Restoring a 1973 BMW R75/5 Motorcycle

November 2, 2007

B’bye Bing!

Filed under: /5, 1973, airhead, BMW, BMW motorcycle, motorcycle restoration, R75, R75/5 — Penforhire @ 3:31 pm

Yes, beaten into submission by my carbs. If you don’t already know, my new insane vehicular project can be found at and that is one reason I haven’t been “here.”

You probably thought that over the weeks since my last post I’ve been sailing all over Southern California on my blue dream machine. You would be wrong. The next weekend after I thought I was road-worthy Joe and I had a club ride planned. I was all excited to take the R75/5 on its first serious road trip. Come the morning of the ride it would not start for nothing! I ran the battery down, trying all variations of choke/no-choke/throttle/no throttle because I could not believe it wasn’t just about to fire up.

I dumped the fuel in the float bowls. No help. Do I have spark? Yep, pulled a boot, inserted a loose plug, and watched/heard it arc as I cranked (you have to provide a normal spark gap or a short to prevent damaging the Boyer electronic ignition). I pulled the plugs and they looked okay, a bit black-sooty but nothing that didn’t wipe off. Double-checked the gaps. The biggest hint I got was not much smell of gasoline when I pulled the plugs and they were not wet. After cranking that long the plugs should have gotten wet, if not flooded.

Next I messed with both carbs, pulling the jets with the bodies left on the bike. Blew some carb cleaner where I could spray. That didn’t get me started. Hmm, pulled the valve covers and did another complete valve adjustment. They had moved a little since my last check but nothing radical. On a warm afternoon I did get the bike to start again, though it still took some cranking to do it. At this point I double checked the timing. Nope. Perfect. I did away with the points so no gap to check there.

I was SO disappointed as I was forced to mount up on the FJR for the ride. If I recall correctly it was a HOG of Glendale club poker run benefit for Guide Dogs of America. Not the big annual run but just a side run. Still had hundreds of riders show up, final count was something like 400! It ended at the Route 66 Grill in Canyon Country. Those Harley groups know how to party! They had a live band, slow ride contest, beer stein holding contest and more. No pictures. I was too bummed. The R75/5 would have stood out in that ocean of chrome. Yes, Joe won stuff… again. In addition to many raffle prizes (I won some crap) they also had a jar of candy corn and a jar of something else and you had to guess how many in each. Joe guessed closest on both. There had to be two hundred other entries! Can you believe it?

Regarding my R75/5, the only thing I can think of is the Bings have a reputation for difficult cold starting and that morning was the earliest (coldest) attempt I made to date. All my confidence in this machine escaped like air from a balloon. How could I travel anywhere except from-and-to my garage if I was worried I’d run the battery down trying to start it later? Grrrr.

Okay. I was faced with several choices. I could replace some parts in my Bings, maybe even send them out for service since I had several whacks at doing it myself. Or I could replace them with a Mikuni carb conversion that several 5 United members spoke highly of, sold by Stan at Rocky Point Cycle (see link in my big list at upper right). Brad, an old dirt biker, even suggested Mikunis since those were a common swap for Bings back when.

I know in a prior post I said I would not be beaten by my carbs. I was mistaken. I am utterly vanquished by my Bings. But I got even. They’re stored in a box now.

The Mikuni carb conversion cost $375. That is less than just new parts required to attempt to fix my Bings. And owner history on Bings is spotty. Sometimes they work and sometimes they never work. It has to be some manufacturing tolerance issue because it is so widely reported. Not one person complained the Mikunis did not work. They are reported to improve throttle response, ultimate power, and reduce gas mileage. Hey now, THAT’s a tradeoff I can live with! They don’t look perfectly stock but I took photos. Decide for yourself.

Here is the kit from Stan. I broke down and got it this week, taking vacation Friday to get a jump on installation.


It includes two Mikuni VM series carbs (pre-jetted for your application), extra jets (to tune if needed), rubber intake adapters, hose clamps, a length of fuel hose, and custom throttle cables. Very complete! You have to discuss your order with Stan so he can pre-jet correctly and send you the right type of throttle cable. He sells a choke cable conversion also, another $40 IIRC, but I decided to try as is (separate choke levers on each carb) since I can always add it later. Stan developed this conversion to use on his personal motorcycle, a 1971 R75/5, so he has high confidence in the R75 application.

The carbs come stickered “Not for Aircraft Use.” There must be some tiny airplane engines out there if someone might consider using one of these on it? Stan includes a one page instruction sheet and a copy sheet of an exploded carb view but to be honest the instructions are pretty weak. I suppose you could say that if you cannot figure these out you should not be installing ’em. But I struggled slightly anyway.

The first conundrum was “where do the throttle cables go?” The Bings have an external lever that rotates a butterfly. The Mikunis have no butterfly. The throttle cable attaches directly to the main jet’s needle piston through the lid. Here are some views of the carb with the lid off and the guts.



There is also no diaphragm that pulls up on the slide like the Bings, just one big return spring that does double-duty forcing the carb back to idle and also providing the throttle control return force. The float bowl is screwed on, presumably needing to be removed less frequently than the Bing bowl? There are three small brass hose barbs and one big one. The big one was obviously fuel input but what about the other three? Well, one is the carb overflow, something flush-mounted on the Bing body. The other two are vents above the bowl. I’m not sure why two vents are needed but, as confirmed by Brad, don’t block those vents!

Now you know we can’t get through a retrofit like this without monkey-boy showing up somewhere, right? Well, my examination of the carbs led me to think they were two of the same model and lay-out, meaning no left and no right, unlike the Bings. That is not entirely true. Yes most of the body, and for instance the choke lever, is the same on both carbs. But I failed to notice that the idle adjust screw location varied from carb-to-carb. I ended up mounting them with the adjust screws pointed toward the body of the bike, harder to adjust on a hot bike. It was not a complete disaster though because the air mixture screws did NOT vary, meaning one was always going to be harder to access no matter how I mounted them. Strange.

Here is one on the right side of the bike —


Looks okay to me. My only real complaint about the set-up is the custom throttle cable’s length adjusters (to set the 1 mm or so throttle slack on each side) are positioned on the cables so they are hidden under the tank, almost right at my rubber cable loom. It would make more sense to have them exposed on the run of cable between tank and carb.

Yeah, it was annoying to mount up the carbs again. Same twist-dance getting everything on at once. I had a problem with gas leaking at my in-line filters. They were working fine before but now they required tiny hose clamps. Oh well, that’s a hazard of using 1/4″ filters in 7 mm fuel line. One trip to the auto parts store and everything was set right. No new puddles of gas on the floor.

The moment of truth? Rrr, Rrr, Vroom! Started right up. Immediately flipped off the chokes and it idled fine at a slightly open throttle setting. I tweaked idle adjustments so cold idle wouldn’t stall and went for a warm-up ride. It ran just fine up to redline and idle rose after it warmed up. Got it back in the garage, backed off the idle, and tweaked the air/fuel screw on each side for max RPM, keeping idle near 1K RPM. When I was done I took Stan’s advice and turned the screws in a quarter turn (slightly richer) for year-round safe riding. I shut everything off and came in to write up the experience. Took me longer than I thought it would. Call it six hours of work.

I may need to tweak them some more and if I want to balance ’em better than by-ear I’ll have to drill ports for my Carbtune but it seems unlikely I will need to.

So if you see a broken down blue R75/5 on the side of the road somewhere in So Cal, that’s probably me. Happy trails!


September 16, 2007

Bags make it complete

Filed under: 1973, airhead, BMW, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5 — Penforhire @ 2:20 pm

So I’m out riding around still doing the whack-a-mole game with mechanical gremlins and I find myself on a major freeway (that’s toll-free highway to you who don’t live in CA). I’m humming along just fine around 65 MPH and I’m starting to relax and take in the scenery. All of a sudden I feel a small thump on my right shin. I look down but I don’t see anything. And everything is still humming along without a glitch. That means nothing off my carb, right? Hmm, large juicy bug? Nope, when I pull over later —


Yep, gorilla snot failure. Well, technically not snot failure. The adhesive is gripping the tank’s paint so fiercely that if I pull on it bits of paint come off with it. I know why this happened. In order to restore my old sun-faded rubber tank panels I soaked them in “Formula 2001” or some similar plastic dressing for months. I tried to protect the backside but I’m sure I failed. So some of this dressing was either on the surface or worked up from inside the rubber and ruined adhesion. I have no idea where, exactly, I lost that panel or I’d go hunting along the freeway. Fortunately these are only $20-something parts and a replacement is on the way from Bob’s.

I’ve only had one really strange thing happen so far. I had fully shifted into second gear after turning a corner on the street. I was then accelerating when the tranny slipped completely into neutral. I don’t know if it was a “false” position or if the neutral indicator was lit. I was too busy throttling down and deciding what to do in traffic with no go. I upshifted and I think I got 3rd gear though I would not bet a large sum on it. Could have been 2nd, I was a touch rattled. But after that shift all was fine. Very strange.

Now that I’m thinking of travelling further on this machine I’m glad my bags arrived. These are semi-replicas of the old Krauser bags, produced today by Hepco & Becker.



They fit onto my stock BMW frames with just a little extra grunting. The right side bag is very tight to the rear turn signal. My right rack loop seems to be shifted to the rear or bent back an 1/4″ or so. This is only significant because the seat will not open completely with the bag in place. I just have to plan on gentle prying each time I want to remove the bag. I say these are semi-replicas because they are better than the originals. They are a little deeper, supposedly will accomodate a full-face helmet, and the latches are improved. There is probably no need to wrap a bungee around them to prevent ’em from flying open on the road.

I had them up to 75 MPH with one bag loaded with about 7 lbs of stuff and no new instability detected. I am very pleased with them. I am the “be prepared” type and that means I feel more secure hauling around a bag or more of crap like tire repair kit, 12 V inflator, flashlight, fuses, full-size tools, first aid kit, GPS, electric jacket liner, water, protein bars, and the like. I know, laugh at me. But I’m usually the guy who pulls out the widget needed to keep me or my riding buddies going.

I added one more long distance touch. I showed you the Throttle Rocker before. While I really like that device, the stock BMW grip length is such that it interferes more with manipulating the front brake than on my FJR. Not only is the FJR grip much longer but I only need a couple of fingers to stop the FJR. I need a full-hand grip to stop the R75. Well, many years of BMW airheads come with a sort of cruise control, a screw with a nylon insert that applies friction to throttle rotation. It is most useful for tune ups or cold warm-up in the garage because it is difficult to manipulate on the move. I stumbled across the perfect solution while searching the 5 United board for distance-riding complaints, the Schneider Flip-a-Lever (see ). It screws in place of the stock set-screw and provides a toggled friction lock, very similar to the Vista Cruise, NEP lock, and similar friction-based cruise controls.


I’m holding the original set-screw for comparison. Here is the lever cammed over into a “lock” position.


The friction applied can be easily overcome with twist of the wrist. There is enough cam that there is zero drag in the “off” position.

I have one more improvement on the way, reduced-strength throttle springs advertised in Airmail (Airhead Beemers Club newsletter). This fellow claims 25-35% reduction in force and for $10 I’m trying ’em. I know my throttle complaint is spring-based because if I twist to full throttle and release the grip it snaps back to idle instantly. This is not a cable or gear friction issue.

My favorite Marsee Rocket 11 liter expanding magnetic tank bag seems to fit fine on the R75. I need the map pocket for route instructions. Joe, are you ready for a Slow Ride (cue Foghat’s music)? There is a new poker run at the end of the month to benefit Guide Dogs of America put on by the Golden State Glendale H.O.G. Chapter (see ). If the hogs can ride it my little R75 can make it!

September 3, 2007


Filed under: /5, 1973, airhead, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5 — Penforhire @ 11:51 am

Joe is good. One look at the fork seal photos in my last post and he calls me up, “um, you know those seals are in backwards?” Sigh.

I whip out my Clymers repair and I can’t tell the orientation. Surprisingly, the thin Haynes repair manual is explicit is stating the spring-side goes in first. Could have something to do with that fold-over failure, eh? Monkey-boy strikes again.

Oh well, two fork seal flips coming up. Geez, I just went another 20 miles and everything was feeling right. And it sure is hot in the garage…

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

August 26, 2007

Improvise. Adapt. Overcome.

Filed under: 1973, airhead, BMW, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5, restoration — Penforhire @ 4:16 pm

And in the end, surrender.

Joe spotted a big error in my costs from the last post. The third item, wheel work for $594, is counted twice, once as that lump sum and then again spread out into the parts and labor. So that total is more like $8600. Joe says I really do need an accountant.

Since last week I retorqued the heads, gapped the valves, cleaned the carbs again, added clear in-line fuel filters to each side, discovered the horn I worked so hard to clean is kaput, and then came the big puzzle — adding the Boyer Microdigital ignition. One reason I got it started with the OEM points system is I can switch back to the stock system in maybe 30 minutes if it fails on the road. It was important to know it worked.

Here are the Boyer rotor & stator mounted in place of the points.


The stator has two coils epoxied in place 180 degrees apart. The rotor has a matching pair of magnets. When they rotate past the coils a voltage is generated. The stator plate rotates just like the stock points plate to adjust timing. The advance curve is built into the Boyer and is not adjustable. I believe it is a little slower advance than the stock curve, hitting full advance at 4K RPM where the OEM weight system hits maximum near 3K RPM.

Here is the recommended spot to mount the Boyer brain.


This is just behind the coils and in front of the tool box, strapped in place with tie wraps (provided in the kit). You can see a laminated schematic (from Prospero’s Garage) rolled up and slipped into the frame tube under the tank. Seems like a good place to keep it.

Oh, the Boyer instructions were simple enough but after I wired it up I had the strangest problem. The starter would only turn over for a second or so. I’d have to turn the ignition off and on again to get another second of cranking. It was as if the starter relay was opening. I was going nuts trying to figure it out. If I removed the Boyer’s black wire to the coils it’d turn over fine but I had no spark. If I removed the unnecessary wire from the coil to the now-unused condenser the starter wouldn’t budge. The schematic told me something was way wrong since that condenser lin is an isolated run.

Well, I could have sworn I used my ohm-meter to confirm that condenser line was what I thought it was but I’d have been wrong. Double-checking my assumptions, I found that black wire was actually the voltage sense line going to the Enduralast regulator. I had hooked it to a coil for switched power. So I moved it to an open switched-power lug on the starter relay and everything was peachy. I had as much starter as I needed and the ignition started the bike with minimum fuss. Woo-hoo!

I sent a help e-mail to Stan and I posted my trouble on the 5 United board. So then I had to fess up and tell everyone I was an idiot and how I solved the problem.

Next I grab my trusty timing light to set the timing on the Boyer. I haven’t used my timing light in over a decade and it is, um, not so trusty. No blinky-blinky. I pop it open to look at the circuitry but considering how I did with the Boyer ignition I know I’m just fooling myself. I don’t see any blown fuses or crispy black spots and it was as cheap a light as one could buy back then so I went out to buy a new trusty timing light. Blinky-blinky.

I had set the stator plate midway in its adjustment range and this turned out to be slightly retarded at idle (“S” mark). So naturally my first adjustment overshot the other way. Boy is that adjustment sensitive! I got it close to centered. I saw some odd jumping of the timing mark but that could be due to too many things. The Boyer instructions only mention checking for full advance at 4K RPM. I haven’t broken in the new engine parts yet so I only briefly took it to 4K and observed the “F” mark centered just right. I could also see it moving smoothly up and then down again as the throttle was adjusted. That’s good to go.

I tried balancing the carbs beyond the crude adjustments Joe and I did before but something is not right. I’ve got a Morgan Carbtune (non-mercury carb sticks) that I use to balance the four cylinders of my FJR. Nothing is as smooth to watch as mercury but this does the job. So I hooked ’em up to the Bing vacuum ports, normally sealed with a screw-and-fiber washer. Another odd thing happens. I get tons of vacuum on one carb and zero on the other! Since I know both carbs are working (heat in the headers & idle moves with either throttle adjust) that means the port is somehow blocked in one carb. I blow through the port and I can hear my compressed air blowing into the body of the carb. Still zero vacuum. I swap Carbtune lines just to be sure it is not the gauge. Grrr. By the way, the carb that shows vacuum maxes out my gauge at 42. I think that is supposed to represent inches of mercury but absolute values are not important when you’re balancing.

I’m also experiencing other odd carb behavior where the idle will rise to 2k or more for ten to thirty seconds, or stick there after goosing the throttle, then drop back down. I’ve got the throttle stops at lowest idle point just above dying, around 800 RPM. Lacking the vacuum port access to balance the carbs I will have to resort to the older cruder method of running the engine on one cylinder at a time, maximizing idle-screw-adjust RPM, and then setting the idle throttle stop. I am a little hesitant because of the Boyer ignition. I believe I will have to rig up a solid grounding clip for whever plug is out of the engine but I need to do some research to make sure.

It idles and revs good enough to work out some kinks on the road so I move on to the front brake which is way out of adjustment. Here is the front arm.


The arrow shows one of two sets of punch marks I added to the arm before I took everything apart. You can see I put it together just the way it came apart. But it was clear to me that the front arm angle was incorrect. There was no adjustment range and the brake shoe was already touching the drum. So I pulled it off and moved it one tooth clockwise, increasing the range of adjustment. That made it trivial to remount the brake cable. The threaded rod now fell fully through the arm with no extra effort. Some of you probably wondered about all my struggles and now I know why. But getting this arm angle right exposed my next problem.

Yes, despite clear instructions and conscious attempts to follow those instructions I somehow loaded the two brake springs backwards. Or so I thought. The symptom was the front arm moving the shoe to the drum before starting the rear arm. The rear spring is supposed to be weaker so that is the opposite of what is supposed to happen. Grrr. I call Joe to whine about it and maybe prime him for spring-assistance (again). He’s up to his hips and fully occupied by multiple toilet replacements. Hmm, since he didn’t ask for my help maybe I’m getting the better end of this deal. I’ve done toilets before. So enough whining. Time to spin wrenches and sacrifice flesh.

Off comes the front wheel. You’ve seen this part before.


The good news is, I did not load the springs backwards. The bad news is, I used two of the same spring (both the stronger spring). Yeah, these don’t look the same in the photo do they? In fact, both are 2.5 mm diameter wire. The proper rear spring, which I had hidden away in a bag, is 2.3 mm diameter. You might not think 0.2 mm would make that much difference but it sure does! I struggled for a while to get the incorrect spring off before I pulled out my go-to tool, a cut-off wheel. Not like I was ever going to use that spring again, right? And the new spring was soft enough for me to grunt it on with vice grips and some body weight.


On goes the front wheel and the brake action is now correct. As I pull the brake lever the rear arm spreads the shoes until they touch the drum and then the front arm starts spreading. Yay!

The next trick is adjusting a little cam near the front arm so the arm has just 4 mm of motion (measured at the cable) between no-brakes and full-on. It is a pre-spreader for the front brake lever. What makes this tricky in my case is the interior-hex-head adjusting bolt was already rounded off by a mechanic that must have been denser as I am (is that possible?). You must pull the brake lever to remove pressure on that cam before making ANY adjustment. Otherwise the turning pressure is too great and, well, you round off the adjuster socket. When I hold the brake lever in I can spin the adjuster cam with my bare fingers. So why is this an issue? Because the lock nut on the adjuster wants to turn it slightly as I tighten it. So I pull the hand lever. Then I move the cam so it is right there, at the contact point. If I could tighten the lock nut here I’d have zero motion of the front arm. By the time I get the lock nut tightened it rotates the cam so I have maybe 7 mm of front arm motion before full-on braking. That means total brake lever travel is more than it needs to be, most of the the way to the handle before the brakes are fully engaged.

Adjusting the rear brake arm is easy. Just spin the adjuster on the end of the brake cable until the rear arm starting position is where you want it. The front brake arm isn’t going anywhere until the rear arm pushes its end of the shoes fully against the drums. So this adjustment is independent of the front arm adjust. I tried to make up for my overly-long front arm engagement by making the rear engagement as short as possible, and snugging up the brake control adjust at the handlebar, but you really want 4 mm of rear arm motion.

Another not-so-subtle point is maximum brake power (leverage) is obtained when the two brake arms are full-on at 90-degrees to the cable. In my case this is a go. The angles are good. Duane’s article on brakes is a good one and he says if you have any error you want the angle to be slightly more than 90 degrees (arms not moving quite enough) because as the brake wears and the arms move further you will gain leverage instead of losing it.

I have an idea to cut a short length off an Allen key and epoxy that into the rounded cam adjuster (happens to be a 4 mm socket). I think if I do that correctly I’ll be able to improve my front cam adjustment. Put that on the list of things to do.

Okay, here’s the decisive moment. Time for me to take this heap around the block and see if I have a motorcycle or just expensive antique mechanical art. The front engine cover is still off, you know about the front brake, … blah, blah, blah. I’ve got enough excuses to keep this beast in the garage for at least another eight hours of work. My insurance card just arrived in the mail so screw that, we’re going for a ride!

I air up the tires to my best-guess starting pressures (38/40 PSI) for my unknown tires. Inner tubes lose air a lot faster than my tubeless tires! I put on my best “pavement luge” outfit (boots, pants, jacket, gloves, helmet) because who knows what’ll happen. I hop on and roll it off the center stand. Geez, I don’t remember this bike being so tall! I’m barely on the balls of my feet. I work the side stand to get a feel for it while the bike is still cold and I’m calm. Geez, that’s a bear to operate — your left leg presses against the (hot) cylinder to complete its forward motion! I bounce on the seat and notice my rear suspension is too stiff. Maybe it’ll break in and I’ve got too much preload also. Heh, at my size that rarely happens! Deal with it later. Start the engine and paddle out onto the driveway. Look both ways and step into first gear while grabbing the front brake for all I’m worth. I’m not getting any of the jump I remember it had, going from neutral to first. Good vibrations.

I can’t remember being so nervous getting on a bike. I slowly ride it down the street and stop next to my two-houses-over neighbor Davis, getting the first feel of the brakes. I’m hearing a box-of-rocks noise when I move. I’m thinking tranny thoughts to myself, “that can’t be good.” Davis rides a recent Road King and a real-deal Triumph chopper. His HD’s modified exhaust is merely annoying. His chopper’s open pipes make my teeth feel loose. He gets a chuckle while helping me out. He says the engine sounds great but that box-of-rocks noise is probably my center stand, which is dragging along the ground. Yes, I rolled off the stand but forgot that I have to lift it up until the spring ‘cams’ over to hold it (unlike any of my other bikes). Rookie error. Well, I’d rather have brain fade as a cause for noise than an evil tranny.

So off I go around my cul-de-sac neighborhood. I’ve got a triangular loop with almost no traffic that is only 1/3rd of a mile. Not too far to push in case it dies, right? Gosh, the front end sure dives under braking and I do have to give the brake lever a he-man grab. Rear brake could be adjusted higher but it seems okay if I step on it hard enough. Clutch engagement is right at the end of travel, I’ll need to adjust it. I do one loop in first gear with a bunch of brake checks. The engine shudders slightly off idle as I feed it gas when I engage the clutch. Not too unexpected. That’s the big symptom of unbalanced carbs. Oh yeah, the handlebar pinch bolt for the clutch lever is slightly rounded (another internal hex socket) so it is not tight enough, so go gentle on the twisting there. Second loop I lift it into second for a short stretch but I’m instantly going too fast for the confidence I have in my brakes. Back to first and back in the garage. Onto side stand then center stand.

Phew! I had no idea my first ride would feel so traumatic. Can you believe I forgot to write down the mileage when I started? Now it says 72,592. I need to know this for break-in and fluid change purposes. I still have no idea if the speedometer works. I was so focused elsewhere on my little jaunt I forgot to look!

I later got the front brakes properly adjusted. Here is the relaxed position —

And the full-on position —

Okay, with the timing also settled it is time to get that front engine cover on. You may be wondering why I griped about it repeatedly in this blog. Well I found out why I was having abnormal difficulty. I think it is the Enduralast rotor frame. It must be slightly deeper than the stock rotor because it was just barely hanging up on a casting post inside the cover.


I could get the cover locating pin inserted or the bottom of the cover seated, but not both at the same time. Eventually I wore that little shiny spot in the cover trying and failing to get it on. Here is the rotor frame bolt I think I was hitting, though there is no matching mark I could find on the bolt head or frame.


The problem was solved by a little sanding on the offending casting post of the cover.


This is not a structural point, the rib that runs through it is still intact, so the cover should not be any weaker. The cover slipped right on, at least compared to the fruitless sweaty grunting that came before.

I made a trip to the hardware store, used an extractor to get the old clutch lever pinch bolt out, and replaced it with a nice $0.89 stainless bolt (6×1.00 mm by the way). I think someone got lazy in the past because the highbeam/lowbeam handlebar switch gets in the way of properly adjusting the pinch bolt. You have to remove it or else work the bolt at too much of an angle. Okay, clutch lever is a go and I moved the clutch engagement point more to my liking.

Now my sense is only two things still stand between me and the open road, my left and right Bing CV carburetors. I soldered an old spark plug electrode to a wire with an alligator clip on the other end to use as a grounding tool to balance the carbs using the “one-plug” method. But I never got far enough to use it. I let the bike warm up in the garage with a strong fan blowing air over the cylinders. No matter how I messed with the idle adjusts and throttle-stops I could not get a steady idle. It’d hold up between 2K and 3K RPM for a minute and then suddenly drop down and sputter. Left alone it would die. All with me doing nothing!

I dropped the float bowls. Nothing nasty floating in the gas. I pulled out the idle jet and idle adjust needle from both carbs. There was some o-ring damage that was easy enough to repair (see pics below). The orifices were clear. I blew everything out anyway.



It didn’t make any difference. My impression is the big issue is my left carb because that throttle-stop screw sometimes initiated wild swings in RPM (not always). I might have a sticky slide. There is still the complete lack of vacuum at the test port to consider. That port is supposed to be connected to the chamber after the butterfly, so at idle the piston’s sucking through the intake port against a mostly closed plate produces a theoretical 30″ of mercury (according to Snowbum’s tech article).

Maybe I’ve got a leak between the carb and the intake? I can try spraying brake cleaner (or WD-40) over the area while running. Sudden RPM change indicates a leak. Of course, I’ve got sudden RPM changes without any spraying and that collar is seated nicely. I can believe a small leak at the air box but that won’t affect idle like this. I think the next thing to do is to pull the slides and polish the surfaces. Maybe take the carbs apart completely again and try more cleaning. It is possible I put one or both butterflies back incorrectly. Beyond that I’m a bit mystified.

Oh, and not that it matters but if I use the choke I can’t get ‘er started. I have to crank it a bunch times and feed some gas with the throttle. Since the timing, valve adjust, plug gaps, and coils are good the many-cranks-to-start strongly suggests the carbs are misadjusted or faulty.

For tonight, I surrender.

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