FCR - Cannot Dial in Idle-To-Quarter Throttle, Tried Many Jets/Settings - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 28 (permalink) Old Sep 12th, 2019, 1:32 am Thread Starter
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FCR - Cannot Dial in Idle-To-Quarter Throttle, Tried Many Jets/Settings

Hey y'all,

First off, a question for racked FCR 41 owners with velocity stacks or pods and stock bore + compression with altitude below 7,000 feet / 2133 meters:

What size is the pilot (slow jet) and how many turns out from bottom is the slow air screw and fuel mixture screws?

--

Onto my issue:

Stock motor with Termigs.

155 main jet

EMT needle, third clip from top

60, 65, 68 pilot

200 main air jet

Blue velocity stacks

--

I have a recently rebuilt set of racked 41s that function very well on my M900 with W heads. But on my SuperSport's V heads with of course different sized jets and settings I am not able to dial in the idle-to-quarter throttle. I recently did a valve adjustment, compression test, ohm'd out the pickups (a little low at 102.3 or something per pickup) and the CCW coils, replaced the fuel filter, tested the fuel pump for 3.5 PSI, and replaced the fuel with fresh 91. For the record I am testing idle after 10 minutes of hard riding.

Initially the revs would hang sometimes up to 1-2k RPM above idle for 4-5 seconds with a 60 pilot. The motor would also heat up very quickly from being dead cold. In my eyes these two symptoms occurring together pointed toward a lean condition. Even after screwing out the fuel mixture screws until they nearly fell out the revs would still hang. I then installed brand new, larger pilots.

However, 65 was still hanging and so are 68 pilots. have only marginally remedied the hanging or perhaps it didn't even help at all...my head is messed up from inhaling fumes . I have been tinkering with all kinds of combinations of the slow air screw and fuel mixture screws with the 65 and 68 but it still hangs. It won't hang much if I almost completely screw in the slow air screws, but then it will barely idle and eventually shut off. I suppose I could increase the idle speed to maintain proper ideal RPM but I would have it screwed in completely and that seems like a band-aid fix.

To make matters more confusing to me, the sparks plugs are sooty, and I must occasionally burn the carbon off them before using them again. How can they be sooty, which is a symptom of being too rich, if hanging is a symptom of a lean condition? It doesn't make sense to me that the rich condition is coming from the needle position or the main jet, but this wouldn't have been the first time I'm wrong.

I'm waiting for 62 pilots. It's the only size I skipped.

Files attached for anyone who needs them.

Cheers.
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Last edited by samadhi; Sep 12th, 2019 at 1:50 am.
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post #2 of 28 (permalink) Old Sep 12th, 2019, 1:52 am Thread Starter
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I will post a link to a video of the condition tomorrow morning.
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post #3 of 28 (permalink) Old Sep 12th, 2019, 3:05 am
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i'd try 50 or 52 pilots and emr needle.

know all wanker
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post #4 of 28 (permalink) Old Sep 12th, 2019, 6:21 am Thread Starter
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Quote:
Originally Posted by belter View Post
i'd try 50 or 52 pilots and emr needle.
Hoorah, someone who actually knows something read this long post lol. I just emailed in an order to Sudco.

I might have these by Saturday / PST. I'll update as soon as I install them.

Many thanks!
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post #5 of 28 (permalink) Old Sep 12th, 2019, 7:55 am
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Hesitating to comment as I lost the paper Chris provided on the specs for the FCR41's he prepped for me years ago but the symptoms you describe on the hanging throttle is how mine acts - but I have noticed one consistent thing.

It is never actually "cold" where I live but I have to turn the idle cable up a 1/4 turn when warming it up. Three minutes or less and I drive off. Within minutes, three stoplights later I dial it back to where it started. For the next 1/2 hour of stop and go with speeds up to 60mph at times, I will get the hanging idle 2krpm. Very annoying, revving it sometimes helps it to drop, it will on its own after about 90 seconds.

Now this is why I am offering any of my experience - if I ride for more than an hour, with some consistent freeway speeds, the hanging idle goes away for pretty much the rest of the day, even with parking it for an hour or so. No more fiddling with the idle cable adjustment. If I drop 200 miles in the desert or the mountains it runs perfectly, idles smoothly at stops, starts instantly. Then the next day its back to same old crap.

I use both the stock airbox lid on K&N filters and one that is cut out across the two snorkle holes - doesn't seem to make much difference other than intake noise. Plugs are always perfect, neither lean nor rich with Remus CF slipons.

*Edit - it has been doing this for 24k miles, I figure it is the nature of the beast, but I'm willing to swap out a jet if it works!
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Last edited by RockAZ; Sep 12th, 2019 at 8:09 am.
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post #6 of 28 (permalink) Old Sep 12th, 2019, 8:50 am
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This'll help... https://www.ducati.ms/forums/57-supe...ml#post7020355 - all jetting etc. in links in post.

'95 900 Superlight IV #064

Mods are: K&N filter, airbox lid on with rubber trumpets removed, MBP collets, full SMI (SilMoto Italia) spaghetti exhaust with 'open' carbon cans, 41mm FCR's - #182 mains!, Barnett alloy clutch basket, Oberon slave cylinder, Ohlins fork springs, Ohlins rear shock, Verlicchi swingarm.
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post #7 of 28 (permalink) Old Sep 12th, 2019, 7:11 pm
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I read those links, some good sources. As I still use the stock airbox battery assembly it is a hassle to pull the carbs out. Having said that I would if I could understand how changing any jet would cure the first hour symptoms without creating problems the rest of the day?
24k miles, track days and lunitic days it runs great after that first hour.

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post #8 of 28 (permalink) Old Sep 13th, 2019, 8:42 am
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Quote:
Originally Posted by RockAZ View Post
I read those links, some good sources. As I still use the stock airbox battery assembly it is a hassle to pull the carbs out. Having said that I would if I could understand how changing any jet would cure the first hour symptoms without creating problems the rest of the day?
24k miles, track days and lunitic days it runs great after that first hour.
Does any of this stuff get any easier if the stock airbox is replaced with a pair of filters on the carb inlets? Wait .. not ~easier~ (obviously it would be) but rather would it be worth the effort to replace the stock airbox with a pair of filters? After all that I have read about these carbs so far, it seems like using a pair of filters is almost a given, what with the better access to the carbs and so on. Part of the territory. Que no?

1996 900CR.
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post #9 of 28 (permalink) Old Sep 13th, 2019, 11:05 am
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I admit I like the look of twin filters instead of the airbox especially when a well made battery holder is installed. I don't like the extra intake noise, and I am not interested in tuning for peak horsepower..

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1995 900SS SP #746
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post #10 of 28 (permalink) Old Sep 13th, 2019, 11:15 am Thread Starter
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Quote:
Originally Posted by RockAZ View Post
Hesitating to comment as I lost the paper Chris provided on the specs for the FCR41's he prepped for me years ago but the symptoms you describe on the hanging throttle is how mine acts - but I have noticed one consistent thing...
Based on Salvisberg's Mikuni tuning guide for idling and low RPM a rich condition worsens and a lean condition improves as the bike warms up.

With my carbureted Ducatis they definitely run better when hotter, but I have never had them hang up except for my current issue. I wonder if the pilot is one size too small, or the fuel mixture screws are 1/4 turn away from ideal, or perhaps the slow air screw is every so slightly turned out too much?
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