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Discussion Starter #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.
 

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Discussion Starter #3
are the snorkles in the airbox lid?
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.
 

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You are in main jet territory at those rpms. You might want to bump up a size in the main jet to see how it runs. I assume by your address that you are close to sea level.
 

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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.
 

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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!
 

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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.
Seems odd that the bike runs perfectly to 6000 rpm if this is the case.
 

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Discussion Starter #9
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!
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.
 

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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.
As far as I recall the canisters under the seat are only for the crankcase ventilation. The Carb diaphragms breath through the small triangular breather boxes on the left side of the bike. The image of this has been posted previously when someone couldn't work out how it went back together.


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.
I have found that with the FCR's on my bike it's happy to putter along in slow traffic where the speed is changing a lot in 2nd or 3rd ( at 3k or lower)vs 1st where you have to be so gentle on the throttle opening or closing to avoid jerking. It doesn't complain at all. I run out of the box pilot adjustment which is on the rich side but it avoids hanging idle. Maybe it actually reinforces your point as I probably use more throttle as the bike is fighting a bigger battle against the gearing than in a lower gear.
 

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Discussion Starter #11
just tape over more of the hole progressively until it runs well.
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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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.
 

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Discussion Starter #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.
 

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+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.
 

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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

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.
 

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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.
Hmm - maybe I got lucky with my reluctance to actually chop up my airbox lid. I'd just looked at the size of the inlets through the snorkels, and eyeball calculated that the total area was less then the area of one carb mouth - thus restrictive. Removing the snorkels, gave me two holes, either of which was well bigger than a carb mouth - thus no (or little enough to not bother about) restriction. Plus, coming from a dirt bike background, the removal of snorkels and opening up of airbox intakes was one of the first performance mods made to any of my bikes.

It still seems kinda weird that taping up the hole a bit made that much difference - but it's obviously worked for you, so I can't say it's a rubbish idea. ;)

As an addendum to that idea - rather than tape - have you thought of gluing some flat plastic into the appropriate position? Then you can just trim the edges to obtain the original hole size.
 

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Discussion Starter #18
As an addendum to that idea - rather than tape - have you thought of gluing some flat plastic into the appropriate position? Then you can just trim the edges to obtain the original hole size.
Yes Steve I will be buying a stock air box and snorkels or fixing this one.:smile2:
The tape was just a 2 second fix to test out Brad Blacks suggestion which no big surprise was 100% on the money.
I am a ex pat kiwi so some number 8 wire did cross my mind! But no bush mechanics I will fix it properly.

I have no desire to re jet this gutless old heap for the extra few anemic Horsepower it would net and I don't want to read my newspaper through the drilled out air box
top either.I just want it to run properly so when it rains I can ride it to work so I don't have to dirty up my good bikes.
Happy days mission complete.
 
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