Author Topic: BMW R100RS  (Read 1003 times)

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Offline Tim2005

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BMW R100RS
« on: March 14, 2023, 02:42:34 PM »
This write-up is about a BMW that I did last winter. It's an interesting bike for not only the usual reasons, but also because it had been in a collection for 15-20 years, so much of what I found isn't specific to a BMW and is likely to be encountered by anyone who is working on many of the bikes that haven't been ridden much and appear at the big auctions etc..  It's a pity I didn't take any photos of the various jobs to make this more interesting, as otherwise the before & after photos of the bike are pretty identical.

 I usually do a project bike of some sort most years, though haven't actually done a Honda SOHC4 for a while, in part because I've kind of nearly run out of ideas for them, in part because they are rare/expensive project bikes to buy around here now, and in part because it is interesting to try something different and learn how other bikes work.  In recent years my project bikes have included a 1993 Ducati Monster 900, a 1996/7 Suzuki GSXR750, and a 2000 Kawasaki ZX12R. All 3 are great bikes in their own ways.
« Last Edit: March 14, 2023, 02:47:35 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #1 on: March 14, 2023, 02:42:56 PM »
While each of those 3 have their uses, the ZX12R is the least fragile and tending to be the one I used in wintertime, but its performance is a bit much for greasy winter roads to say the least.  So I figured I needed something with a decent fairing, preferably pre-1981 (tax-free here), and not too sporty or exotic.  Looking through some old 70s bike mags I was drawn to the R100RS BMW,  it seemed to have the right attributes, it's not too rare or pricey, looks alright, etc. Also I was curious to see what was so good about it that new back in 1977ish they cost £3100 while a new Goldwing was £1800.

It took a while to find one (I wasn't trying very hard either) but in Autumn 2021 one came up on the other side of the country. 1980 bike, it looked ok in the photos and was sensibly priced. It had been in a private collection for 15 to 20 years and then been recommissioned (partly, as it turned out). I had a chat with the seller and it seemed all ok, though when I suggested I got a train over and then rode it the 5-6 hours home he tactfully steered me away from that. Anyway we agreed a price & I sorted out a courier to deliver it to me.

It arrived aok early one evening and I gave it an initial inspection. Overall it seemed decent, very clean and noticeably lacking in corrosion. The first issues I noted were just tyres that were very hard and perished (so I ordered a new pair of Avon Roadriders & Michelin tubes that evening), a throttle that was like a ratchet, and various lights weren't working.  Oh, and it had clearly been polished a lot while in the collection, the seat was so slippery it was almost impossible to sit on.
« Last Edit: March 14, 2023, 02:47:46 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #2 on: March 14, 2023, 02:43:22 PM »
First jobs - clean seat. I tried various chemicals and ended up using surgical spirit, this helped after a few applications. The ratchety throttle, I thought it would be cable problems, but stripping it out I found the cables & carbs were nice and smooth, however the twistgrip has an odd bevel gear arrangement inside it which was worn out, I found a useful BMW dealer who had those in stock, although it was not far off the price of another tyre.  Not many options though, no used ones around and to replace the whole twistgrip/switch/brake lever arrangement with something off a different bike was going to be complex & even more expensive.  The tyres quickly arrived & were pretty easy to fit, though the rear wheel is surprisingly awkward to remove, there's so little clearance that it's best to deflate the tyre to squeeze it by the swinging arm. As for the front wheel/brakes, more later on that topic.  In the meantime I also found a disconnected plug under the seat which was why none of the rear lights worked, a splendid easy fix. If only the instrument lights were so easy, more later on that too.

OK, I now had an intact and seemingly roadworthy bike. Test ride time. It starts easily, full choke, hit the button and away it goes after a few turns. It kind of shakes at tickover like something heavy is going on. Clunk into gear and away I go. Just gently along my road to the junction at the end, a brake check didn't seem to do much, then I so nearly overshot the junction. Dreadful brakes, almost nothing there yet firm lever & pedal. I tried riding along a bit dragging them, to burn off any crap, which maybe helped a little, but it was a short slow ride with a stop at the local petrol station as it was on reserve too. I then headed back home, other than the brakes it was riding pretty well, but they needed sorting before I explored it further. Something seemed amiss with the transmission too, I'll come back to that later.
« Last Edit: March 14, 2023, 02:48:03 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #3 on: March 14, 2023, 02:43:53 PM »
A quick cuppa and I went back out to the workshop to discover another problem, a growing pool of petrol on the floor. There are fuel taps on either side of the tank, with hoses down to a balance pipe connector then onto the carbs.  One of these hoses was completely rotten and leaking, maybe the new fuel had finished it off.  Anyway, an easy fix by replacing the hoses, and adding new inline filters to them too.

Onto the brakes. Starting with the rear, it has a conventional twin piston brembo caliper, which was rather seized and a pretty easy job to dismantle and clean that out. However, getting it working was harder as the master cylinder wasn't pumping from it's reservoir, I worked that out by filling the system through the caliper bleed nipple. No leaks but there was an internal blockage. Alas the rear master cylinders are neither rebuildable nor available in stock anywhere.  Fortunately  I remembered a post on here about a similar issue and attached another master cylinder instead of the caliper and pumped fluid back into the master cylinder, after a bit of pressure built up it came free and the blockage cleared. Caliper refitted and bled & all was good.  Or as good as R100RS rear discs are-  they are actually pretty crap, so much so that one company offers a rear drum conversion for them.
« Last Edit: March 14, 2023, 02:48:09 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #4 on: March 14, 2023, 02:44:19 PM »
I then tried a quick clean of the front  brakes next, the discs were really badly contaminated with something (polish I suspect) but even after cleaning & test riding they were pretty poor. Dismantling them I established someone had already been in there, nice new fluid and the 4 pads were secondhand ones of 3 different brands. I ordered up a set of the grippiest pads that  I could find, made by EBC. These old BMW front brakes are an odd thing, the pads have an angle to the surface and the caliper mounts on a cam arrangement, so to get the pads parallel to the discs is quite a chore, you have to draw a line across the disc surface (both sides of each one) with a sharpie then rotate the wheel dragging the brake, and see where the sharpie line has been worn away & where the pad has missed it. You then adjust the caliper on the cam until you find the sweet spot where each whole line is worn away. All seems very unnecessary to me. Anyway, the brakes were all finished now (oh, I had to replace a cracked disc too, never seen that on a Honda) so I then got on with a trying a few rides.

A couple of observations about my initial experience of working on it. Tools. Firstly, put away your 8mm sockets. Everything is built heavily, I tended to use tools that I usually use working on the car. 17mm & 19mm bolt heads seem to prevail. But good news, as the bike had had an easy life and been in a collection for years, there was no corrosion on it anywhere, I didn't encounter any seized bolts at all.  Parts availability is really good, there's a particular supplier with a comprehensive online shop that everyone seems to recommend, but I found another (James Sherlock's) that is cheaper and has a very helpful guy on the end of the phone, which to me is much better than a web-shop. You have to be careful ordering parts to get the correct ones, there were lots of detail changes even within a model year. Parts aren't cheap, every phone call seems to be £100 or multiples of that, but the availability is impressive so it's a good trade-off.  That said, a full restoration/overhaul would be very expensive & far more than these bikes are worth here. There is also tons of tech info online, you can read pages and pages about many tasks and watch countless videos people have carefully produced, it's better than I've found for any other vehicle. Though sometimes you can spend quite a while looking into something before finding that it's a 3 minute job anyway.
« Last Edit: March 14, 2023, 02:48:16 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #5 on: March 14, 2023, 02:44:38 PM »
A few rides now followed. It ran reasonably well, even better when I fitted new plugs that were the same as each other, unlike the old ones. A carb strip showed someone had been in there, new jets and very clean carbs inside , new airfilter too, and an oil change soon followed too. The oil filter is surprisingly awkward to do, much easier if you remove the right side exhaust. The clutch needed adjusting, for which  the process is rather simple if you follow the manual, it is opposite to a Honda, you adjust the cable then do the pushrod.

I put a couple of hundred miles on it after this. It's a heavy beast, with limited turning circle and some awkwardly narrow handlebars, so getting it out of the shed & turning it around is not so easy. Once on the road you soon get used to the bars, the riding position is quite sporty but comfortable enough, though I really dislike the way that the rear brake pedal is positioned as I seem to have to catch my ankle on the carb to use it. The fairing is fantastic, it keeps the weather off you so well. Add in warm feet from the engine and the very good headlight and it's an awesome bike for blasting along a fast road on a cold night. Fuel capacity is huge and economy is pretty decent too, so you've 200+ miles of range easily. Top end is meant to be 130ish,  overall I'd say it's a bit quicker than a CB750 despite being a fair bit heavier, however it is nowhere near as smooth. The suspension is decent and it corners really well, however it's not what you'd call an involving bike to ride, there doesn't seem to be much feedback at all, but it sorts everything out itself and shifts along pretty quickly anywhere. The braking is poor by modern standards, it can stop pretty quickly but you need a big handful of lever. Overall I think if you fitted a R100RS fairing & headlight to a good stock-ish CB750 then you'd have a better bike than the BMW.
« Last Edit: March 14, 2023, 02:48:22 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #6 on: March 14, 2023, 02:44:58 PM »
Two problems remained still after the  rides. These turned out to be the most time consuming jobs too. Firstly, I noticed early on that when riding it the gearbox had something missing. Yes, no 5th gear.  1-2-3-4-neutral.  This was not so good. Some research online established this is not uncommon on these bikes when they have been stored for a long while. Apparently sometimes after an oil change and a decent ride and 5th will eventually appear, I tried all that but got nowhere. The problem is that one of the selector forks sits well above the oil in the gearbox and when stored it all dries out and sticks on its pivot shaft. So, I had to take the gearbox out and free it off. This isn't too bad a job though, unlike a SOHC4 the gearbox is a completely separate unit that can be removed from the bike while leaving the engine fitted. That job just took an hour or 2. Stripping the gearbox has a reputation of being difficult as the output shaft flange has to be removed first, this involved making up a locking device and undoing the retaining nut, then taking the box over to an engineer friend who owns a 10 ton hydraulic puller. Legend has it that that's what's needed to remove the flange from the tapered shaft. In my case, luck was on our side as the engineer called me next day to say he was just measuring the flange to fit the puller and it fell off the shaft.

So, on with the gearbox strip - pretty easy really, take off the end plate, spot the stuck fork selector and with a bit of hand pressure it soon freed up, I did take them all out and clean their oil holes, checked the rest of the box (which was immaculate, and seems to be built to cope with a decent V8, everything in there is huge), fitted new seals, neutral switch etc and reassembled & refitted it. Around £100 for the parts. At last, all 5 gears were working. It shifts really nicely too, much better than I had been lead to expect.
« Last Edit: March 14, 2023, 02:48:28 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #7 on: March 14, 2023, 02:45:20 PM »
So another test ride or 2 beckoned, however this brought me into another problem area. The first long & fast run - well 40 miles each way on the highway - and although it was riding really well, when I stopped I found that the rear wheel & tyre was covered in oil. Luckily I hadn't discovered that the hard way. This oil was coming out of the breather on the rear hub, mysteriously. A series of investigations followed, not helped by having to do a long fast run every time (with possible rear tyre oiling) to see if the problem had been resolved, but basically the BMW transmission has 3 lots of oil- one in the gearbox, one in the rear hub, and one in the shaft in between, and whilst the bike was stored some corrosion had occurred on the pinion surface between the shaft & hub, presumably there had been some condensation in there to cause it. . That had worn out the oil seal and then oil from the shaft was getting pulled back into the rear hub, overfilling that so it flowed out from the breather. A rather fail-unsafe design imho.  A new pinion and seal etc cured it nicely, and was only £100 or so, of course.

Onto the final issues- just the instrument & warning lights. When I bought it, these were mostly non-existent with just the indicator/flasher warning light stuck on all the time too. Dismantling them was curious, for some reason there must be about a dozen screws that hold the 2 halves of the clocks together.

A few bulbs needed replacing and contacts needed cleaning, though it still wasn't enough.  The neutral light needed a new switch as well as a connector re-connecting.  The indicator/flasher warning light was a pain, the relay is weird as it provides earthing for the indicators but current for the warning light (or maybe vice versa), the options to replace it are either a 2nd hand 40-year-old one (£100 of course) or a modern relay and some extra diodes soldered into the connecting wires. This was a solution I found online, it worked great though I'm no electronics expert & didn't really understand it entirely. One nice feature I found is that inside the headlamp shell there's a connection board with a selection of spare terminals to power things off.
« Last Edit: March 14, 2023, 02:48:49 PM by Tim2005 »

Offline Tim2005

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Re: BMW R100RS
« Reply #8 on: March 14, 2023, 02:46:53 PM »
The final thing to fix about the clocks was the voltmeter, this was odd. It didn't work at all. Inspection showed there were 3 wires to it. 1 with power from the lighting circuit for the nighttime illumination, 1 earth, and a live. Someone had carefully cut off the live at the harness, and made a figure-of-8 type connector up (very nicely too) to connect the live and the earth terminals on the voltmeter together. I've no idea what they were thinking of, any ideas welcome. Anyway I put it back to standard and it works perfectly.

So this is mostly the whole project finished. It's a strange project bike as I didn't need to do any cosmetic work at all, no painting or anything, didn't have to deal with any seized bolts, and few previous owners' bodges.  I sense I'll probably sell this one sometime, though first I need to think of something to replace it with...
« Last Edit: March 14, 2023, 02:48:37 PM by Tim2005 »

Offline ekpent

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Re: BMW R100RS
« Reply #9 on: March 14, 2023, 04:22:05 PM »
Entertaining write up. Think this was the model of BMW that supposedly saved the company and breathed new life into it. I think they are attractive bikes. Never have dipped my toe in the BMW water yet though there is a guru who lives just west of me who knows these things inside and out.  Enjoy the toy !

Offline Tim2005

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Re: BMW R100RS
« Reply #10 on: March 17, 2023, 02:22:20 PM »
Entertaining write up. Think this was the model of BMW that supposedly saved the company and breathed new life into it. I think they are attractive bikes. Never have dipped my toe in the BMW water yet though there is a guru who lives just west of me who knows these things inside and out.  Enjoy the toy !

Cheers. If you fancy dipping your toe, try to find a post-1980 model, they had different front brakes (Brembos) with normal 2-bolt mounts on the forks so they can be upgraded to more modern stuff too, plus the engine was changed a bit to make it more free revving.

Offline calj737

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Re: BMW R100RS
« Reply #11 on: March 17, 2023, 05:15:57 PM »
Actually it was the R90/S that was credited with saving BMWs bike group. I had an ‘83 R100RS and did a full cross country US trip. Was an absolute delight. Still have a ‘74 R90, ‘87 K75S, an ‘07 R1200S and my beloved ‘09 K1300S. I’ve logged hundreds of thousands of miles on them and still prefer them to anything else day in and day out. Lots of other great bikes out there, but push to shove, I’ll take a BMW for the trouble-free riding enjoyment.

Quirky by other standards to service, but once you get acquainted with them, you appreciate the engineering thought that goes into them. Or at least I do…
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline ekpent

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Re: BMW R100RS
« Reply #12 on: March 17, 2023, 05:49:49 PM »
If I ever get a BMW I am going to make sure I make Cal my new best friend !!  :D































Offline RAFster122s

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Re: BMW R100RS
« Reply #13 on: March 17, 2023, 06:20:24 PM »
If I ever get a BMW I am going to make sure I make Cal my new best friend !!  :D
































It is a good idea to have Cal as a friend always Eric.
David- back in the desert SW!

Offline simon#42

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Re: BMW R100RS
« Reply #14 on: March 18, 2023, 04:47:45 AM »
i have a r75 /5 and a r80st . i have done thousands of miles on both . more than 200 thousand on the 80 and i still love it . despite having owned many supposedly better modern bikes i always go back to the bm .  the 80 is now a 1070 and not much is standard but to be honest it was a good bike before i messed with it .

Offline C317414

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Re: BMW R100RS
« Reply #15 on: March 18, 2023, 05:41:21 AM »
In the late 70s worked PT at my buddy's motorcycle shop, while going to college.  People would often stop by wanting to sell their bikes.  One day a guy showed up with a beautiful R90S in the back of his truck.  He said it wasn't running and needed a valve job.  My buddy made him an extremely low-ball offer and he accepted it.

The next day I started working on the bike.  I discovered that three valve adjusters were screwed in too tightly, and the valves weren't closing; plus one Dellorto carb accelerator pump diaphragm was bad.  After I adjusted the valves, replaced the diaphragm, changed the fluids, and did a tune-up, the bike ran great.  My buddy rode it for a few months, and sold it for about 3x what he paid for it.

Offline MauiK3

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Re: BMW R100RS
« Reply #16 on: March 18, 2023, 08:31:20 AM »
That blue/silver is a nice paint job.
My brother had and R75 a long time ago. Nice bike for the day. I stuck to my CB750 and later my GL1000
1973 CB 750 K3
10/72 build Z1 Kawasaki

Offline Tim2005

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Re: BMW R100RS
« Reply #17 on: March 26, 2023, 03:11:47 PM »
That blue/silver is a nice paint job.
My brother had and R75 a long time ago. Nice bike for the day. I stuck to my CB750 and later my GL1000

I've been doing some tidying on the bike the last couple of days and the paintjob sprung up a surprise.  I always knew it wasn't original, this blue/silver combo was produced in 1984 as a limited edition called the Classic Series. My bike was a few years old then and the story goes that then owner liked that paintjob so much he had the bike resprayed in it from it's original gold. It would have been a pretty expensive job too, as it's been done with the silver base then the blue that is faded-in over it is actually a candy paint. It looks pretty special in some lights.  However, I have just noticed there is something major wrong, which is only obvious when you know it. Though I've owned the bike for over a year without spotting it, and been to various bike meets & lots of mates have seen it, without anyone else seeming to have noticed it either.  Can you see it? Answer below if not...
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What's wrong? The fairing is a completely different blue to the rest of the bike. It stands out a mile now I know about it.




Offline ekpent

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Re: BMW R100RS
« Reply #18 on: March 26, 2023, 05:55:02 PM »
Doesn't stick out like a sore thumb in the pics, all seems to blend together pretty nice. Now you will see it every time but hopefully not enough to bug you too much.

Offline RAFster122s

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Re: BMW R100RS
« Reply #19 on: March 26, 2023, 06:00:17 PM »
I noticed it but thought it just might have been the gloss.
David- back in the desert SW!