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Discussion Starter · #1 ·
Having a problem with idle. Idle is set at 1200, but it has been jumpy and goes up to approx. 2800-3000. If I blip the throttle it sometime settle down. When I have been riding awhile it does this when I come to a stop. Also it seems if I lean the bike to the left at stop it starts. It really doesn't show much rhymne or reason to when these are just some of the times. The bike is a 93 900SS with 94 carbs on I'm told. Still have original. Am new owner, air box has K&N with lid cut full open top. Exhaust is original cans but may have been cored. Seems louder than it should be for factory. Vertical cyl spark plug is clean with a red hue to porc. Horiz. cyl spark plug has a little carbon on porcelain and a slight red hue near electrode. Could this be a simple float issue or does it sound deeper? Any help is appreciated
 

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Sounds like throttle cables not running smooth,
esp since it happends when you move bars.
Check and ev lube.
 

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If it is not the cables try synching throttles and adjust idle speed 1200 RPM is good, but are you going by the tach on the bike or a real tach?
 

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I vote to lower the idle about 100-200 rpm and synch the carbs.
 

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Discussion Starter · #5 ·
Thanks I'll look into the cables, and was already looking into carb syncing. Was using bike tach, but the difference is very noticable.
 

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printman said:
Thanks I'll look into the cables, and was already looking into carb syncing. Was using bike tach, but the difference is very noticable.
I meant for setting idle speed. You want an accurate tach for that.
 

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The ignition modules start to advance the timing around 1500 RPM. If the engine speed reaches that point the advance makes it take off.
Setting the idle lower will help but it needs to be low enough that it won't idle 'til it gets warm.
Setting the idle mixture really close to what it likes will help a lot, but that requires different pilot air bleeds.
It's a constant battle.

Doug
 

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ducpainter said:
I vote to lower the idle about 100-200 rpm and synch the carbs.
Are you saying to lower it that low just to do the sync or for good and why?
 

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>Are you saying to lower it that low just to do the sync or for good and why?

Unfortunately, you need to lower it that much to keep the digital ignitio from advancing and causing it to pick up idle speed.
A stock pilot system calibration is going to be lean at idle even if you put a larger pilot fuel jet in. There are two pilot air bleeds, one in the carb mouth (0.7mm I think) and one under the diaphragm, a 1.4mm.
The one under the diaphragm is shut off by the 'coast down' valve (the diaphragm on the side of the carb) at high vacuum so the pilot circuit gets a lot richer on over-run. Unless they do that it will insist on not idling down.
In normal operating mode like idle, the two air bleeds are vacuum breaks meaning that the idle fuel jet feels little vacuum. The off idle ports (on the airbox side of the butterfly) are also vacuum breaks in that mode. That is why putting a larger idle fuel jet in doesn't really make the idle a whole lot more stabile. It still has a limited vacuum in that system.
If you change the top idle air bleed to a 1.2mm Mikuni air bleed, it will make the idle a lot more stabile and allow a bit more fuel to be adjusted at idle.
The stock system is quite lean at idle and removing the air box lid or un-restricting the air box in any way aggravates that situation.
When an engine idles really lean (and this holds only with a carburetter) the hotter it gets,the richer it runs. The fuel is better vaporized, so the same amount of fuel causes richer combustion.
It's the same effect that requires a choke. If you start a cold engine the A/F meter will indicate lean until the engine starts to build temp and then it gets rich.
Anyway, as the mixture goes from way lean toward best power mixture, the engine responds by idling higher. When it hits that 1500 RPM point the ignition starts to advance and first thing you know it's idling at 3000.
By getting a mixture closer to best power mixture at idle, the tendency of the engine to pick up speed is greatly reduced. It hates way lean, so almost rich enough gets you a good idle speed and it doesn't pick up quite so much as it gets hot.
Another problem with that engine is that it idles at 6 degrees BTC and the vacuum is real weak. When you have to retard it a couple for a high compression kit the idle gets even softer and the vacuum goes down more.
The reason the vacuum goes down is that, instead of the engine making the power to idle from more ignition advance, you need to open the throttle to make the power to overcome the friction. Opening the throttle drops the vacuum.
With a 944 HC engine you might even have to open the throttle at idle until it runs off the first off-idle hole. In tha case you won't be able to adjust the idle mixture through the full range because you only have cojntrol over the idle port.
There isn't much you can do beyond jetting and constant adjustment, unless you by the Silent Hektik ignition and add some timing at idle.

Or, maybe I'm wrong?

Doug
 

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moperfserv said:

Are you saying that the idle will raise about 1000 rpm between a cold start to warmed up to where it should be (which I've always heard is about 1200)?

Also, as a side question, do you all year rounders adjust the idle in the winter then put it back in the summer?
 

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Aguacate said:
Are you saying that the idle will raise about 1000 rpm between a cold start to warmed up to where it should be (which I've always heard is about 1200)?
Nope, you'll see a 2 or 3 hundred rpm rise between cold and warm, _BUT_ if you've set your idle high enough that the warm idle gets as high as 1500rpm then the ignition advance cuts in and that will push the rpms up to 3k or so. You need to make sure your warm idle stays down around 1200 (so you've got a bit of leeway before the ignition advance kicks in), which means you need to get the cold idle comfortable at 900-1000rpm. There's a bunch of reasons in Dougs post that expplain why getting that 900-1000rpm cold idle can be difficult (all of which is new to me, thanks Doug, I think it may have given me some hints about a minor issue on my Honda).

Also, as a side question, do you all year rounders adjust the idle in the winter then put it back in the summer?
Not me - I use a little more "choke" in winter, and little to none in summer...

big
 

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Man I feel like an idiot. I read "lower about 100-200" as "lower to about 100-200"...

Good info all around.
 

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Discussion Starter · #13 ·
Thanks to all, I am still waiting on some parts to sync the carbs ( carbtune coming from england ). Originally removed throttle cables and inspected, lubed and replaced. It still continued, then I got the bike properly warmed up and reset the idle. Down just below 1000 ( still on bike tach ) where it would sustain idle while driving but only when warmed up some.Need to get a true tach still, but can only do a little at a time. Problem seems to have gone away. I'm taking it that it was close enough to the ignition advance to bounce in and out of advancing. Was stuck on thinking it was a fuel problem thanks for guiding me in how the idle speed can effect ignition advance!!! Thanks to all for your help. [thumbsup]
 

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It's always good to be able to do something that has an effect, even if it's bad.
Don't assume that it isn't quite lean as well as the timing issue.
There are things you can do that will make it much more user-friendly, but as a long time service guy, that's what we faced constantly.

Oh, and it never stops needing 'tweaking'.

Doug
 

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Discussion Starter · #15 ·
I enjoy the tweaking. It is something I've done since I was a teen. However I have taken a long hiatus ( think that's how it's spelled ). Was once Suzuki certified, but like I said a long time ago. Everyone used to say that a Ducati is so hard to work on. But I would have to disagree. Its not harder just different and with this message board I'm pretty confident I can solve any problem. Still planning a bump in the pilot jet ( small one ), seen a lot of threads about how enriching the pilot gets rid of the low end surging. Gonna see how a carb sync will effect the low end surging ( slight ) before getting a jet kit.

Don
 
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