DR750 & DR800 owners thread

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

  1. motolab

    motolab Long timer

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    At 21K miles, I'd be surprised if the slide guides (and therefore the emulsion tubes) were not worn out (unless someone has already replaced them). 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, and therefore the emulsion tube 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. Emulsion tube wear sets in somewhere around when the surrounding plastic is worn flush with the floors of the depressions, and causes overly rich mixtures in the lower portion of the rpm range (see https://store.moto-lab.com/articles/bst33-slide-guide-wear-indicators).

    Check the slides for vertical grooving on the downstream side, as well as for enlarged/ovaled needle holes (see https://store.moto-lab.com/articles/slide-wear-indications).

    Have the slides been drilled? You can tell if the insides of the holes are dull rather than satiny, or if the diameters are larger than 2.5 mm. Slide guide wear is pretty much proportional to lift hole area, so drilling the slide will dramatically increase slide guide, emulsion tube, and slide wear.

    Replace the jet needles if they're worn. This will be evident because missing anodizing will reveal a different color underneath. This is most commonly found right in the area where they pass through the slides (see the needle on the left at https://store.moto-lab.com/articles/jet-needle-wear-example). Also replace the needle if the clip no longer fits it tightly. The clips are easy to lose, so it's worth getting spares.

    Make sure that the white plastic spacers fit the needle tightly.

    Have the slide springs been clipped? You can tell by whether they have an open coil on one end. If so, they should be replaced, as a clipped spring causes low rpm richness issues. Also replace if the sides of the coils are worn (as this changes the spring rate).

    Inspect the tip of the float needles with and without magnification to see if they have witness lines where they have been contacting the seats. If so, they should be replaced. Float needles with witness lines cause a higher fuel level, which in turn causes a richer mixture (and at some point overflowing).

    Replace the float seat o-rings if they do not fit snugly or are hardened/deformed/shrunken/cracked/otherwise damaged. Leaky float seat o-ring cause a higher fuel level (and at some point overflowing), which in turn causes a richer mixture.

    If both the float needles and needle seat o-rings are in need of replacement, it's probably best to just get rebuild kits, as the rest of the rubber parts are likely to also be in poor condition (for example it's virtually guaranteed that the slide guide o-rings and fuel screw o-rings will be smashed flat).

    Selecting o-rings that are made from, or kits that contain FKM (aka "Viton") o-rings will significantly extend the service life.

    Inspect the float hinges for wear (pivots should not be loose, pins should be round instead of lozenge shaped, bores should be round instead of oval).

    Make sure the floats are not fuel logged (should weigh 6.1g or fewer when separated from the cage).

    I recommend replacing the pilot jets with a fresh stock sized ones (40) as a matter of course. It's difficult to be sure that they're clean and they're cheap.

    People often strip the heads of the pilot jets. This can be avoided by using a driver with a round shank that closely fits the well and has a hollow ground blade no wider than the head of the jet. There is a purpose-made driver available.

    You will need to set the float height before the float bowls go back on. Use a float height setting tool (ideally itself set with the tail of a caliper) to accomplish this accurately. The stock setting is 14.6 mm. Instructions with pictures: https://store.moto-lab.com/articles/float-height-setting-tool-instructions-bst33-carburetors.

    You can replace the easy to strip and prone to corrosion JIS float bowl, diaphragm cover and throttle cable bracket screws with stainless steel socket heads.

    You can add an the inlet fuel filter inside the inlet hose barb (like the DR650SE has).

    Adjust the idle mixture via the following procedure:

    Start the engine, let it warm up and ride the bike around until it's hot. Using the idle speed screw, adjust the idle speed to 1300 rpm (for accuracy, use an aftermarket electronic tachometer - you don't have to mount/permanently install it if you don't want to). Set the CO to 3-3.5 %. If gas analysis is not available: coming from the lean side, adjust the fuel screws so that the strongest idle is achieved. You will notice there is a threshold where it becomes rich enough (enough turns out) to run strongest, beyond which no change is noticed. Adjust the idle mixture screw ~1/8-1/4 turn out from this threshold. Adjust the idle speed back to 1300 rpm (as it is likely to have changed). Leave the mixture to the leaner side of these settings if the bike will be seeing altitudes much higher than the one it was set at. Set to the richer side if you would like the engine to idle well earlier during warm up. Extended/remote fuel screws simplify the process and allow adjustments on the fly if you will be riding at extremes of altitude. Report back with the resultant settings.
    BST33 carburetors have cold start enrichment circuits instead of chokes. A choke is closed when an engine is cold and opened when it is warm, whereas an enrichment circuit is opened when the engine is cold and closed when it is warm. A choke works by restricting the air flow and by lowering the pressure inside the venturi, which causes the existing jets to meter more fuel, whereas an enrichment circuit adds extra fuel without restricting the air flow. A choke requires a fast idle cam or something akin to it (or you have to hold the throttle open manually), whereas an enrichment circuit adds a little extra air simultaneously with the extra fuel, so the fast idle is already built in. Chokes are not commonly found on motorcycle carburetors nowadays. The last one I can think of (on other than some type of minibike) is the Keihin VB42C found on the '83 CB750 Nighthawk.
    Aftermarket metering components often flow differently than genuine Mikuni, even if the are marked as being the same (even this may not the case with kits sold for variations of the same model across multiple markets). The resulting running problems have been reported in various threads.

    The o-rings will be Nitrile (a.k.a. Buna-N) instead of FKM (a.k.a. Viton), which means a shorter service life.

    A good exhaust will cause the engine to suck on the carburetor harder, requiring the same or slightly smaller jets.

    Regards,

    Derek
    olsburk and ihaha like this.
  2. Ladder106

    Ladder106 It's a short cut, really

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    Derek,

    Many thanks for writing out that summary......again.

    Hope things aren’t too chilly in Iowa.
  3. motolab

    motolab Long timer

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    Sir, you're very welcome!
    Not too bad right now.

    Regards,

    Derek
  4. Julesp

    Julesp n00b

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    Location:
    England
    Yes.. thanks for all that...very informative...I have returned the sr43 kit and am awaiting an sr42 one made by Keyster ...should be good after I change all the bits...I'll report later.
  5. Ladder106

    Ladder106 It's a short cut, really

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    You'll likely find the Keyster kits do not include the grey slide guides nor will they usually include new needles and emulsion tubes.

    You should be able to source the grey slides from Moto-lab or from a local BMW dealer. They were used on the BMW Funduro model. There are also available from KTM but dont remember the models they were used on. The dealers might be able to help by just giving them the BST33 carb description.

    I think the needles and emulsion tubes can still be sourced from Suzuki.

    Also shipping from the USA to the UK is, I think, much less expensive than the other way 'round.
    Julesp likes this.
  6. motolab

    motolab Long timer

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    There is no BST33-equipped KTM I am aware of.

    Regards,

    Derek
  7. Ladder106

    Ladder106 It's a short cut, really

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    Hmmm...Someone mentioned that a particular KTM model used those slide guides. Cant find the post so possibly wildly inaccurate.

    I do know that you can get them from BMW since I've done that before I found Motolab and your amazing carb parts inventory.
  8. opanos

    opanos Been here awhile

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    Rotterdam, Flatland
    Does anyone know the intake/exhaust valve size and the cam specs of the 800? This is quite an interesting engine and I wanna see what potential is left on the table with some head work. Thanks!
  9. Ladder106

    Ladder106 It's a short cut, really

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  10. stavrouliasp

    stavrouliasp Adventurer

    Joined:
    Oct 16, 2009
    Oddometer:
    28
    Hello.

    I am about to buy a German SR43B dr800. Is there a way I can find out if it is a restricted model or not?

    If it is, can it be derestricted?