize. Too much hubris. But you never know. The photos and text in this blog were helpful to have for my own assembly. But it adds hours each week to do. I’ll leave you with some old English car jokes, sure to warm the hearts of BMW airhead enthusiasts —

Why do the English like warm beer? Because Lucas made their fridges.
Lucas denies having invented darkness but they still lay claim to sudden unexpected darkness.
The Lucas motto? “Get home before dark.”

Thanks for coming along on the ride!
Eric Arnold



  1. Wow, what a cool restoration. Thanks for the details and taking the time to put this on the web. As someone who enjoys time in the garage, your honesty about mistakes is refreshing…been there, done that. Not only is the restoration great, but the sentimental value of the bike makes it even more enjoyable. Thanks again. :=)

    Comment by photomd (from stn) — September 21, 2007 @ 5:36 pm

  2. This is a great story… not only becuase it has an emotional/
    chronological conection, but because you use photos with commentary at each little step. Most resortation tales fall apart because they have no progression; rather they jump from a pile of parts to completion with no intermediat content. This was worth all the effort to the readers-thanks.

    Comment by Corey Adelman — October 9, 2007 @ 8:33 am

  3. Thanks guys. I know why you don’t get this in every resto tale. the effort to do all this is a big deal. I had grandiose thoughts to video tape some specific work and You-Tube some segments because I know video would have helped me.

    Sh’yea right! Doing up all the still shots is hard enough.

    Comment by penforhire — October 9, 2007 @ 10:11 am

  4. In gasoline engines used new mechanism for gas distribution to changing phases of intake. This has increased their power and torque significantly raisedin the area of low and medium speed…

    Comment by Smit — December 1, 2007 @ 6:05 pm

  5. Well done! Truely an inspriation for my own restoration project of my father’s 1973 R75/5 LWB.

    Comment by David — January 9, 2008 @ 1:55 pm

  6. Hello & best wishes from the UK. I have really enjoted reading your /5 rennovation notes, as I am also in the process of a similar, though less extensive, job of my own R75/5. I’m sure others will have mentiond this to you, but just in case, it would appear from your photograph that the leaking and damaged fork seal pictured is fitted upside down! The flat side should face upward and the grooved side down. Fork oil forced into this groove in the seal exerts sideway pressure on the slider and fork leg, which helps to keep the seal in place and oil-tight… best to you. MP

    Comment by Mark Peel — June 18, 2008 @ 12:56 am

  7. Hah! Good catch but where were you when I was doing the work?! If you keep reading you’ll see in my next post that you were 100% correct (as was BMW Joe who first told me).

    Comment by penforhire — June 18, 2008 @ 12:26 pm

  8. Hi. great story. I’m working on a 71 r75/5 right now and am interested in replacing the old carb tubes, between the air box and the carb. would you know where I could get some good replacements?

    Comment by Andrew — October 10, 2008 @ 6:26 pm

  9. Hi Andrew. If I recall correctly I got a replacement for a cracked tube from Bob’s BMW but it also available from most parts suppliers.

    That is one the hardest things to reassemble on this bike! The best way for me was to loosen the rest of the carb-to-cylinder connections and sort of insert both ends of this elbow simultaneously. It is an awfully snug fit and I have no idea why BMW didn’t use a soft elbow here!

    Comment by Penforhire — October 11, 2008 @ 2:41 pm

  10. On those fork seals. They were put in upside down. This is why the lip turned in when placing the tube into the lower leg.
    The spring ALWAYS goes to the side of the oil.
    You are right about mistakes, I’ve made most of the mistakes you have made except for the seals.
    Now I’m restoring a ’71 R60/5


    Comment by "Beemer" George — March 26, 2009 @ 9:08 pm

  11. Before you call something a ripoff over the internet you might want to make sure that you have your facts straight. First off, the rebuild that Bing does for the $322 price includes: all gaskets, o rings, throttle shaft screws, diaphrams, float needles, jet needles, needles jets, pivot pins and floats. They soak the carbs in Saftey Kleen and then run them through and ultra sonic cleaner to make sure that all the cleaner is removed out. That also includes the price of shipping back to you. All of the parts they get are from Germany and with the current exchange rate those prices are quite resonable in my book. I’ll never understand why people spout thier mouths off like they have a grasp on something when, in all reality they dont have the first clue! Oh ya nice bike.

    Comment by John — February 7, 2012 @ 2:01 pm

    • Given that I also posted the prices for NEW Bing and a conversion to NEW Mikuni carbs (which get the job done just fine) I stand by my statement that the refurb price is excessive. You make it sound like Bing’s parts are made of German unobtanium and magical elves do the work. Y’all can make up your own minds on their value.

      edit – Just to add, this blog isn’t a democracy so please post any continued intangible whines about MY whine on YOUR popular site. This here is my soapbox.

      Comment by Penforhire — February 7, 2012 @ 2:50 pm

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