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Discussion Starter · #1 ·
I’m at the wit’s end!

I installed the 944 kit on my 900ss sp. I’m still using the original Mikuni CV carbs. It runs really well once it gets going, but right off idle, below 2000rpm at throttle just cracked open, it bogs and stumbles, sometimes even pops in the airbox. I’m guessing it’s where the transition from the pilot circuit to the needle is, am I right? It idles well, and passed that, it’s all fine, just that area is problematic. I tried different pilot jets, I trieD different fuel screw settings, and I can’t dial it in properly. The carbs are synched perfectly. I’m totally lost as to what to do. Could you please help!
 

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I’m at the wit’s end!

I installed the 944 kit on my 900ss sp. I’m still using the original Mikuni CV carbs. It runs really well once it gets going, but right off idle, below 2000rpm at throttle just cracked open, it bogs and stumbles, sometimes even pops in the airbox. I’m guessing it’s where the transition from the pilot circuit to the needle is, am I right? It idles well, and passed that, it’s all fine, just that area is problematic. I tried different pilot jets, I trieD different fuel screw settings, and I can’t dial it in properly. The carbs are synched perfectly. I’m totally lost as to what to do. Could you please help!
On my 900SSFE, I had the stock carbs rebuilt 3 times, by the local Ducati shop. They are very, very, very hard to get 100%. I had a problem with the floats, jets & needles, which finally after a bunch of adjustments, the bike idled and ran the best ever. Then I sold her!
 

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93 900ss,2019 Triumph 1200XE, 2017 CRF250L RALLY, 2003 F650GS DAKAR, 1993 KLR250, 1986 HONDA TLR20
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You didnt mention rechecking the float level?
 
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There is probably a video, belter or someone, of how to do the basics.
Every last hole has to be clean, not just clear. Down to metal , without removing metal . Use guitar strings, tiny drill bits, whatever, but all passages must be completely clear.
Make sure when you buy a rebuild kit that you get a complete one with diaphragms. They aren’t cheap. Depending on the rebuild kit you may also need to buy a jetting kit so you can fine tune the carbs. You will want NEW needles and seats for the float bowls. Make sure the kit has everything or buy it all before you start.
learn how to set the floats
Make sure you have put all the new parts on, including o- rings.
Make sure every little bleed hole into the Venturi is totally clear. Find the source, and spray cleaner through it should SPRAY out , not drizzle, or it’s not clean.
Check your carb to intake connections. You can’t tune carbs with an air leak.
Most tuning kits come with good instructions and their are videos.
 

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I clean all carbs in an ultrasonic carb cleaner with a heavy duty degreaser & heat around 80 C--I run them for about 2 hours, then rinse in water very well then blow them out with air --checking all passages while doing this--then reassemble with all new o-rings & seals, normally at this time I also replace the float needle & seat, all jets & emulsion tubes--and if needed slide needles. And of course check & set float level
 

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The important part is that no matter how you clean the carb, check to make sure every orifice flows the same on both carbs when you’re done before re -assembly . Also, make sure when you reinstall the carbs that the diaphragms opens both slides the same. That’s why you should get new diaphragms.
 

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Discussion Starter · #8 ·
Hi,

Thanks for all the input,
the carbs are as clean as I can make it on this side of the ultrasonic. I do have access to one, so I may do that. No air leak or any that sorts. The jets, needles, emulsion tube, and all o-rings are new, I put brand new factory pro kit (I just happened to have it on hand, along with a new Dynojet kit). Only things I haven’t replaced are floats, slides, and diaphragm. Float height is currently set at 14mm. The timing is set at 3 degrees retarded from the factory setting (per recommendation for big bore kit). Ignition coils are from ca cycle works. Again, it idles perfectly, I can even ride it, but between idle to 2000, small throttle opening is extremely jerky, and transition to higher rev/wider throttle opening is very abrupt, and really can use improvement. Once passed that, it’s totally fine. I’ve tried different pilot jets, fuel screw settings, and even played a bit with needle clips, but I can’t seem to find the cure.
 

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i'd advance the timing as far as the slots in the pick up plate will allow and put an ignitech on it to give you control. or modify the flywheel lump if you don't want the ignitech.
 

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Discussion Starter · #10 ·
Belter, I remember you mentioned that to me on the other thread. Could you explain the logic behind moving the pick up vs. modifying the flywheel lump? Also, do you think my issue is ignition related? I sure am having hard time dialing it in with carb settings right now. If that’s the case, I can clearly see the benefit of programmable ignition. I’ve never done it before, is that difficult to do?
 

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Just to clarify you are not expecting to ride the bike at 2000 rpm.
The bike is stumbling as you come off idle to pull away?

Running a Ducati at 2000rpm is about the equal of running a Harley at 20,000rpm, there is no benefit to either the owner or machine. If the machine will not pull away from idle to normal, that's different. Running rpm 3500 rpm's are about the bottom for a 900 one well set up may do 3000 if forced but cruising at under 3000 is for other brands.
 

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Discussion Starter · #12 ·
Ducvet, exactly as you described, it’s the problem with pulling away from idle. That where hinges are jerky, and extremely abrupt. It wants to bog down, and it even puffs through the airbox occasionally.

I thought of something, so I hooked up the vacuum gauge again. The carbs are well balanced, idle seems to be as good as any. What I notice was when I just crack open the throttle from idle, before vacuum increases, it momentarily loses vacuum a bit from level at the idle, and I’m wondering if that’s where the problem is. Or maybe not, I’m losing my mind!
 

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popping into the airbox is not abnormal --When the bike is cold, mine does it as well --but once it's hot & up to operating temp that goes away. --while riding keep it spinning over 3500 rpm and it may be happier.
 

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I’m at the wit’s end!

I installed the 944 kit on my 900ss sp. I’m still using the original Mikuni CV carbs. It runs really well once it gets going, but right off idle, below 2000rpm at throttle just cracked open, it bogs and stumbles, sometimes even pops in the airbox. I’m guessing it’s where the transition from the pilot circuit to the needle is, am I right? It idles well, and passed that, it’s all fine, just that area is problematic. I tried different pilot jets, I trieD different fuel screw settings, and I can’t dial it in properly. The carbs are synched perfectly. I’m totally lost as to what to do. Could you please help!
Would FCR's help, or hurt?
 

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IMHO, FCRs will help a LOT. Been biting my tongue trying not to mention them as this thread was about working with the OEMs, but you opened the door.... ;-)
 

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Would FCR's help, or hurt?
FCR would be great, if you can tune them, or if you buy a set from someone “ready to go” brand new , out of the box. But don’t expect perfection. Someone with knowledge and an understanding of your modifications and altitude should be able to get close.
 

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I have bits out of several different tuning kits in my stock carbs. and it was tuned at as close to sea level as you can get without getting wet. From that to about 800 ft elevation I haven’t seen enough difference to warrant any kind of jetting change.
 

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I will say this. If I were to want to change carbs on my SS, I would probably not buy FCR’s unless I bought a whole kit . If I wanted to change I’d use something a bit simpler.
 

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I'd try richer pilot jets. On slide carbs that's where the slide cutaway comes into play, and the transition holes just in front of the throttle plate[ or back side of the slide on slide carbs]. they can clog even with the idle hole[closest to the engine] open. Also, are the valve clearances correct? Too tight will really confuse things. Especially tight intake openers.
FCR's will really wake things up though!
 
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