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Jetting Start Point – 900/966ss.

Discussion in 'Technical Help' started by Nasher, May 15, 2017.

  1. Can anyone suggest a starting point for re-jetting the Carbs on the 900SS mongrel I’m building?

    I’ve got to the point where I’m going to strip and clean the carbs then fit them to the engine.
    It will be ages yet before I can think about starting it up, and I will get it to a Dyno eventually, but whilst I’ve got the carbs apart I might as well get to a starting point.

    Spec
    M900 bottom end with V2 900SS Heads.

    966CC big bore kit from California Cycleworks.

    The head inlet tracts appear to be a lot smoother than I’d expect as standard, and are exactly the same diameter as the inlet tubes I’ve picked up, so I suspect they’ve had some minor work.

    Standard Inlet and Exhaust Valves.

    Standard Exhaust Headers.

    Carbon end cans of currently unknown origin.

    I’m using standard Carbs that when I brought them were said to be from a 900SS, but I’ve not checked what’s in them jet wise yet. (I’d love a set of Flatslides, but can’t justify the cost for a road bike)

    Thanks.

    Nasher.
     
  2. Have you got Dynojet needles and springs or OEM? Establishing that would be a good starting point.

    If you haven't looked inside, then you won't know, obviously; but as most carbed SSs got Dynojet kits fitted, there's a good chance you may have one.
     
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  3. upload_2017-5-15_14-41-7.png upload_2017-5-15_14-41-7.png
    Setting the float height can be tricky to do accurately, so make yourself one of these.

    According to Al, the 14mm dimension that everyone said to use at the time is wrong but the principle is the same and I'm sure he'll give you the correct dimension
     
  4. I don't know why it got attached twice... :/
     
  5. Unfortunately there isn't a specified dimension to set them at - just one that is right - 14mm IMO is wrong because it makes the fuel level too low and thus the pilot circuit doesn't work well. I would start at 12.5mm. But the only way to get them spot-on is to get it on a dyno.

    I don't know why I got attached four times - must have been mad.
     
  6. Four times?!? Wow.... :/
     
  7. Funny you should say that.
    I know my M900 is fitted with a Dynojet kit, and I've now opened up all three sets of carbs I've ended up with for the SS and all three have Dynojet Needles.

    Set one is the pair I was going to use, which as mentioned were reported to come from a 900SS.
    I brought these off Ebay last year when I just had the engine for the SS.
    They are very dirty on the outside, although the air screws are fine, but having opened them up now I'm shocked at how bad they are inside the float bowls. They look like they've been underwater, even the Dynojet needles are badly corroded.
    So they are only good for spares.

    The second set came with the 600SS donor bike I brought.
    These are in reasonable condition on the outside and inside, but unfortunately bear the scars of somebody trying to drill out both the seized air screws. I managed to complete the process to get one out, but the other had been butchered and the boss the screw fits into is broken away at the side. I'd say it's useable, but with no adjustment.

    For the above two sets I might just strip them right down, get the first set of bodies Ultrasonically cleaned and build up a spare set out of the two as spares to keep for either the SS or my M900.

    The Third set are a 750SS pair I took a real risk on recently on Ebay because they were so clean on the outside but the plastic tops were both broken. They were very cheap though.
    They have turned out to be spotless inside, the airscrews are free to turn, and the jets are not seized in.
    So I think I'll use these, with caps from above, and buy a new Dynojet kit intended for a 900SS.

    Nasher.
     
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  8. So the 600 and 900 use exactly the same carbs, just with different jetting?
     
  9. Personally when rebuilding carbs for projects i set to std settings and go from there
     
  10. These engines struggled to meet emissions standards for both exhaust and noise, so were pretty strangled as standard, which is why most were fitted with Dynojet kits. Not just for more power but smoother running too.
     
  11. Not quite.......

    ....there, that's thrown y'all......

    Carbs.jpg
     
  12. Nasher - Are you sure they are Dynojet needles? What tells you they are?

    Also what aboout the springs? Are they Dynojet as well?

    And - the jets, of course - their sizing is different to OEM jets.

    Note that the 600, 750 and 900 Dynojet needles are all different to each other.

    But if you write to Dynojet enclosing the old needles, they will provide you with new needles FOC.

    BTW - If you are absolutely certain you have a Dynojet kit fitted (which I can't be sure of unless I have seen a pic of spring; needle and a main jet) given the specification you have outlined in your original post, IMO the starting point for a DJ main jet would be a 155 as the standard 900SS Stage II DJ jet is a 150 which you may just get away with so it's worth having both sizes handy.
     
    #12 Ghost Rider, May 20, 2017
    Last edited by a moderator: May 20, 2017
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  13. Thanks Arquebus and Old Rider.

    OK, it looks like I could have been a bit daft here then.
    I'd assumed because the needles were adjustable, ie they had several circlip grooves, they would be Dynojet.
    Perhaps I'm wrong in that respect.
    I'll have to strip them all back down and get some pictures.

    I realised the needles would be different for each size engine, and as the only 900 set I have are corroded I'll need to get new anyway, but didn't realise I could get new from Dynojet FOC, thanks for that.

    I was under the impression that the carbs were the same on all three engines, and were just jetted differently, and the different part numbers reflected that.
    If they are not the same I need to rethink my way forward, so can somebody confirm it one way or the other?

    Also, showing my ignorance, what are a 900D and 900CH?

    Thanks

    Nasher.
     
  14. You aren't showing ignorance - I have as yet to discover what the two are - I suspect D means normal Desmodue and the CH may mean the Superlight; however the other reference you will come across is SP - Ally swingarm and CR - Steel swingarm.

    Sorry, got diverted when I was typing this.
     
    #14 Ghost Rider, May 20, 2017
    Last edited by a moderator: May 20, 2017
  15. Couldn't wait, so popped back out to the garage and took an image of the three different needles and springs.

    Perhaps none of them are Dynojet, as all three needles have the Mikuni Square within a Square logo stamped on them.
    They also taper quite uniformly, and I seem to remember Dynojet needles having a bit of a curve to the taper.

    What do you think Arquebus?

    The 600 ones markings were difficult to read, I think it's 5CJ1
    Both the 750 and 900 read 5C19, but are in different circlip positions.

    needles.jpg
     
  16. None of those items are Dynojet - and the square logo is the OEM Mikuni mark.

    Here is another extract - this one from the early-ish workshop manual.

    Carbs2.jpg

    Don't take my word for it, but IMO all the BDST38 carb bodies are the same - I think the different carb types Ducati refer to is actually a simple form of part number for each engine capacity etc.

    It may relate to the emulsion tube type and/or the pilot jet type - some have a single hole all the way through; but others have cross drilling as well which pre-atomises the fuel (and they make the engine run richer).........but they are respectively the same in overall dimensions and shouldn't be mixed ie one of one type and one of the other.

    I know that both the 750 and 900 ones I had, were the 'solid' type.
     
    #15 Ghost Rider, May 20, 2017
    Last edited by a moderator: May 20, 2017
  17. I suspect that D is for Germany and CH is Switzerland where the emissions regs were probably different from the rest of Europe, certainly in Switzerland anyway.
     
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  18. Well D and CH are the national ID plate symbols for those countries, equivalent to GB, so that does make sense.
     
  19. Well sh*g me backwards with a stiff bent poker - I never thought of that!
     
  20. Ouch... :eek:
     
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