Restoring a 1973 BMW R75/5 Motorcycle

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.



  1. Jessie

    Thanks for the info. By the way, I am a big fan of your site. Keep up the great work.

    Trackback by Jessie — February 1, 2008 @ 11:17 pm

  2. I enjoyed reading the articles on the Mikuni and Boyer installations.

    Would you mind if I linked to your article from my website?


    Stan Smith

    Comment by Stan Smith — August 1, 2008 @ 5:03 pm

  3. No problem Stan. If it can help anyone else it is all good. I remain quite happy with the Mikunis.

    Comment by penforhire — August 2, 2008 @ 8:24 am

  4. I would like to buy the Hepco Becker saddle bags for my 73 R75/5. How much were they and how can I order them.
    You have done an excllent job on your BMW and this website!


    Comment by Peter Dove — August 10, 2008 @ 5:32 pm

  5. Thanks Peter! I’m pretty sure I got them from Bob’s BMW. If I recall correctly price was somewhere around $500 (ouch!). I already had the racks needed, otherwise you’ll need to add those.

    Comment by penforhire — August 11, 2008 @ 7:04 am

  6. Hi Stan,
    Inspiring reading about your R75/5.
    I have a 71 R60/5 and have experienced the gearbox automatic neutral selecting a couple of times – natures way of making life interesting dontyaknow?!
    I also have the same panniers as you – I would suggest that you hold the front end of them onto the frame with clips as they had the reputation of going walkabout at times.
    Hope that these comments may be of interest, safe riding mate. George C

    Comment by George — September 22, 2008 @ 9:32 am

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