There's no way to stop the wear that I'm aware of but I would expect them to last maybe 15,000 miles. I had to change them at 18,000 although they would probably have been better done a couple of thousand earlier.
I followed up on the info on the site that Derek linked to and found some more detail. From what I can glean there’s a design flaw in that series of carbs. https://store.moto-lab.com/articles/bst36-slide-guide-wear-indicators This is taken from the above link - this is what a new slide guide looks like: And below is a very worn one: The throttle slide itself is made of hard black plastic - harder plastic than the guide - so the guide wears out and starts to allow the emulsion tube/needle jet to wobble around - wearing it out prematurely. This occurs on other bikes with the same design of carb but I think the issue may be exacerbated by the vibey nature of the Ducati Twin. The rebuild kit doesn’t actually include the slide guide or needle jet so it wouldn’t solve this issue. It does include the slide itself and I would check that for any wear grooves too.
@Cammmy would you consider editing the thread title to replace ‘not the common culprits’ with Mikuni Carb Info or something similar? This is turning into a very useful thread on Mikuni carbs as used on Ducatis and it would be helpful for anyone in the future searching for such info (eg me ) if those words were in the thread title. It would be a shame for it not to come up in search requests where others have a similar issue. I, for one, never even considered the slide wearing in CV carbs, needles yes, but not the slide and its housing.
Up to the O.P. as ever, but you could just start a thread devoted to Mikuni info and ask it to be "stickied" Paul? Also - if you use the Search function, you will find over 30 items on Mikuni BDST 38 tuning within this Forum alone.
Okay, back on this. Just pulled the carbs and the jets say 144. From reading a doc previously provided in this thread, that's waaaaaaay too big? Seems the 900 wants a 140 and the 750 a 132.5? Looking at the emulsion tubes, I see a little polishing but they are definitely round to the eye. So, stock bike with SC Projects exhaust. Just go back to standard jet or dynojet stage 1?
If it were me I’d stay with the stock jets for now, otherwise you’re introducing another variable. Once I’d sorted out the rich running issues (and anything else) got it running sweet at idle, mid range and full throttle, then I might think about adding after market parts to ‘improve’ it. Jets are about €10 each plus postage https://www.stein-dinse.biz/index.php?cPath=12174_12293_12631_12633 admittedly that may end up a bit more expensive landed into the UK but it’s still a lot cheaper than the Dynojet kit. As you have the SC exhaust end can I’d go larger than the standard by 1 or perhaps 2 sizes, without a gas analyser (which you may have access to) it’s trial and error. I’d say 144 is too large and is the reason you find it running very rich.
Thanks. I don't have an opened up lid but am happy to do it if it actually makes a difference? I have a gas analyser but not one that hooks into the ports on the manifold, so I can't read individual cylinders or read it under load but can check combined idle values. I might order stock size plus the next two up. Cheers Edit: seems that the 144 is two sizes up. I may just go back to stock. I can't imagine an exhaust would justify going to the same jet as a 900?
I agree with Paul that the 144 main jets are too large the 900 runs a 140 and I found that even with open cans and an opened up air box the mains didn't need to be bigger. I wonder though if the jets are Mikuni ones? Mikuni jets (and Factory Pro) are numbered according to the orifice size. i.e. A 140 jet has a 0.140mm hole through it. Dynajet however use their own number system which doesn't correspond with Mikuni numbering. Although the emulsion tubes look ok eyeballing them so did the ones I took out of my bike but there was a very marked difference in how it ran when I replaced them. It seems to take very little wear to throw the mixture out.
Actually I'm wrong about Mikuni numbering being based on the size of the orifice. It appears that Dynajet are. Here are 2 sites comparing sizes between Dynajet, Mikuni and Keihin. https://faq.ninja250.org/wiki/Dynojet_-_Mikuni_-_Keihin_jet_sizes_conversion https://www.angelfire.com/pa5/cannondale/Mikunivs.dynojet.html
Thanks Derek I think what I'll do is go back to stock jets and keep in mind a full rebuild with new slides if it still doesn't run correctly. The idle screw adjustment should have an effect across the entire range, correct? I.e. dialling in should be possible in a small range? I would guess that's why Ducati say best power is made at Co readings higher than those dictated by emissions regs. Cheers
The idle screw will only affect the mixture at idle and very small throttle openings, up to maybe about 1/8th throttle.
Okay thanks, does it somehow shut off at higher load or is it just insignificant in the scheme of total flow at that point?
Hmmmm, a 'full' thread! @Cammy46 - I haven't seen anything about you setting the idle mixture on both carbs. If you have, was it easy to achieve? You've said a few times how good the bike is at full chat, so the (rich) 144 mains would explain that along with the black-ish plugs. I agree with the previous comments that one up on the mains should be enough for the more-open pipes. There's a lot of info on here somewhere about it being a waste of time making holes in the airbox lid, so I'd hold off doing that for now (especially as unmolested ones seem to be rare now!)... Hopefully you can persevere to the solution
Hi Cammy, you may be suffering like hell... But this thread is almost like a public service offering. You could almost consider it as "Taking one for the Team". Thanks!
Thanks. I've been on my back at the side of the road many a time adjusting the idle mixture. Had it as low as about 1 turn on the idle screws all the way out to 4. Never ran right.
Ha ha, put it down to new rider enthusiasm. It's been either fix this or don't ride. Though I have a second bike now but I feel I'm close to sorting it, which helps.
132.5 and 140 jets ordered with new o-rings. They also have clear float bowls available, though checking it isn't hard on the bike.
