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Carbs.Run out of ideas and things to try.

3K views 25 replies 10 participants last post by  Addicted24/7 
#1 ·
I bought a set of Mikuni BDST38 CV carbs of ebay from japan which where listed as 900ss.
They are brand new.
The bike runs perfectly from idle to 6000rpm at which point the bike starts missing and struggles to 7500 rpm.
I pulled them apart and found 37.5 pilots,140 mains Y2 needle jets and some unmarked needles that were shorter than the 5C19's which are now in there along with 42.5 pilot jets.
So now these carbs are brand new with new jets the float heights are 14mm.
The Diaphragms don't have any holes in them and there are no damaged or missing seals.
The carbs have been balanced and are spot on.
It has a stock air box and filter and stock exhaust.
I know it is the carbs because I fitted the FCR's off my 89 and it ran perfectly all the way to red line.
Any idea's appreciated.
 
#3 ·
No.
I googled Ducati 900ss air box snorkels too see what they look like and your 2 articles came up and it seems they are important !
My previous 93 900ss has FCR's and when I fitted the stock Mikuni's at one stage it ran fine with no air box lid and a K&N filter.
They had been fitted with a dyno jet kit (Which I had forgotten about until just now) so in my mind the air box lid on this one looked stock to me.
Now I have seen what a stock air box looks like the snorkels are missing from this one and the holes have been joined together by cutting a bit out.
A lot of bikes would not be affected by this too much but it makes sense that some do and early T300 triumphs run like crap in exactly the same way with the snorkels removed so I now know Ducati 900ss are the same.
Thanks for the lead Brad at least I have something to try now.
 
#5 ·
You need to remove the air cleaner lid and element and rev the engine. Make sure your vacuum slides are opening together without fluttering. Did you replace the o rings under the vacuum covers ? The air lines that breath for the vacuum slides run back to the canister under the seat on a stock bike. They have small screen filters in the lines that plug up with crud. Air goes in and out of the vacuum slide through these lines and they are thin and prone to collapsing and ripping causing the vacuum slides not to work properly. I replaced mine with fuel line and fuel filters that won't collapse and fuel filters will pass air but not plug easily. If air can't pass your vacuum slides open slowly or not all the way which screws up both fuel and air flow.
 
#11 ·
Thanks Brad (again!)
Just taping it up this far was enough to restore the top end power.
Finding a stock air box with snorkels is on the to do list.

Would be great to have a permanent thread on this site with "THE TOP 10 Dumb things NOT to do to your carby 900ss"
Randomly cutting holes in your air box to increase air flow would surely make that list!
 

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#7 ·
duc96cr - I learned something new today. My own bike carburates horribly at small throttle openings (super annoying around town) and I say the slides fluttering and just thought it was the big pulses from the cylinders. I will check mine this weekend. Thank you!
 
#9 ·
I would not get your hopes up Lambs foot I am new to Ducati's but all the carbed 900ss's (4 in total) I have ridden with the stock carbs fitted don't like small throttle openings at low rev's.They still stutter along with FCR's but they smooth it out and make the around town riding a lot nicer.I think it is just the nature of the mikuni's when fitted to these bikes but if I am wrong I would like to know the solution.
 
#12 ·
I'm kinda puzzled by this "don't run a 900 with standard carbs without the snorkels" thing. :confused:

That was one of the first things I did to my 900 when I bought it - brand new. Within a couple of weeks, the snorkels had been pulled out, and stuffed away in a bag (with a small gain in performance - those snorkels are very restrictive) - and it stayed like that for the whole time I had the CV's in it - and I never had any problems with it. OK - I fitted a K&N filter the first time it needed a new one, and then chucked a dynojet kit into it as well (which perked it up a bit) - then finally got around to a set of FCR's and exhaust, which really livened things up. :)

The only thing I didn't do was chop the holes in the airbox any larger - they seemed big enough to me. I did try running it without the lid - but the intake noise was too irritating, and I didn't notice any real difference in performance, so the lid went back on.
 
#13 ·
I'm kinda puzzled by this "don't run a 900 with standard carbs without the snorkels" thing. :confused:

The cutting of the air box by itself as a stand alone performance mod is the problem as just proven by me.I know it's ok with a jet kit or with FCR's because I have run my bikes like that.
Cutting up the air box is an extremely common modification on 900ss's because I had trouble finding a picture of a stock one.
The "THE TOP 10 Dumb things NOT to do to your carby 900ss" idea was poking fun at myself because even though I did not cut the air box I also had no idea it could effect the running of the bike so dramatically.
It takes someone like Brad Black who obviously knows these bikes inside out and is willing to share his knowledge to save people like me from my own ignorance.
I could have been farting around with this carb problem for a very long time if it was not for some Duct tape and the right information.
 
#15 ·
+1 ! On the top of my " goofy theory list" my air filter top looks like Swiss cheese. You could read a newspaper through it. If you believe proper jetting is just a theory, best leave it stock.
 
#16 ·
carbs

A quick and simple test is to try and remove air filter and box cover and drive it, If it runs worse, then it is too lean, if it runs better, you are too rich on main jet.
If it is too lean, then straighten out the snorkle issue before jet change.
Mark
 
#21 ·
i'll link this again: Brad The Bike Boy: Ducati 900 Carb models with and without airbox lids

i think it's actually the needles. i've run ss with open airbox lids and 140 mains and they've been fine, but only with jet kits fitted.

the cv carbs seem to react to changes differently to slide carbs.

again, it's a good example of making one change without the required second change (tuning it) and going backwards. or, in this case, going half back to std. the tape across the air inlet thing is just a tuning trick, you do it to assess problems/solutions.

if you put a jet kit in the carbs it will probably be nicer to ride at lower speeds too. that's what we used to find. so it'll be a nicer rainy day bike.
 
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