Starter Covers & Airboxes

Discussion in 'Airheads' started by gsjoel, Feb 20, 2013.

  1. robtg

    robtg Been here awhile

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    SU carbs have very little in common with Bings. They have no idle circuit. no main jet, no low speed or idle jet. Everything is controlled by the needle and needle jet. The slide is oil dampened to provide acceleration enrichenment, the slide spring assists. Hundreds of different needles adjust the mix through the range.
    #21
  2. Rob Farmer

    Rob Farmer Long timer

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    SS,
    The square air box with the 32mm carbs was nigh on perfect on the dyno. The American spec gs I bought that had been imported from the states was spot on as well its just the 40mm carbs on the square air box that doesn't seem to work properly. moorespeed make a higher volume air box to get round the issue and I believe that Hpn raise the lip of the air box lower by 1cm to increase the volume of the box below the filter.
    The clamshell reduced mid range torque but flowed more air at the top end, to the extent that on the standard jetting the mixture went so weak it felt like the ignition was being turned off, the lack of readily available jets in the larger sizes meant that it was difficult to get the jetting right. I fitted 170 main jets but these were too big, 160 were the next size down but these were weak.

    Personally my ideal would be the square air box with 32mm carbs. After spending lots of money and dyno time I cannot see any advantage to the 40mm carbs. The bikes run so much better and have a much cleaner midrange with the 32mm carbs and the top end doesn't appear to be compromised for normal riding.
    #22
  3. Plaka

    Plaka Brevis illi vita est

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    The main jets go 170, 165, 162, 160, 158.....

    With some wire gauge drills or reamers you have even more sizes available.

    Speaking of Mikunis, the smaller carbs are known for higher intake velocities and better mid range torque. The larger ones for more high end horsepower. I would gather the same holds for the Bings, albeit with more lag.

    I'm dealing with 5000'+ altutude for all my riding. I need raw breathing and everything I can get. I stick to the old butt dyno. if I can't feel it I don't care about it.
    #23
  4. Plaka

    Plaka Brevis illi vita est

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    I was talking about stiffer springs in Bings.

    I used to have a '66 MGB. I remember the SU's.
    #24
  5. supershaft

    supershaft because I can

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    I think the 32mm Bings work better too. I am not much of a fan of the 40mm Bings. I am a big fan of bigger than 32mm carbs, just not Bings.

    Whatever the issue is with a square box/40mm Bing combo I don't think it is volume. If 40mm Bings pulled that much more air than 32mm Bings they would run that much stronger with 40mm Bings and they don't. Besides, I know from experience that the stock airbox can pull a lot more air than 32mm Bings on a stock engine for my bike making way more power with a stock square box and 38mm Dellorto's, a sport cam, mild port work, and a sport exhaust.
    #25
  6. Plaka

    Plaka Brevis illi vita est

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    Methinks it's the dellortos. just sayin'...
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  7. Beemerboff

    Beemerboff Long timer

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    So, if I have a GS with 40 mm Bings and I want to fit 32mm Mikunis what inlet stubs do I fit - is it just a case of getting the 32mm carb stubs from an earlier bike or is the thread in the head bigger too.?

    And my previous post was wrong - SU have fifteen different slide springs not eight, although in practical terms probably no more than half a dozen would be relevant to any given carb set up..

    The needles are also not plain tapers but are ground in 16 stages to enable precise selection in the midrange.

    Does it all matter? - well, when Truimph did a 24 reliability trial on three stock production SU equipped, unfared, Thunderbird's they managed over 90 MPH and over 90 miles per (imperial) gallon - a bit better than any Bing equipped Airhead is ever going to do.
    #27
  8. Plaka

    Plaka Brevis illi vita est

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    Bing needles are also a complex taper. Lay one along a straitedge.

    Look over the kits from Sudco. They have all the parts and you want not only the transition pieces but the jetting already set up and the correct cables. Ditto Rockypoint Cycles. There are a number of different Mikunis to choose from. I think Rocky point only has the VM. Sudco has the TM as well. San Jose BMW also has experience with the TMs. You might consult them if you go that way.

    A friend of mine put some TMs on his 90/6. I rode that bike a few times---it was like there was a stiff steel wire connected to the power. Very crisp and nice. But in the end he was disappointed. The Mikunis didn't handle crummy gas the way Bings will and he got stranded a few times. He eventually gave up on them. I've gotten wet gas from a bad gas station and the bike was missing a lot but it kept going until I could replace the fuel (I gave it to a gal in a gas station who was filling some cans for free--she was very pleased). Bings are so easy to service that popping a float bowl and draining the water every 25 miles is trivial.

    If you tell me that a 1600cc open bike running carbs got 90MPGi at 90 MPH...um...I'm gonna ask you what you smoked for lunch. That's about 76 miles/US gal. poking around on Triumphs site, the 2012 tbird gets 38/56 mpg but they don't mention the fuel delivery system. The 2013 Tbird is also listed at 38/56 and is fuel injected. I've never known any carb setup to beat fuel injection. My ancient type III VW squareback always got 40MPG with stock Bosch injection and this was long before "economy" cars. Small station wagon, 1600 cc air cooled motor, high altitude.

    At 90mph at sea level my RS was getting high 30's low 40's. If I cut speed to 55mph mileage was in the high 40's low 50's. I used to run from LA to well north of San Francisco (Vallejo) up Highway 1 a lot. It's a 525 mile run and if you have to stop in someplace like Carmel for gas you get seriously bent over. I can carry about 8.6 gallons in the tanks and it's a slow road for the most part so I could make for cheap gas on the peninsula to finish the trip---roughly 400 mile range.

    BTW, I'm not sure those intake spigots are threaded. I got a couple lying around and they ain't got threads on them. They do have slots for a flat bar so you can twist on them to help get them out. Also, you might consider going to 38mm rather than 32mm. I gather you have a 100GS. Consult the people that sell them, they will know more than anyone else and they have no particular interest in selling one over the other.
    #28
  9. chasbmw

    chasbmw Long timer

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    "If you tell me that a 1600cc open bike running carbs got 90MPGi at 90 MPH...um...I'm gonna ask you what you smoked for lunch. That's about 76 miles/US gal. poking around on Triumphs site, the 2012 tbird gets 38/56 mpg but they don't mention the fuel delivery system. The 2013 Tbird is also listed at 38/56 and is fuel injected. I've never known any carb setup to beat fuel injection. "


    I think that he is referring to a mid 1950s 650cc single carb Triumph twin!

    Some people fitted 750 Norton Commandos with single SU carbs with good results in terms of fuel efficiency and tractability.
    #29
  10. supershaft

    supershaft because I can

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    The later 32 and 40mm carb spigots have the same threads into the head.

    Bing CV's jet needles have a straight taper.
    #30
  11. Beemerboff

    Beemerboff Long timer

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    SS , Thanks for that - I suppose I could have pulled both and checked, but with so many suppliers offering special stubs I assumed that something special was needed.

    And Chas , yes it was the early 50's 650 T.birds which managed the
    90 MPH/MPG feat.

    It was 60 odd years ago when I first read the article on the test, but our local old bike mag is rehashing old "Motorcycle" articles(without any acknowledgement) and I had the chance to read it again in a freshened up form.

    I still have a clear recollection of the original article, but I cant remember just what I have done with the new mag------.
    #31
  12. supershaft

    supershaft because I can

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    No, in this case it's a BMW PN. I have a set if you need some.
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  13. Rob Farmer

    Rob Farmer Long timer

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    The standard PD engine has too low a compression ratio to make the most of the 40mm carbs at higher RPM and the standard airbox (tops and all) cause a depression midrange that takes the mixture way too rich effectively killing the power delivery. The 32mm Bings were great and kept the mixture within a very tight band making good power all the way through. I was lucky at the time in having a two GS's, both the same year, one with 32mm carbs and the other with 40mm that I could run back to back to see what the differences were. The 32mm equipped bike was so much better all the way through.

    What I do find strange is that BMW chose to produce the R100R and 100GS in Europe with 40mm when the rest of the 1000cc bikes came with 32mm carbs. Makes no sense when the GS especially is not intended as a high speed machine.

    The clamshell airbox doesn't cause such a midrange depression in the 40mm carbs mixture as the standard airbox allowing the bike to run cleaner but it cause the mixture to run very weak at the top end. I know Plaka lists a range of jets but I could only buy 160 and 170 in the larger sizes. 160 was too weak and 170 too rich so I ran with the 170 mains along with variations in airbox designs for a while before putting it all back to standard. What the clamshell did do was give a nice progressive power delivery, unfortunatly the GS doesn't have the compression as standard to make the most of it at the top end. At the end of the day there's little point trying to hotrod a standard GS when the chassis, weight, tyres and brakes are the biggest limiting factor. Personally I think the biggest single improvement to the 100GS if you don't want to be messing with high comp pistons, airboxes, exhausts etc is to fit a pair of 32mm Bings.
    #33
  14. supershaft

    supershaft because I can

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    Which top did you have. It sounds like the one with both horns being the same size? My 8.5:1 Dell/sport cam setup had a big flat spot in the midrange with that top on the box. One large/one small horn fixed that perfectly with almost no penalty on top but the cap with two large horns killed midrange with absolutely stock setups as well. IMO, the GSPD types are too heavy but the G/S's are light. Tires and brakes issues are easily fixed. The single biggest thing I did to my bike which had the same engine as yours excepting lower 8.5:1 CR pistons (I think your came with 9.5:1 pistons?) and coming stock with 32mm Bings was putting a 336 cam in it. Huge step in the right direction! If I was going to go back to 32mm Bings I would rather instead go back to 36/38mm Dell's or Mikuni's. 32mm Bings are great at what they do. My only problem with that is that they don't do much at all to start with!

    Bing makes mains for our CV's from 70' to 300's. I keep them at my shop from 120's to 170's. I keep needle jets from 260's to 274's. You can't get 260's and 262's through BMW.
    #34
  15. motu

    motu Loose Pre Unit

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    My 650 Triumph would do 100 imperial mpg with an HS2 SU. But it wasn't a T'Bird, with a 9 stud top end, 9:1 pistons and Bonnie cams....and it wasn't a Triumph either, but a Triton and weighed a lot less than a stock bike. Still, how come my 650 Triumph could do 100mpg, and my 600cc Norton also with an HS2 could do over 90mpg....and yet my 650 BMW of similar hp can't get anywhere near those figures ?
    #35
  16. Rob Farmer

    Rob Farmer Long timer

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    I tried no horns, cut down horns and different sized ones but they all still had the vacuum depression mid range.

    The standard pd engine has 8.5:1 pistons (mine are standard in the pd).

    I have another gs running high comp, moorespeed oversized valves, guides etc but is running 81 heads, which richie isn't keen on due to the port size. It has a standard air box on at the minute and still has the stumble on the midrange where the mixture goes way too rich. You can barely feel it on the road but it shows in the dyno.
    #36
  17. chasbmw

    chasbmw Long timer

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    try this one....

    [​IMG]

    or maybe not, because it still has the dip between 4000 and 5000 revs. max torque is at 3750 rpm

    [​IMG]
    #37
  18. Rob Farmer

    Rob Farmer Long timer

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    It's a pity they didn't plot the air/fuel ratio on there Charles, that dip coincides with mixture dropping as low as 10:1 on my plots. The 32mm carbs kept a nice mixture all the way through so made a lot more power.
    #38
  19. chasbmw

    chasbmw Long timer

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    Mine is with 38MM dells, i should have asked for the printout to include the mixture.
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  20. Rob Farmer

    Rob Farmer Long timer

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    Dells? Now that's interesting.
    #40