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/5 Bing Slide to CV Carb Conversion

Discussion in 'Airheads' started by korinthias, Sep 23, 2019.

  1. korinthias

    korinthias Been here awhile

    Joined:
    Apr 29, 2019
    Oddometer:
    257
    Location:
    Devon, UK
    Following on from a conversation with fellow Airhead Fillippp after I added a comment to a thread by JoyRide, I have been prompted to post some information on this conversion I made a while ago to my 1972 R60/5.

    At the time, I was having great difficulty balancing the original 26mm Bing slide carbs and in truth they were already, at 29k miles, showing quite a lot of scoring and wear to the slides making movement in the bore quite erratic. In pondering the matter, I came across a reference to an article on the BMW Club UK forums describing a conversion to 32mm CV carbs, written by fount-of-all-knowledge and general Airhead guru Mike Fishwick. After a bit more research, I managed to get in touch with him directly and received some more specific first-hand advice and a copy of the article itself. If I am able to obtain his permission, I will follow up this post with a first-hand copy of the information he originally gave me. The 32mm CVs – as fitted to the R80 etc. are, for sure, a larger diameter, but Mike had determined that their ability to self-adjust once jetted accordingly, would provide more reliability and durability.

    First thing to say, with the general transferability of parts between different airhead models, most of this process is straightforward and just requires standard off-the-shelf items. However, the /5 R60 (and R50) do have a different carb stub screwed into the cylinder head and a different thread to screw into, so this is where a special sleeve has to be fabricated.

    Here's a pic of the original slide carbs:

    IMG_5291.jpg

    A photo of the original carb stub upon which the carbs are mounted, once removed from the head; the slots are already milled into the stub, making removal from a pre-warmed head an easy job:

    IMG_0385.JPG

    And now here's a drawing of the part required to fit the larger diameter 32 Bing CV carbs:

    BMW R50-60 Carb Stub Converter.jpg

    Having no access to a lathe, I had to find someone to make up these components, and got in touch with Carl Wadkin-Snaith at TurnerTech. He made these up for me; including a clever tool for fitting:

    R60.jpg

    More to follow…
    #1
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  2. korinthias

    korinthias Been here awhile

    Joined:
    Apr 29, 2019
    Oddometer:
    257
    Location:
    Devon, UK
    A while watching eBay turned up a suitable pair of 32mm Bings, grubby but complete. I had initially considered the less common flat-top carbs, but Mike advised the more common dome-top type where the aluminium slide moves up and down on a steel tube, for greater durability. His words were:

    I would steer clear of any flat-top CV carb, as the piston is guided directly by the body, rather than a hardened steel column. This means that the body wears, as with a normal slide carb, so any used flat top carb is likely to be worn out. The domed top type is far better, not suffering from body wear, and being more sensitive to changes in air flow. That is why the flat top carbs only lasted a year – 1978 – being a failed attempt at cost-cutting.

    The 28 mm size was fitted on the R45, but may be OK on the R60 – but not in flat-top form! As so many different models, from R65 to R100 used the 32 mm carbs, there are lots of them around. They work well on the R60, and even on the R45, as to a great extent CV carbs are self-regulating – so there is no real point in bothering with the smaller CV carb. So – my advice is to keep looking for 32 mm dome-top carbs. It should not be a problem to find a pair.

    The pair I bought cleaned up well, and although I was initially planning to get them cleaned ultrasonically, it turned out they weren't too bad internally.

    s-l1600-30.jpg

    Mike recommended that they should be rebuilt using new needles, needle jets, and float needles etc. to the settings used by the R65. This meant 45 pilot jets, 145 main jets, and 2.64 needle jets, with the needle fitted in its number 4 (highest and therefore richest) position.

    IMG_0406.JPG IMG_0409.JPG IMG_0410.JPG

    Jets, needles and other new parts needed were bought from Motorworks UK. I decided to stay with the existing floats because the PO said he had recently replaced them. In addition, I needed a choke lever which Motorworks supplied from used stock (the boss to fit it to was already there on the left air-filter housing), the old slide carbs only had ticklers, which made starting and cold-running in cooler weather sometimes difficult… The rest of the parts, throttle and choke cables, inlet ducts etc, were standard pre 1980 airhead stuff. I did manage to get secondhand genuine BMW stainless clamps for the inlets from Motorworks. Coated in all sorts of crud they cleaned up perfectly and help with an authentic finish, even though the whole project is decidedly incorrect.

    More…
    #2
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  3. korinthias

    korinthias Been here awhile

    Joined:
    Apr 29, 2019
    Oddometer:
    257
    Location:
    Devon, UK
    Once the job was complete, I sought advice on the web for balancing the carbs , but ultimately used Steve Doyle's instructions for set-up here: http://rossmz.blogspot.com/2008/12/tuning-40mm-bing-cv-carburetors-for-bmw.html
    Okay, they were for 40mm carbs, but they worked for me, and amazingly the default settings described were damn near spot-on! Choke on, the bike fired up instantly and it just needed a small amount of tweaking after the requisite 15 mile run to thoroughly warm the engine. From my point of view, I don't think they look entirely wrong, I could have gone the Mikuni or even Dellorto route, but at least the Bing CVs are still classic airhead in spirit! The bike now starts first-press, runs up smoothly from cold and seems a much more lively and responsive motorcycle to ride. I hope this proves useful to others who may experience similar issues and are seeking a solution.

    Couple of pics of the completed job:

    DSCF3429.jpeg

    IMG_5019.jpeg

    43274474-779A-4BFB-A38B-4C4C7DF32008.jpeg

    IMG_5143.jpeg
    #3
  4. PaulBarton

    PaulBarton Long timer Supporter

    Joined:
    Apr 14, 2015
    Oddometer:
    3,611
    Location:
    Seattle
    Thanks for posting this. I think this is a very appropriate upgrade for anything other than a concourse restoration. The CV carbs are an improvement over the slide carbs both in performance and maintenance. Nice work!
    #4
    korinthias likes this.