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 xkeresto.wordpress.com 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.

2394-mikuni-kit.jpg

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.

2396-carb-body.jpg

2395-mikuni-guts.jpg

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 —

2401-mikuni-on-bike.jpg

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!

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22 Comments »

  1. any updates?

    Comment by Monk — June 2, 2008 @ 10:31 am

  2. Hi Monk. I probably will post some pics in my XKE resto blog.

    I changed the mirrors to CRG Hindsight LS’s, bar end style. I was having a hard time seeing behind me and bar ends are sexy.

    I added a plain rectangular sheepskin cover for the seat. Hah! It completely covers it so I might as well not have bought a new seat! I also slipped my old Airhawk cushion under it because the hard bench seat was rough on my butt. It helps.

    I also have the bike running pretty well. Carbs will never be like fuel injection but, knock on wood, it hasn’t failed to start yet!

    Believe it or not, I’m thinking of selling my FJR so I’ll be forced to spend more time on the R75. I thick 80 MPH highway miles will be hardest, with no wind protection.

    Comment by penforhire — June 2, 2008 @ 7:59 pm

  3. Great! I’m about to start my own R75/5 resto and have been reading yours for inspiration.

    I have to finish the inlaw appartment first though. Priorities.

    Thanks again for a great series.

    Comment by Monk — June 6, 2008 @ 9:43 am

  4. Penforhire,

    My name is David Gerulski, in September of 2006 I broke the World Record for the fastest motorcycle trip from Prudhoe Bay, Alaska to Ushuaia, Argentina (34for40.org). I am planning a much less stressful and longer trip in South America or Australia in 2009.

    I would like to get in email contact with you to talk about your R75/5 restoration. Would like to get your feedback and some advice about running such a bike on a long adventure ride. There is one local that I am thinking of buying and restoring myself for the trip.

    In ’06 I rode an F650 Dakar for the record. However, I would like to have a bike that is more roadside serviceable and part friendly for my adventure. This has lead me to the R75/5.

    Couple of points that I see as problematic without having any knowledge of the R75/5. It appears to be awfully low to the ground for an adventure bike required to cross streams and flooded roads. The HP is about the same as the Dakar, but the weight is about 50lbs more.

    Would like to discuss these topics and a few others regarding modifying the R75/5 to be more enduro friendly.

    Please contact me.
    dave @34for40.org.
    Take the space out of the email. Put it there to avoid spiders finding my address. Hope to talk with you soon.

    dave

    Comment by David Gerulski — July 4, 2008 @ 12:32 pm

  5. Hello David. Awesome record on something very cool to do!

    I am seriously the wrong guy to ask about reliability. I feel lucky to get back into the garage whenever I go for a ride!

    You should post on the “5 United” board. They’ve got members with hundreds of thousands of miles and intimate mechanical knowledge of the /5’s weaknesses. I only know about what I fixed and, if you read here, I don’t know much!

    In general these machines had THE highest reliability of their vintage. But today that’s not saying as much. My FJR, recently sold, was absurdly trouble-free (like 26K mi valve adjustments that rarely need adjusting even then?!) and if I wanted to go on a zillion mile pavement months-long ride I’d pick that FJR every time!

    Comment by penforhire — July 4, 2008 @ 3:32 pm

  6. Hi! I stumbled across this page while searching for Mikuni Carb info. I just picked up a ’74 R90 s with the same carbs installed. Someone (me) messed with the fuel / air screws, and now the thing won’t run without the chokeson, no matter how I adjust. D you remember approximately how many turns you backed off the screw? that could put me in a ballark range. Thanks! noahnoahc@hotmail.com

    Comment by Noah — August 22, 2008 @ 12:48 pm

  7. Hi Noah!

    I think I started at one and a half turns from closed. You do need to run maybe a minute or more on-choke to get the carbs warm enough to run off-choke. And mine takes some throttle to keep running for far longer than that.

    You might have a problem with the main jets. If I understand them correctly, the choke is an additional “circuit” in the carb. So maybe your main jets are gummed up? You said the previous owner messed with the screws. Maybe he damaged the seals and they are leaking air?

    The real expert on these is Stan over at Rocky Point Cycle. Give him a call to talk about it.

    Comment by Penforhire — August 23, 2008 @ 7:16 pm

  8. Hi, I like your blog and found it while trying to find some information for my husband about his 1973 Honda motorcycle. He’s having some problems regarding the timing or the points, if you have any ideas or can help or anything…he says the bike won’t go over 50 miles an hour. 🙂

    Comment by Kim — August 25, 2008 @ 5:55 pm

  9. Hi all- I’m getting ready to jump into my restoration project. ’72 R75/5 toaster. Thanks for the ideas!

    Comment by Eric — September 2, 2008 @ 4:28 am

  10. I have a question… I have a bad fuel line on my R75/5 its the hose below the petcock that runs into the engine compartment and I’m assuming to the other side/carb/fuel assembly. Is this true? Is it hard to replace? Do I need to take apart the engine compartment? Any help would be great I can’t find a diagram of this anywhere

    Comment by shanman — September 8, 2008 @ 7:37 am

  11. Hi Shanman!

    You are correct that the line just connects left-and-right through those T-fittings.

    If I recall correctly you just need to remove the engine cover over the air filter and pull out the filter to get to it. I’m thinking there is just one long bolt (all the way through to both sides) holding the left side cover on.

    edit — I have a photo of the area with that cover off! Look back a few pages for my post “Sentimental Value” (https://penforhire.wordpress.com/2007/08/05/sentimental-value )

    Comment by Penforhire — September 8, 2008 @ 5:39 pm

  12. Great thanks, great photos, this really helps a lot. I too have a badge that came loose. You mention Gorilla snot, as an adhesive. Are you referring to gorilla glue? I tried really heavy duty double stick tape and that only lasted a few days. So is G GLUE the way to go???

    Comment by shanman — September 18, 2008 @ 9:53 am

  13. Shanman, no, I do not mean Gorilla-brand glue. That might not have the thermal cycling capability. I just meant how the adhesive looks and feels. It is a gooey yellow thick glue. If I recall correctly I used an “automotive trim adhesive.” You should find a selection of similar adhesives at your local auto parts store.

    I do not know the exact basis of those glues but they are similar to extra-sticky rubber cement. They cure to a rubbery finish.

    Comment by Penforhire — September 20, 2008 @ 7:29 am

  14. I’m thinking of putting Mikunis on my old R100. What did you have to do to make the hoses — 1)from carb to airbox and 2) from carb to intake manifold — fit the Mikuni carb? Does Stan give you new hoses? thanks

    Comment by Don — October 11, 2008 @ 10:10 am

  15. Hello Don! Stan provided a complete kit, including replacements for the throttle cables. I’m not sure if your R100 has the same design. Check with Stan since he is responsive and easy to talk to.

    My only complaint about his set-up is how the length adjusters get hidden under the tank. I’ve put in enough miles and I think one cable stretched a little more than the other. So now I need to lift the tank to adjust them! I’ve already suggested the adjusters get located closer to the carb but it is not a huge deal.

    Comment by Penforhire — October 11, 2008 @ 2:44 pm

  16. I just finish installing the carb kit from Stan ( Rocky point cycle ) on a R60/5 ’71 . I got euro bar , so I’ve got some extra cable and I route the mid adjusting barel cable expose just under the tank for an easy reach

    Comment by pierre — May 14, 2009 @ 6:21 pm

  17. Can you tell me how to get in touch with the guy that sells the Mikuni conversion kits?

    Comment by Matt — August 18, 2009 @ 11:46 am

  18. Sure, check out Stan’s website and contact info at http://www.rockypointcycle.com . This is also given as a link in my Blogroll at the upper right corner of my blog.

    Comment by Penforhire — August 19, 2009 @ 3:05 pm

  19. I just finshed putting the Mikunis on my R60/5. My adjusters hang below the tank so no access issue there, put to get the proper free play in the throttle I’m pretty much maxed out on the mid point adjuster and carb end adjuster. I hope the cable doesn’t stretch. Also the fuel line I was sent was too large in OD to run through the airbox. It was also too large in ID for the Karcoma petcock, but proper for the carb inlet. I had to but smaller hose locally and force it over the carb inlet. I don’t think I’ll have any fuel restriction issues and the bike starts and idles nicely. Haven’t done any rides yet due to weather, but I say go for the kit. I wish the kit had a piece to replace the stock airtubes – you need to cut your originals down since the Mikuni hangs out farther.

    Comment by Dale — April 28, 2011 @ 8:14 am

  20. Out of curiosity – how’s she running with the new carbs? Everything still good? I’m thinking of changing the Bings out on my ’72 75/5.

    cheers,

    Neil

    Comment by Neil — April 17, 2012 @ 5:52 pm

  21. Peter reported he was happily riding it all over San Francisco, no major gremlins, last I talked to him. I give Stan’s Mikuni conversion kit two thumbs up.

    Comment by Penforhire — April 18, 2012 @ 9:54 am

  22. ANY UPDATES HERE? I’m installing the kit on my 1973 R60/5 tomorrow.
    John

    Comment by John — November 9, 2017 @ 8:00 am


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