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Mikuni Carb Jet Block Questions (1994 900ss)

Discussion in 'Supersport (1974-2007)' started by Pete W, Nov 15, 2019.

  1. I'm doing a full stripdown and clean of my standard 900SS Mikuni carbs, (BDST 38 - B67). All is now cleanish but O rings etc were knackered or perished so will be replaced, also I'll replace the main and pilot jets and their o-rings.
    However, the brass bolt holding down one jet block was broken (snapped inside the emulsion tube, but I got it out ok) meaning one jet block wasn't held down tightly (!!). Bit odd but there you go.
    First question: to hold down the jet block is there any reason why I cant use some other screw or bolt with the same thread rather than the original Mikuni part which was hollow?
    Next question: what is the thing inside the jet block under the 'pilot' jet? in the attached picture? Can it be removed to be cleaned because I think it might be clogged up (not absolutely sure).
    Thanks for any help, carbs are not really my thing and the terminology appears designed to confuse - one time it's a pilot jet, then it's called an idler jet, then something else is a 'jet needle' as opposed to a 'needle jet'. Ridiculous, pass the nurofen Jet block.jpg
  2. Pilot/Idle jet.......probably marked 42.5. Should have a screwdriver slot....is that an O-ring in there? Shouldn't be.

    Needle jet would be 'emulsion tube' .....the brass bit the main needle (jet needle) slides into.

    You should also say if you have any numbers or markings on the main needle and the jets.

    Stock Mikuni will have a size number and a little square logo.

    Dynojet don't have anything on the main needle and only numbers on the main jets.
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  3. A needle jet is also called an emulsion tube (I think)
  4. See my post above yours, OR.

  5. Yes, just seen you posted 3 minutes before me.
    Tis all rather confusing with different names for the same thing.
  6. If you managed to remove the float bowl screws undamaged, you did better than me.
    They are very tight, which is presumably why Dynajet supply stainless replacements, included in their jet kits
  7. I’ve still to remove a seized mixture screw from one of my spare carbs. Ultra-Sonic cleaning hasn’t released it. Any suggestions before I attack it with a drill and Torx bit?
  8. Thanks for the repiles, and apologies because I made a mistake right from the off, in my first photo it's the thing beneath the 'choke jet' (Mikuni R70) in the jet block which I wondered about, not under the pilot or main jets. By the way all my jets are standard according to the spec sheets I've seen, no mods.
    The brass bolt which holds down the jet block and screws into the emulsion tube is hollow - is it hollow so the tip of the needle can go into it, which means I cant use any old small bolt with the same thread?
    Thanks again

  9. Here are a couple more pics of the hollow brass 'bolt' in question, (one of mine snapped which is why I'm interested in it).


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  10. I got one out using a low power battery drill and using lateral pressure to almost polish it out, with the side of the drill bit, after I’d drilled down the centre.
    I did go up a size or two after the initial hole but didn’t attempt to drill it “vertically” all the way with the cutting edge of the drill bits.
    Worked a treat for me and for someone else on the old SS yahoo group after I posted about it :)
  11. I’m guessing your bolt is a jet but Al is much better on carbs than I am.
  12. Keep soaking it..........if you go in too heavy you could screw up the carb body.
  13. Get replacement stock items if the ones you have are buggered.
  14. Have you tried gently tapping it..... with a sledge hammer...:astonished:
  15. Soaking has to be better.
    The reason I resorted to a drill is that I telephoned a carb specialist for advice about what to soak it in and he couldn’t recommend anything that would work.
    #15 Old rider, Nov 16, 2019
    Last edited: Nov 16, 2019
  16. Thanks for the feedback guys, I now know I need the correct brass 'screw' to hold down the jet block and not just any old thing with the same thread. I found one online (see pic) - the Yamaha TDM850 uses the same Mikunis so that made it easier to find. The 'mystery' thing located below the choke jet in the jet block which I asked about when starting this thread doesn't seem to be removable but I've managed to unblock the passage with a bristle from a plastic brush so good news there. All jets and mixture screws, even the air ones, came out no bother thankfully. Cheers Pete

    brass screw.JPG
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  17. When cleaning these BDST38 carbs, the main thing to concentrate on is the pilot circuit......it is usually the cause of rough running and difficulty in balancing......

    ........there is a gallery in the carb body with very tiny fuel ways, which you can't really get into.......some people are brave enough to lever out the gallery plug to get in there, but they usually end up getting another carb body....

    ....this one of the reasons why I use the Wynns carb cleaner, because it is high pressure particularly when using the tube nozzle.

    You can insert the tube (or hold it tight over the holes) and blast the stuff into the circuit and see it come out of the various orifices (some you will have to hold a finger over to block them off, in order it gets into the other areas).

    For example, with the pilot screws out, it will come out of them, whereas when they are in it will come out in the carb throat.

    Having done it several times on each carb, I attached my compressor and blasted 125psi through the galleries.

    But be warned.....mind your eyes when using the cleaner.
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  18. i have used lighter gas (butane) but you cant beat the ultrasonic bath, i gave my miki's 3 sessions the other day at 30 mins a time , perfect it seems to clear those concealed galleries better than anything.
  19. Thanks for the great cleaning advice.
    AL, when you said "........there is a gallery in the carb body with very tiny fuel ways, which you can't really get into.......some people are brave enough to lever out the gallery plug to get in there, but they usually end up getting another carb body", did you mean this thing in the attached photo (referred to as a "pilot merge cavity bung")?
    I have a new o-ring for this on order along with the gasket and o-ring service kit but I haven't tried to remove it yet and it seems very tight.
    Cheers Pete

  20. That is the plug I mean.....a lot of damage can be caused trying to get that out....
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