Best carbs for 77 RS big valve motor?

Discussion in 'Airheads' started by beemerphile, Jul 23, 2013.

  1. AlpineRAM

    AlpineRAM PartsChaser

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    You could put a small non-submersed pump right beside the starter and feed it from the stock fuel crossover under the airbox.
    A microsquirt injection can use the starter teeth on the flywheel for crankshaft positioning and two camshaft sensors to get the correct TDC info. With the BMW cable splitter under the tank you can drive the TPS and you can use a completely gutted Bing40 as an injector-holder+throttle-plate, and route the cabling to drive the injector through the intake air duct from the air filter housing.
    Voila- almost invisible EFI mod for the airhead. (You would see the second set of fuel lines from the fuel pump to the "carbs".
    (I did a project like this for a thumper, needed to be invisible because on my side of the pond it is almost impossible to get something like that through the hoops of bureaucrazy*)

    Cheers

    AlpineRAM

    * bureaucrazy is not a typo- it's a definition for the crazy in the offices of the authorities.
    #21
  2. RGregor

    RGregor Been here awhile

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    Hello Beemerphile,

    if you want to keep it simple, you should go for the Dellortos.
    Cables and adaptation to the air box readily available from BMW.
    If you choose the rubber mounted version, e.g. PHM 40 NS/D (Sinistra for left, Destra for right) and want to connect them to your big-valve heads' spigots you could take these rubber mounts (from the Guzzi 1100 ie):
    http://www.stein-dinse.biz/images/product_images/popup_images/01114390.jpg
    They fit perfectly on the spigot and the carb.

    If you don't want to have choke cables hanging around this may be an option:
    http://www.stein-dinse.biz/images/product_images/popup_images/9619-64.jpg

    Regarding this:
    A friend had Mikuni TM40 on his R90S with the stock airbox (round filter type).
    Connection to the airbox was easy and looked perfect like stock.
    This is the bike in question:
    http://www.motoren-israel.com/images/galerien/Boxermania/picture2033.jpg

    The floats in the TM40 are not independent. No problems here whatsoever. The carbs work great. Jetting is also easy. I could provide a basic jetting as a starting point.
    Readily prepared (basic jetting set for BMW use) Mikunis and cables can be bought here: http://www.mikuni-topham.de/ENGLISHSITE/ENGLISH/Frame_English.html)
    But Mikunis and parts are relatively expensive (at least over here).
    So again, if it has to be simple and cheap, I would prefer the Dells.

    Best regards

    RG
    #22
  3. beemerphile

    beemerphile Long timer

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    Thanks RG,

    In a couple of paragraphs you more than doubled my knowledge of the Dellortos. We are getting somewhere now. I do not mind choke cables, nor do I mind independent chokes, so either solution there would work.

    You mentioned that you could provide basic jetting for the Mikunis. Would you be able to help with basic jetting for the Dells?
    #23
  4. RGregor

    RGregor Been here awhile

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    Hello!

    Yes. Like SS I would recommend the R90S jetting as a starting point.

    Idle jet 60, Atomizer AB260, Needle K4, main jet 155, Slide 60/5.
    For a 40mm Dell I would go up to a 165 main for the first step.
    I would run it with acceleration pumps.
    If set properly engine response with pumps is much better, especially from low revs.

    However, depending on your engine configuration, the jetting your engine really needs may be quite different.
    I'm running 55 idle jets, AB262 atomizers, K6 needles, 155 mains, 60/5 slides.

    Best regards

    RG
    #24
  5. Bulldust

    Bulldust Bulldust

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    Don't need it inside the tank, go with an external fuel pump.
    #25
  6. supershaft

    supershaft because I can

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    In that photo? Those aren't regular TM40's. I wouldn't go bigger than 38's but . . . . Those are smooth bore TM's or XR600 specific TM's or something along those lines. Just regular TM40's have a much larger built in bell mouth that won't allow a late model BMW snorkel to be used like in that photo.

    Floats in 'a' TM40 aren't independent? Which TM40? They are in every regular TM series carb I have ever seen.

    I would use 38mm Dellorto's too over 40's but . . . . I also get them picking up revs and accelerating much harder without the pumps connected as do MANY tuners I know. That is unless you crack the throttle wide open from low rpms with a good load on the motor. The pumps will help you out of that stupidity but why sacrifice picking up revs and accelerating for masking a ham fisted right hand? That's pretty much all those pumps are good for.
    #26
  7. RGregor

    RGregor Been here awhile

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    Hello SS,

    now what is a "regular" TM?
    I'm speaking of TM 40-6 carbs.
    If you are interested you can find them here:
    http://www.mikuni-topham.de/ENGLISHSITE/ENGLISH/Frame_English.html

    And yes, they are flatslides/smooth bore. And yes, they don't have independent floats. If required I can provide a photo, I've got a set of carbs here.

    Sometimes there are things outside we have not yet seen ....

    Regarding the pumps: you have your experience, I have mine.
    I have learned that you disabled the pumps and got more power.
    Knowing how the pumps work there is no logic explanation for that.

    Regarding the wrist: yes, I have a sensitive wrist.
    And still I can feel the benefit of the pump.
    I ran my carbs with/without pumps and with modified levers. So I believe I can compare very well. Without the pumps the bike ran good, with the pumps it simply runs better.

    If it works for you without the pumps it's fine.

    38mm Dells are in fact big enough, I'm using them as well.
    However, regarding the big ports of the '77 heads this will give a strange port shape. The problem are the heads, not the carbs ...

    Best regards

    RG
    #27
  8. supershaft

    supershaft because I can

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    Google mikuni tm. Those are 'regular' TM carbs. I was not talking about smooth bore TM's. If I was, I would say 'smooth bore TM carbs' versus 'TM carbs'. Go figure?


    No logical explanation? Give me a brake with the logic. I have had AMA National Championship winning tuners tell me that they use to do the same thing for the same reasons. Figure out that logic.

    So now you are telling me about big ports?
    #28
  9. RGregor

    RGregor Been here awhile

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    Hello SS!

    1. I'd suggest we let Mikuni decide, what they call TM and what not.
    2. There are no TM40 in the series you mean. The only TM40 is the one I mentioned.
    3. If there is an explanation I would be eager to learn
    4. No, I'm not telling you about big ports, but Beemerphile

    Best regards

    RG
    #29
  10. supershaft

    supershaft because I can

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    The only TM40? There's the TM40 smooth bore, TM Pro Series which come in 39 and 41mms, TM40-6's, and more.

    Explanation? Better mixture.
    #30
  11. RGregor

    RGregor Been here awhile

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    Hello!

    Oh man, I came up with a carb type that fits very nicely to our beemers.
    And as a reaction we discuss about your self invented categorization "regular" and finally, if TM39s or TM41s of the Pro Series are also TM40s?

    Something like "They fit, cool" would have been sufficient.

    My very best regards

    RG
    #31
  12. RGregor

    RGregor Been here awhile

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    Hello again!

    Just to be sure I got it right.
    1. The improvement concerns WOT high rpm?
    2. You did not remove the pump jet? Just set the adjuster screw for zero delivery?

    Best regards

    RG
    #32
  13. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    RG, thank you for your contributions (here and elsewhere) and bless you for your patience. It is always good to see your posts.

    --Bill
    #33
  14. beemerphile

    beemerphile Long timer

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    ^^^ What Bill said ^^^
    #34
  15. chasbmw

    chasbmw Long timer

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    +1
    #35
  16. Jon-Lars

    Jon-Lars Jon-Lars

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    Hear, hear!
    #36
  17. supershaft

    supershaft because I can

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    You didn't just come up with a carb. You said that my advise about TM carbs was wrong and it wasn't. You can't use a late model snorkel on TM carb like in the photo you posted. You can a TM smooth bore but I was talking about 'regular' TM's. I suspect you might make better sense sticking with forums in German? Self invented? I didn't invent TM's, Mikuni did. They also invented TM-6's and TM Pro series and TMX's and TM smooth bores and so on. How do you miss an entire point over and over? TM Pro Seires are NOT TM's. They are TM Pro Series just like TM smooth bores are not TM's. They are TM smooth bores. BIG DIFF between TM smooth bores and TM's! Clarify.
    #37
  18. supershaft

    supershaft because I can

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    You almost never get the meaning of what I write correctly so what's the use? Remember when I mentioned picking up revs and accelerating? What has that got to do with WOT high rpm? Nothing is the correct answer. The improvement concerns accelerating from low and mid rpms.

    I removed the pump arms so the pumps do not function just like a lot of tuners do.
    #38
  19. Carl

    Carl ,,,

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    Hey!
    #39
  20. Jon-Lars

    Jon-Lars Jon-Lars

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    What Carl said.

    Times two.

    If there ever was a reason to ban Supershaft, this is it.
    #40