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Dynojet Woes With A Helping Of Sooty Plugs

Discussion in 'Supersport (1974-2007)' started by Woolfie, Sep 14, 2021.

  1. Long story short - 900SS that has never run properly in the time I've had it and has been sooting up plugs to the point of them getting wet with petrol and misfiring.

    The bike as I got it:
    Termi pipes with baffles inserts in tailpipes
    AS Uotani ignition box and coils
    K+N filter with airbox lid fitted inc trunks
    Mixture screws 3 turns out
    On carb inspection I discovered dynojet needles (on 4th clip down) and diaphragm springs but still running Mikuni 140 main jets! Also the float heights were 12 and 13mm
    Bike would start without choke

    Ah hah! thinks me, the mikuni jets will be the problem. So I ordered the 150 jets from dynojet, removed the airbox lid and exhaust baffles and set up the carbs as per dynojet instructions - needles 4th clip from top and mixture screws 4 turns out. Set the float heights to 14mm.
    Increased the plug gap to 1mm as I believe I can get off with it with the ignition set up.

    Test ran the bike for a few miles but the plugs still sooted up, although not to the point of misfiring any more.
    I've since dropped the needles to the 3rd clip down and set the mixture screws at 2 turns out but it didn't make any difference on the plug colour.

    I've also inspected the choke for sticking on but it isn't so I've ruled that out.
    I don't know if the dynojet kit is a stage 1 or stage 2, I see there's one number difference on the needle type - would this make such a difference to the running of the bike?

    Also the bike feels a bit stumbly and rough sitting at a steady speed but smoother on acceleration, I think the dry clutch rattling maybe makes this feel worse than it is though lol.
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  2. Hopefully someone will come along soon for the full diagnosis Woolfie, will flare up my tennis elbow if I attempt to, but for starters:- you wrote that you started off with plug sooting and wet plugs - but then you've decided to fit larger main jets. :thinkingface: I can't remember all the Dynojet stages and jet recommendations but I found people tend to fit them to std/near std engines expecting a noticeable power gain but this is rarely the case, and unless work or changes have been carried out to porting/cylinder heads/valve diameters/C.R. and camshafts, then Stage 1 makes the most sense "just as an experiment*". A personal opinion but the kits are often more trouble than they are worth, for several reasons. Have a look for back-to-back rolling road tests and don't take my word for it.

    *at least worth a try with your free-er flowing exhaust and K&N.
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  3. In my experience, it could equally be the result of an IGNITION issue, so check all that aswell
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  4. That sounds exactly like my superlight. Black plugs etc after a few miles and limping home.
    Mine has a K&N filter in it and came with a box of dynojet bits that had come out of it. A standard filter makes no difference. It does have high comp pistons and a full system. The one time it did work for 30 miles along a sweeping A road it was epic!!!.
    I have no idea about carbs etc so will be taking it somewhere, probably Baines racing early next year for them to sort.
  5. What you're saying makes perfect sense and I don't know why I didn't see this for myself before now.
    I seemed to have it in my head that it must be a stage 2 kit for some reason, I think what I should probably do now is get back on the phone to dynojet and order the appropriate jets for their stage 1 kit and give that a try. I'd rather give the stage 1 setup a proper go before I give up and go back to standard as I'd propably still have to fart about with jetting for the pipes/filter setup anyway.

    As a side note I was lead to believe the dynojet sizing was completely different to mikuni, so I thought a 150 might be leaner than a mikuni 140. To the naked eye there was basically nothing in it although the dynojet had a steeper taper in it.

    Thanks for your help so far!
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  6. Thanks, I'll maybe go down Chris' route first though as I don't think there's an ignition issue. Tickover is pretty good and so is WOT. Increasing the plug gap seemed to be help the plug fouling slightly too I think.
  7. what's irritating is that the Dynojet needles have no part numbers on them, so it's down to either skilful recognition or in my case carefully measuring in increments for reference, as i did for a young member on here when he first got his 750SS. ;)
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  8. Also consider that that there may be oil getting into the cylinder, (it won't be the issue, but it can contribute). The valve guides on these engines were made of cheese - Baines stated mine were worn after 6K miles!

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  9. No sign of oil though - thankfully!
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  10. agree with all said above, regret no experience with dynojet kit but my Factory Pro kit did wear the emulsion tubes oval and gave similar symptoms.
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  11. Thanks for the link. Are the Keyster kits good quality? I'm actually asking for the reason that they also do carb rebuild kits for the Suzuki RG500 and I need to buy needles and needle jets for that too... :thinkingface::laughing:
  12. I haven't bought a kit for a while but tentatively. yes - good stuff. :upyeah:I still have some needles kicking about but haven't fully stripped a pair of Mikunis for about 8 years, I bought a brand new pair when the price was reasonable so at my age, I would probably just fit these if the need arose.
    i've no idea what's a "correct" price though, as so many changes now.
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  13. Might be worthwhile checking the emulsion tubes as they will go oval with use and give you all sorts of running issues
    The keyster kits are really good quality and include a replacement tube, if you want anything I use NRP CARBS, really helpful and really good price wise
    Another thought since you’ve been changing main jets is that the gaskets of the jet mounting plate can fail and draw in fuel causing rich running problems
    If you rebuild with new gaskets and seals your problems may disappear
    Keep us posted
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  14. UPDATE:

    I took Chris' advice and ordered YJ124 main jets from Dynojet which is what they specify for a stage 1 kit with K+N/exhaust combo.
    Big mention here too for Dynojet, they were super helpful and friendly on the phone, didn't charge much and despatched stuff super quick. Allens Performance on the other hand didn't even bother their arse replying to me when I emailed them about RG500 Mikuni parts.

    Anyway, fitted the jets, cleaned pilot jets again thoroughly (just to make sure) and rechecked float heights with the tube method.
    Road tested it and found a huge improvement right away except for it holding back a bit under heavy load in higher gears like it was getting starved of fuel a bit. Plug colour was spot on though.

    Tried the easiest thing first - replaced the airbox lid, did another road test and now it's absolutely fine. A very happy Ducati owner here now :blush: (until the next thing goes wrong with it :laughing:)
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  15. Excellent news!!.
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