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Discussion Starter #1
Got my 39 mm FCR's on, and it is running like crap.
Things I have done:
Checked float height: both floats at 9mm
Checked fuel mixture screws: were at 1/4 turn from seated, turned to 3/4 past seated, slightly helped but still not running good.

What I can describe:
The motor does not freely rev at all.
Hard to start. Have to start with throttle cracked about 1/4 turn.
Full throttle causes it to choke and die, or really cough and not want to rev.
1/4 to 3/4 throttle revs very slowly.
Not rideable as it chokes down on take off

Any suggestions would be great. Thanks a bunch.

-Patrick
 
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Did these carbs supposedly come jetted for the bike? I'm not sure if I'm reading your description right, but it sounds like you're running out of fuel (lean) when you crack the throtle and it dies. Lack of fuel might also explain the inability to rev freely and your other symptoms. Does anybody out there have jetting specs you can compare to? Also I'm assuming these are new carbs and not somebody's take-offs with a heap of time and problems in them.
 

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I vote for fat. Check your plugs, if they are black & wet looking, drop your needles 1 notch.
 

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maicoktmducati sounds like he's on the money, where did you buy the carbs & were they set up for your bike?

Little things make sooo much difference with jetting & setup, mod's, like pipes, open air box (did you cut the top out of the airbox), headwork ect.....

Without knowing the exact config of your bike it's impossible to make any reccommendation.

I've got 41's on my Duc & the carbs were lean when i installed, but i didn't know that the previous owner had done some head work. So a bit of re-jetting and it's now awesome.

playing around with the different circuits on the carbs probably isn't a good thing.

I got mine from Chris, & with his precise knowledge of the units we got things honking by remote controll (e-mail USA-PNG) in no time.

I'm no expert but did manage to get my FCR's functioning the best they can (with a little help of course)

What Mod's have you got & what guide (or steps) did you take to install, ie. blocking vaccum lines, cutting airbox lid, have you installed a K&N filter or do you have the std airbox with snorkels intact?
 

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you know the old saying, "read the manual" whell i should have looked at your details at the bottom of your post.

Looks like everythings in place, What size mains are in it?
did you block the vaccum lines?

I don't want to start making any W.A.G's but "process of elimination" is a good place to start.
 

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Open idel screw about another full turn to a total of 1 3/4 to start. Then try either raises the c-clip on the needle one or two notches or going down with the main jet a size or two. The reason you have to crack the throttle to start is lack of air. Opening your pilot should fix this. Your other problem to me sound really rich.
 

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Discussion Starter #7
Ok guys, thanks a bunch. I"ll start in on these tips tomorrow. These carbs were bought new and I thought that they came from the factory jetted and set up for the 750. I thought SUDCO was doing that now. Anyways I checked the baseline settings such as float height and fuel mixture before install. That is all I knew to do, but I'll try some of you guys inputs.

Also I'm sick of taking the air box on and off, and would love to just cut the box between where the coils mount and the back of the airbox. Then throw on two K&N's. Make adjustments easier plus no more airbox removal to work on the carbs. What do you guys think of this modification?

I look forward to getting these working. Hopefully some changes will get them working. About the jetting, did you guys have to order more jets from SUDCO? How did you know what sizes to get?

-Patrick
 

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Sizes on jets rise and decrease in twos, I.E. 148, 150, 152, 154. You should be able to buy them from any dealer even dirt bike dealers. Just tell them what carb you have.
 

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Patrick/Andrew,
I would have thought that the idle & off idle settings should have been fairly close out of the box?

I never had to touch the base settings & was warned against it.

Patrick, really need to know what main jets are in the carbs.
 

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Hi Patrick,

Glad to hear you got some FCRs. You WILL end up a very happy person. The reason that I double check the baseline settings when I resell the carbs from Sudco is because they are not always set to their own specification. The reason is that they tell Keihin the settings and Keihin is supposed to build them to order. I find that the human adjustable settings (float height, fuel screws, and slow air screws) are occasionally "off". Actually, the slow air screws are typically pretty good, but the float heights and fuel screws are often whacked. Sudco occasionally will double check their baseline settings in the carbs, but they're busy enough that I do not fault them for not doing it every time. I have the baseline settings recorded in my FCR FAQ as well as a link to where the adjustments are. edited to add: The baseline is same for 39 and 41 for 750 or 900.

Note that on the FCR FAQ I go to great lengths to point out how the different circuits in the FCRs are completely unrelated and dissociated from each other AS WELL AS the fact that tuning the FCRs is 100% about throttle position while under a load.

First off, diddling with the throttle while not moving means nothing. If the bike isn't moving, you're concerned with idle and idle only. What affects idle: mix screws, slow jets, and needle root diameter.

Next: futzing about with needle clips or main jets sounds like they have nothing to do with your problem as they affect 1/2 and full throttle operation.

Main jets are not in #2 increments. They have 4 sizes per 10. they'll be like 150, 152, 155, 158. They always have 0's and 5's.

I have a pretty good selection of jets in stock. BUT, I'm on vacation in SF Area right now. If you can wait, watch for me to "un close" my biz at www.ca-cycleworks.com and call me so we can talk about it. The baseline should work for you unless someone ported the heads. :)

Always take notes of the changes you perform and always do only one change at a time. :) Stick with it and you'll get there.

Good luck,
:) Chris
 

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Discussion Starter #12
Thanks Chris,

You sold me the throttle cables, tube, and air box adapters. I hope you are having a good time in SF. I do need to get the ball hex driver off of you too. I'm going to go fool around with the carbs today. I'll post where everything is.
Merry Christmas guys!

Patrick
 

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Oh Hey, Patrick. If anything, the de-personalization on the DML BBS is the only thing I do not like. :) Oh well. :) Yeah, you can get ball hex driver "anywhere", just call ahead first. Local hardware store is faster then waiting for me to get home. :) Have fun and merry christmas.

:) Chris
 

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Discussion Starter #14
So I've been tinkering some. Heres what I have been finding.

Cables are tight and only a bit of free play in the throttle
1/8th to 1/4 throttle are just ok
1/4 1/2 throttle is just fair
full throttle it bogs down and revs very slowly, actually revs faster and more responsive with less amounts of throttle input
started fuel mixture screw at 3/4 turn from seated. turned back towards seated for best throttle response yet (which is still really crappy)
Under load it revs very slowly, full throttle has no effect on accelaration.

I have not messed with needles nor the jets. I guess its running real rich, there is no popping or anything just bogging of the motor when throttle is given. It actually cranks right up and idles ok. Tries to die when cold but idles pretty good. Problems arise when throttle input is given.

I really don't want to have to order jets and play with those, but it sounds like that might be where this is leading.

Next thing I am going to do is see what jets it has in it right now. I'll post that and the needle clip position tonight.

Anybody know where I can get SUDCO jets quickly. Thanks for all of your help.

-Patrick
 

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Hmm that sure sounds odd. That or someone has ported your heads and you don't know it. The main jets are "Keihin long hex main" jets. 155s should be in there. Have you pulled the plugs just after one of your WOT throttle runs? If they're light, then you'll need to go richer. start with like 165 or 168 and then go up from there. A dirt bike shop will have the jets, as will any shop that does FCRs. Don't call them Sudco jets, as they're not Sudco jets, they're Keihin long hex main jets. :)

Also, I will agree that you need to mess with the mains. Overall, I'd say you sound lean. When you're too rich, you an't really tell, as the FCRs atomize fuel SO well that you'll run well (albeit very rich) with 120 hp worth of fuel running through the engine! I've done this... ::)

Perhaps you'll also want to add some more richness via the slow air screw (turning it in 1/8 turn at a time) but I'd say to wait until the mains are happy to mess with this.

Good luck, call ahead to shop for the jets, and have fun! Oh, and remember that you can change main jets pretty quickly with carbs mounted on bike... turn off fuel, get aerosol can lid (without hole), 14mm wrench for the main jet cover, and 6mm socket for jet itself.

:) Chris
 

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Discussion Starter #16
Thanks Chris.

Lemme go tinker some more.

Patrick
 
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I gotta agree with Chris, your symptoms sound lean to me. A rich running bike doesn't always give up much in the way of bad running symptoms and in fact will often times run quite well. Yours clearly doesn't run well at all. Bump up those mains and it should then take some throttle and get going. From there you'll be able to fine tune the rest. Good luck!
 

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Discussion Starter #18
Thanks y'all. I played around with the mixture for about an hour last night and went from between 1/4 from seated, all the way to 2 and 1/8 turn from seated. The more turns helped some, but its still crappy. Guess I'll be picking up some jets on Monday. I don't know if we have dirt bike shops around here, but do have alot of dealerships. I guess I'll just have to call around and ask for the main's. I'm sure it will get worked out though.

Thanks for all of your help.

-Patrick
 

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Discussion Starter #20
Jets that are in the carbs are 155's. I also checked the needles and they are fine, and I pulled the plugs.

The plugs are very new. I have ridden on them maybe twice before this tear down, and I know the bike was running rich before hand. Tonight when I pulled the plugs they were far from lean. Think this is from previous rides or could they be rich looking just from the 2 times that I have attempted to ride the bike around after tinkering, and from idling while tinkering?

Guess I should get some jets that are under 155 too.

-Patrick
 
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