DR750 & DR800 owners thread

Discussion in 'Thumpers' started by MCmad, Jul 14, 2008.

  1. Yuza

    Yuza Adventurer

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  2. Ladder106

    Ladder106 It's a short cut, really

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    [​IMG]

    Here is a copy of severe slide guide wear. The thing to know is that your needle jet is likely worn out by the time the small circles cast into the guide are worn off.

    Yes, PN #5 in the BMW diagram is what you want. Odd that the O-ring is NLA
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  3. Yuza

    Yuza Adventurer

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    Following your and Stefan's advice I will order and install larger idle jets (1st step upwards) and new needle jets and jet needles with stock parts. Plus the "easy to adjust" mixture screw

    This shall allow a slightly richer mixture at idle and off idle and a better mixture control over the 0 to 3/4 throttle range (at least I hope so!) while still being not an over expensive fix (100€ or so)

    Slide guides seems still ok so I will postpone this expense
  4. Ladder106

    Ladder106 It's a short cut, really

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  5. motolab

    motolab Long timer

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    Are the jet needles and emulsion tubes being replaced because they're worn? If so, the slide guides are likely to be worn as well, as it is slide guide wear that causes emulsion tube wear. Jet needle wear typically sets in last, that is well after the slide guide, emulsion tube, and slide are well past their points of replacement.
    There are six depressions in the face of the slide guide. Four of these are round. If any of the round ones are no longer visible, there is no need to look any further, the slide guide is worn out. There are two more depressions that look like little parallelograms concentric to the bore adjacent to where the slide bottoms. These depressions are .010" (.25 mm) deep when the slide guide is new. With an aluminum needle, emulsion tube wear sets in somewhere around zero depth, and causes overly rich mixtures in the lower portion of the rpm range. Compare the circled areas of [​IMG]to [​IMG] (these pictures are of BST36 items, but the principles are the same). How deep are the depressions now (measure with the tail of a caliper)?

    Regards,

    Derek
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  6. Yuza

    Yuza Adventurer

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    I can measure them but how these depressions and their being worn or not shall affect the carburation? I do not get it...

    Stefan said if there are no cracks there is no need to replace them. I am not saying he is right and you are wrong of course, I'd just like to understand this

    Thx
  7. motolab

    motolab Long timer

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    The wear does not directly affect the carburetion. As already mentioned, slide guide wear causes emulsion tube wear, which in turn causes rich mixtures at low rpm across all openings.

    Regards,

    Derek
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  8. Ladder106

    Ladder106 It's a short cut, really

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    Yuza,

    While you know your budget best, carb removal and disassembly down to replacing the emulsion tube and needles seems a waste of a lot of work if the slide guides are not replaced if they are bad. At least inspect them. Derek has many hours of work with the BST series of carbs. He uses a dyno and gas analysis to do jetting. I've always had good results from following his advice.

    Derek sells the slide guides for about $40 USD. Dunno how that compares to Stefan's price or the BMW price.

    In any case, be very careful when reinstalling the carbs. It's a VERY tight fit. You can make it a bit easier by disconnecting the airbox from the frame and moving the airbox back and upward as much as space allows.

    Then be totally certain that the flexible boot between the carbs and the airbox has a complete seal around the airbox and both carb mouths. Use a good light source and inspect it from as many angles as you can. It's very easy to have a gap or a fold in the connector.
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  9. motolab

    motolab Long timer

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    $33.07 ea. for the BST33.

    Regards,

    Derek
  10. Yuza

    Yuza Adventurer

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    Carb removal is not a problem, I have done it few times already (remember I had overfueling issues?)
    They were ultrasonically cleaned and inspected by a professional mechanic and renewed in the lower part.
    I think they are ok (as said bike starts fine, idles fine and apart from having probably a too lean mixture just off idle also runs fine)
    Anyway thanks for the advice, I will consider replacing the guides as well
  11. motolab

    motolab Long timer

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    Were the emulsion tubes and jet needles replaced because they were worn? Or was there some other reason?

    Regards,

    Derek
  12. Yuza

    Yuza Adventurer

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    I only replaced idle jets, fuel needles and seats and o-rings when I cleaned the carbs

    Rest shall be still stock with 44k km (I bought the bike last september)
  13. motolab

    motolab Long timer

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    So by fuel needles, you mean the float needles?
    Here you specifically said jet needles (I don't know what an "so needle" is).

    With 44-45K km (27340-27962 miles) it would be very unusual if the slide guides (and therefore the emulsion tubes) were not worn out (unless someone has already replaced them).

    Regards,

    Derek
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  14. Yuza

    Yuza Adventurer

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    Hi sorry I missed your reply
    I have replaced pilot jets, float needles & seat, not jet needles, needle jets and slide guide (sorry my previous message wasn't correct) which might still be the stock ones (no idea to be honest if some of the previous owners have replaced them)
    According to Stefan, if guides are not cracked/broken there is no reason to replace them, same for jet needles. He suggested as a first try to just increase pilot jets since it seems to be lean just off idle
    When I replaced pilot jets I ordered from a suzuki dealer and in suzuki catalogue there is just 1 size available for both carbs and it is 45, so I might try 47 or so
    But, in my order to Stefan I have also taken new needle jets and jet needles so I will replace them as well.
  15. motolab

    motolab Long timer

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    I read that the first time you wrote it, and responded accordingly.

    I will attempt to explain one more time (after which I am giving up):

    Normally, the slide guide holds the slide in a position such that the slide's jet needle bore is centered over the emulsion tube's outlet bore. As the slide guide wears, the slide becomes offset in the downstream direction, causing a misalignment between the slide's jet needle bore and the emulsion tube's outlet bore. The misalignment causes the jet needle to bear harder on the downstream side of the emulsion tube's bore, causing accelerated emulsion tube wear. At some point, the slide will become supported by the jet needle in the emulsion tube's bore, rather than by the slide guide. At that point, the slide's needle bore will begin ovaling along with the emulsion tube, and eventually the jet needle will wear in the area it passes through the slide. Emulsion tube wear sets in when the slide guide is worn to a point (or beyond) where the depressions in the bottom corners of the guide adjacent to the bore are no longer visible (see https://advrider.com/index.php?threads/dr750-dr800-owners-thread.362489/page-879#post-35568553). You may not be able to see the emulsion tube wear, although at 44-45K km (27340-27962 miles) it should be pretty obvious (sometimes spinning it between your fingers will help - just like a wow in a wheel can sometimes be seen better when it is spinning). A case where it is not visible does not mean it is irrelevant or has no effect. Keep in mind that Mikuni's needle jet (an emulsion tube is a type of needle jet) size increment is 0.005 mm (0.0002"), and that this is a change that can unmistakably be seen as a change in CO% on the dynamometer. That said, there is in fact no need to inspect the emulsion tube separately - if the slide guide is worn beyond where the bottom corner depressions are visible, the corresponding emulsion tube is guaranteed to be worn out (and causing excessive low rpm richness).
    As delivered, the mixture is typically lean from idle up to 1/4 opening. Raising the needles addresses 1/4 opening, while correctly adjusting the idle mixture address idle. 1/16 and 1/8 openings are likely to simultaneously be appropriately addressed on the overlap.
    Your new emulsion tubes will not last long if the slide guides are worn out.

    Regards,

    Derek
  16. Yuza

    Yuza Adventurer

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    Thanks for the explanation. What is still unclear to me is why the guide worns on the face where the depressions are. I understand it worns where the valve slides up and down million times and how this will affect directly needle and emulsion tube wear
  17. motolab

    motolab Long timer

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    The slide slides up and down on the face where the depressions are.

    Regards,

    Derek
  18. Simosez

    Simosez go ride your bike.

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    This thread needs some photos.
    I got out this Arvo for a few hours.
    IMG_20180812_161315371_HDR.jpg
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  19. Yuza

    Yuza Adventurer

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    I still need to remove and inspect the carbs (maybe tomorrow), but since today I removed the fairing to do some electrical mods (installed a trailtech frame with bulbs over the trailtech dash and ignition lock from SR43), I had a look at plugs and they are both full of carbon deposit

    I also checked meanwhile with the suzuki dealer where I bought the pilot jets and he said suzuki only gives n.45 ( standard on SR423 but SR42 should fit n.40) so it seems I am running already bigger jets

    I am now thinking I am running too rich and not too lean at idle, also given the black plugs, do you think this can be the reason for bad running at low throttle opening I was reporting before?
  20. Yuza

    Yuza Adventurer

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    Ok carbs removed, here is what I found
    Pilot jets are indeed n.45, while SR42 shall fit n.40. I think I will return to stock size

    Main jets are 117.5, stock size. No need to change them for the moment

    Mixture screw is out 2 turns, maybe this combined with larger pilot jets is the reason for bad running off idle?

    Jet needles seem a little worn (shiny spots)
    [​IMG]
    [​IMG]

    Slide guides are also worn (now I understand that the face with the little depressions is where actually the valve slides onto...), will place another order to Hassler
    [​IMG][​IMG]

    Emulsion tubes and jet needles are already in the order
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