Ducati.ms - The Ultimate Ducati Forum banner
  • Hey Everyone! Enter your bike HERE to be a part of this months Bike of the Month Challenge!

81 - 99 of 99 Posts

·
Premium Member
Joined
·
5,441 Posts
By all means try with the cut upp lid as that will lean out the mixture a fair amount enough to see if it improves or gets worse.
 

·
Registered
Joined
·
7,402 Posts
The thing is, the needle and the emulsion tube wear together in the area below half throttle, where most riders at times want to hold a steady rpm. Also, air flow pushes the needle to be in contact with the emulsion tube causing the wear. Egg shape emulsion tube orifice and needle with a flat spot in one area are the results.
 

·
Registered
Joined
·
504 Posts
Discussion Starter · #83 · (Edited)
I finally got to ride it properly today. First of all, i really needed it! It was fun! 944 basically still has the same characteristics, just pulls a bit harder and a bit punchier.
Let’s see…ridden and warmed up, it does not pop or bang, so that is good, a huge improvement. It still stumbles as pulling away from dead stop, especially if you roll on slowly, and the transition is still abrupt, BUT it’s a lot more predictable and basically I can ride around it. I still have no idea what’s causing it, but I’m pretty sure that it’s right where the slides start to open, and vibrate as if there isn’t quite enough vacuum to open them. So I’m pinning my hope on the new diaphragm, and maybe swapping the springs with the one from Dynojet (it’s the softest, and longest of the bunch).
I also tried the open airbox. It distinctively improves throttle response, didn’t make ‘stumbling’ any worse, just more noticeable (so it’s worse then), and higher rpm a bit crisper, so I may still be a bit rich in general. I have to say though, it’s so loud! Can’t say I’m a huge fan.
Boy I needed that ride, it felt really good to do that.
 

·
Registered
Joined
·
7,402 Posts
Good deal ! If you’re still at 3-1/2 turns on the mixture screws crank them out another 1/2 turn at a time and the stumbling will get better.
 

·
Registered
Joined
·
9 Posts
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 93 900ss thé CV carbs are an issue cleaned rebuilt but still nothing good. Smart Carb is my next move. Spoke with Scot P with ItalianIron Classics. italianIron.com. Speak with him. Scot from what everyone says in extremely knowledgeable on carbs.
Doug
 

·
Registered
Joined
·
504 Posts
Discussion Starter · #86 · (Edited)
On my 93 900ss thé CV carbs are an issue cleaned rebuilt but still nothing good. Smart Carb is my next move. Spoke with Scot P with ItalianIron Classics. italianIron.com. Speak with him. Scot from what everyone says in extremely knowledgeable on carbs.
Doug
What’s smart carb? I know Mikuni cv carbs are low side on iq score, but I’ve never met a carburetor that was particularly smart either.

…I take that back, some of the late 80s/early 90s computer controlled carbs on Japanese 2 strokes were pretty smart, actually, way too smart.
 

·
Registered
Joined
·
9 Posts
What’s smart carb? I know Mikuni cv carbs aren’t is low side on iq score, but I’ve never met a carburetor that was particularly smart either.


“The SmartCarb is a precision air and fuel metering instrument designed to push the limits of mechanical fuel atomization. Ultra-fine fuel atomization occurs via proprietary venturi features that concentrate and accelerate airflow past a precision ground metering rod.Feb 19, 2021” Lectron carbs are designed by the same man. There are several YouTube videos on them, check out Scots website. His blogs should be far more informative.
i was hot to trot for FCR’s but a friend in Australia who sells the “hand held dyno’s” he mentioned to me that I need to contact Scot in Tucson AZ. A former UK native. I was even looking at flat slides by Mikuni as a backup to the Keihin’s however I’m at 5200’ above sea level, elevation as high as 7200’ & ride at sea level every now & then with levels in between 0-7200’.
something to look at.
 

·
Registered
Joined
·
2 Posts
I finally got to ride it properly today. First of all, i really needed it! It was fun! 944 basically still has the same characteristics, just pulls a bit harder and a bit punchier.
Let’s see…ridden and warmed up, it does not pop or bang, so that is good, a huge improvement. It still stumbles as pulling away from dead stop, especially if you roll on slowly, and the transition is still abrupt, BUT it’s a lot more predictable and basically I can ride around it. I still have no idea what’s causing it, but I’m pretty sure that it’s right where the slides start to open, and vibrate as if there isn’t quite enough vacuum to open them. So I’m pinning my hope on the new diaphragm, and maybe swapping the springs with the one from Dynojet (it’s the softest, and longest of the bunch).
I also tried the open airbox. It distinctively improves throttle response, didn’t make ‘stumbling’ any worse, just more noticeable (so it’s worse then), and higher rpm a bit crisper, so I may still be a bit rich in general. I have to say though, it’s so loud! Can’t say I’m a huge fan.
Boy I needed that ride, it felt really good to do that.
Really interesting, in my case the open airbox really leaned the mixture as ducvet mentioned, and 99% removed the stumble (yes it is loud and I love it!!). I will keep following this thread and see what else you come up with. I want to mention one more thing I did, which definitely helped: I moved the clip on the main jets up by one notch (from 5 from the top to 4 from the top). That made the transition from idle to the primary circuit way smoother. Good luck with your troubleshooting!

P.S.: and yes once it runs well that engine is exhilarating!
 

·
Registered
Joined
·
3,883 Posts
Maybe the rings on the new pistons and barrels are still just bedding in? Extra resistance at the lower rpm's?
 

·
Registered
Joined
·
3,883 Posts
It does get hotter than normal while the rings are bedding in, that means friction and perhaps uneven combustion. Shouldn't last too long as an impact though.
 

·
Registered
Joined
·
171 Posts
In the 'good-old-days' when those Mikunis were current, I did a lot of my own jet kits.
The first problem is that the needles bang the needle jet larger with miles. Every time the air goes in it pulls the needle in that direction which eventually makes the needle jet metering orifice increase it's area drastically. This makes the off-idle go piggy rich.
That is the reason Factory Pro offers their SS needle jets. (although they are sometimes a couple ten thousandths of an inch larger than stock, they should outlast every other component in the carb.)
Someone mentioned the needle jet and emulsion tube as one part. That is not the case. The needle jet is only the needle orifice. The emulsion tubes are in the jet holder. On a side-draft carb, the emulsion tube is part of the needle jet, but these carbs are downdraft and Mikuni has put the emulsion tubes in the holder with the push-in main jet, the choke jet and the pilot jet.
The problem with the emulsion tubes is that they are too short. In a side-draft carb, the main jet screws into the bottom of the needle jet/emulsion tube so the fuel goes up the middle and the air bleeds in from the jet-well around the NJ/ET.
In the Duc CVs, when the jet well is emptied (the jet well is your accelerator pump volume) the air sucks under the emulsion tube above the main jet. This is like having giant holes on the lower area of the E-tube.
The dynojet kit compensates by making the main jet too big.
The Yamaha TDM 850 uses exactly the same carbs except that the E-tubes are just a little longer and they put the main jet in with the long side up against the bottom of the E-tube. Thus, working like the NJ/ET of a side-draft Mik. On the Yamaha, you can read the main jet number when they're installed, not on the Duc.
And then there is the pilot circuit. The pilot circuit is intentionally lean for emissions but they get there by making the pilot air bleeds so large that it limits the fuel you can get from that circuit. Car guys refer to air bleeds as vacuum breaks.
With a large air bleed (and the Duc CVs have 2, 1-0.6mm and 1-1.4mm) The 1.4 is in the body beneath the diaphragm.
It is there so the 'coast-down-valve' (that device on the side of the body) can shut off the 1.4 when there is high vacuum, decel. That richens circuit (which is down to 1- 0.6mm air bleed) for decel.
Installing a larger pilot fuel jet only partly solves the lean problem, but the circuit is still unstable because there is too much of a vacuum break. Smaller on the 1.4mm air bleed (1.2) gets you higher vacuum and better control of the entire idle/off-idle circuit. I use the stock 37.5 pilot fuel jet and get plenty of fuel (13.5:1) at idle with the mixture screw around 1.5 T.O.
Mikuni air bleeds.
I make my own emulsion tubes with smaller fewer holes and seal the bottom of them with O-rings that the main jet holds in place. Fuel up the inside only and air around the outside.
 

·
Registered
Joined
·
504 Posts
Discussion Starter · #93 ·
I took my bike to work this morning. It was exactly the same, stumbling a bit around 2000rpm-ish, but like I said, I can ride around that. I took a long way home, thinking about various adjustments to try out once the new diaphragms come in. The long way home is about 30 miles, and about 25 miles in, the symptom pretty much completely disappeared. It started to idle a bit higher, at around 1250rpm (it was set around 1100), and the stumbling when pulling away from dead stop was gone. I just rode it, I didn’t make any adjustment, the ambient temperature was pretty dry and about 70, but none of this should affect anything, so it’s a complete mystery to me. I’ll try tomorrow again to see if it’s indeed gone. Still what happened!
 

·
Premium Member
Joined
·
5,441 Posts
Possible it is breaking in or that it was never fully warmed up before. The idle usually does sit around 1100 so it will be interesting to see what you find tomorrow. Also on that note I had a bike in the shop recently (500 pantah iirc) and the tachometer was off about 300 rpms so the owner trying to get it to idle at 1200 was actually struggling because he was actually trying to idle at 900 and it did not like that.
 

·
Registered
Joined
·
7,402 Posts
Was it idling high or was the idle hanging ? Because hanging idle is a sure sign of a lean condition on these carbs.
 

·
Premium Member
Joined
·
5,441 Posts
Hanging idle is also caused by the idle screw being set too high. You see it all the time with FCR's as well where you simply lower the idle a little at a time and at a certain point the bike idles and no more hang up.
 

·
Registered
Joined
·
504 Posts
Discussion Starter · #97 ·
It's still a bit of mystery.
First of all, I totally forgot how long it takes for the engine to properly warm up, so maybe it hadn't quite reached the optimal operating temperature before, that is a possibility. Fully warmed up, the stumble is still there, just less noticeable, and easier to modulate, so there's that. After yesterday's ride, I readjusted the idle down to 1100rpm, and this morning stayed the same, but when I got to work, it again was idling a bit higher, so I readjusted it again. Now it idles around 1000rpm when cold, but by the time I got home and fully warmed up, idles just around 1100rpm. It never felt like rev was hanging up though. But the good news is there's been no popping from the carbs, though I feel like I could make it pop if I kept right where it hesitates as I pull away, so as I said, it is definitely still there.
The diaphragms haven't come yet, and my work is catching up with me, so I probably won't be able to work on the bike for a while, so this investigation may have to wait a bit.
 

·
Registered
Joined
·
504 Posts
Discussion Starter · #98 ·
I had a chance to commute on the bike today. I haven't done anything to it since the last post, but things have improved enormously. I don't know what that is about, the stutter, again, is still there, but barely noticeable now, and the engine revs significantly smoother. So far I put on just about 200 miles since the engine work. Maybe, it's just breaking in? It feels like it. The symptom is kind of weird, but could be that. The new diaphragms arrived, so when I have time, I'll put those in.
 

·
Registered
Joined
·
7,402 Posts
Did you ever try adjusting the low speed mixture screws out further than 3-1/2 turns ?
 
81 - 99 of 99 Posts
Top