40mm bings...

Discussion in 'Old's Cool' started by honcho, Feb 2, 2007.

  1. honcho

    honcho Barley Adventurer

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    assuming i contact the original owner and get permission, i would consider "upgrading" my stock 32mm bings with some 40mms that i found (attached to a whole bike in a friend's basement). what are the advantages/disadvantages of this mod? anything required other than just swapping them out (and possibly rejetting)? if i were to do this, i would probably rebuild mine while using the 40s, then put them back... rebuild and clean up the 40s, and sell whichever set i didn't need... thanks for your help.
    #1
  2. Motomech

    Motomech Adventurer in Latin Amer.

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    You shouldn't have to rejet, but the boots, manifold and possibly, the airbox would have to be changed.
    I have a pair lying around but have not installed them because, realisticly, I don't often spin the motor higher than 5,000 r.p.m. And it's my understanding that that is where any performance gains would be. With a proportional loss below 5K.
    #2
  3. chiefrider

    chiefrider Chrome won't get you home

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    I own a '93 GSPD, Canadian/Euro spec. It has 40's, and bigger valves and different cam, I'm told. I've never had it apart.

    My brother has a '93 GSPD, US spec. Of course, he runs 32's. We weigh in at similiar weights, me being a bit heavier than he. We keep our bikes in pretty good tune.

    Comparisons are inevitable. Top gear roll-ons from 55 mph has him edging slowly away from me until about 80-85 mph, and then I gain a little more steam.

    I wouldn't bother with the conversion unless you were going to match the cam, valves, etc. The gains are minor for the work and expense.

    My $0.02
    #3
  4. widebmw

    widebmw Been here awhile

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    You would have to change the air tubes to the air box, tubes to the heads and the tubes that screw into the heads.
    The 40mm has a longer pull so you need to get new gears in the throttle and new cables.
    #4
  5. bajaburro

    bajaburro Ancient Adventurer

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    did the swap on my 91'pd several years ago.you'll get about 5 mph on top,loose some mid-range.took the 40 mm carbs off and went back to the 32 mm.there are better ways of getting a better running gs.
    #5
  6. Motomech

    Motomech Adventurer in Latin Amer.

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    Yeah, like cutting the airbox snorkels in half.:wink:
    #6
  7. honcho

    honcho Barley Adventurer

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    cool... doesn't really sound like it's worth the effort. although i may still (assuming i get my hands on them) rebuild them and clean em up... good practice, and i can probabaly hock em for more than time spent and the cost of a rebuild.
    thanks fellas
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  8. Mugwest

    Mugwest .

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    dittos on the above-- the 40's are s'posed to shine on the high end, and nowhere else on the powerband. In the neighborhood where most Boxers are sweetest (sub 5000rpm), the 32's are fine, and probably even optimal.

    True, the hotter 'performance' airheads (RS, CS etc) had 40's up to 79 or so (US models, that is), but these had hotter cams and comp ratios too; think about the intended performance envelope for these specials.

    Everyone assumes BMW downsized the 1980's 1000cc carbs to 32mm in the States strictly for emissions compliance-- that may in fact be historically correct, but in my opinion 40mm Bings on a GS that sees dirt and urban combat are inefficient. Euro 1000cc GS's certainly did wear 40's stock, but think about their market.

    Now, 40's on an RT or something that will be an interstate warrior may indeed benefit from such a swap. For my money the GS likes the 32's.
    #8
  9. honcho

    honcho Barley Adventurer

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    i think my bike likes it up above 5 the best... true it has good power all the way through, but it seems to really shine/prefer to run at 4500 to 6000. i don't think i'll go the route of the 40s though. my gas mileage already sucks.
    #9
  10. opposedcyljunkie

    opposedcyljunkie Heavyweight Boxer

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    you serious? what about using a K&N air filter? many say it's an easy and cheap performance upgrade.
    #10
  11. wintonio

    wintonio last of the Dum Dum Daze

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    YES, tell us more about this cutting the airbox snorkles in half.... :ear

    opposedcyl- I am certainly no expert, but lurk around here long enough and you will read many more words of the K&N filter being bad news for airheads, something about the size of the particulates it allows into the carb, into the engine, bad juju. ...I think they are fine to use on Harleys, but only because they usually keep such low mileage, or because nOOb Harley riders like to tip their bikes over with such frequency that the air filters get busted off and have to be replaced, on average, every 30-50 miles or so :lol3
    #11
  12. jtwind

    jtwind Wisconsin Airhead

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    Wintonio is correct about many many recommedations to not use K&N. In a sense it's logical, it's the same size as a stock filter so the only way to allow more air through is to be less restrictive and thats less filtration. The stock BMW filter works great.

    As for modifying your snorkels, I'd do some research before I tried that one also. The stock snorkles are tuned and everyone I've talked to or read on the subject, except for one person, says they should also be left alone. JT
    #12
  13. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    having swapped the 40mm and goodies of my wrecked 77RS to my un-wrecked 78/7 along with the heads and pipes, I saw a much better overall power band...I also lightened the flywheel.
    However, the old 32mm with the heavy flywheel just hummmed and ticked over like a dream.
    I'm delighted that these 90's GS's have the 32's and I'd suspect it has something to do with low rpm running and mid-range needed for off road apps.
    I'm leaving mine be...:norton
    #13
  14. chasbmw

    chasbmw Long timer

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    I was told that the latest 40mm Bings that were on the Euro GSs were the best carbs that Bing produced...............but what I do not know was what the detail differenaces are.

    R100 |CS? RS Rt s etc etc all have the same 308 degree cam, The main differance between the EURO and US models was in the inlet valve diameter and the compression ratio.

    Charles
    #14
  15. wintonio

    wintonio last of the Dum Dum Daze

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    My homeboy Greg, parts manager at the local BMW dealership, gave me a pair of carbs for free :clap, I just need to return what I don't need. My GS is a '92 with 32mm carbs, and he gave me a pair of /9 or 9 series or whatever 32mm carbs. I cleaned one up last night, and realized that there are a few differences between the 2 models. SO, should I switch both, and return him my one working carb, or just switch out the carb that is not working and leave the other alone? I think all gaskets and O-rings will transfer, but the needle set up is totally different, and the jet is slightly different as well. I can't remember now, looking at the Bing blow up pdf which exact model it is listed as on the Bing site.
    Anyone have an idea, an experience, a word of advice to offer here?
    #15
  16. chiefrider

    chiefrider Chrome won't get you home

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    K&N filters on an airhead is not a good idea. Stock paper filters get the real fine grit better than K&N. The grit is bad for the nickasil cylinder bores.

    Also, K&N filters warp as they age. When the seal is compromised, it's like having no filter at all. Very bad.

    I ride in the rain, sometimes downpours. My stock filter in my stock airbox has never been wet as far as I could tell.
    #16
  17. Lornce

    Lornce Lost In Place

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    Your GS doesn't need 40mm carbs unless you're going to put big valve heads on it, too. Wheezie US spec small valve low compression motors get all the fuel they need from 32mm Bings. :norton

    Correctly cleaned and oiled K&N filters work just fine on BMW airheads. :1drink

    ymmv,
    Lornce
    (one airhead or another over a few 100k miles since '83)
    Currently running a 232k km '92 GS/PD with modded '77RS heads, 40mm Bings and 1050 Venolia pistons at 12:1 cr breathing through a K&N filter. Works just swell. :D
    #17