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Discussion Starter #1 (Edited)
Okay I may be the only dumbass out there still trying to make these stock 38cv's work, especially with a built motor, but I'm planning to fuel inject this bike over the winter and just want to get the stock carbs working well enough to get some short late summer evening rides in.

My bike's got a 944cc 12.5:1 piston, open airbox and exhaust. I've got the mains drilled to 162 and the bike is jetting nicely at full throttle and anything close to it. I've got the pilot screws all the way out and it's still surging at 3500-4500 cruising speed.

No idea what size pilots are in there now, but I'm sure they're stock which I think would be #40's. What size are folks having success going to if they need more fuel on the low circuit?

And what clip are you big bore or hi comp guys running the jet needle at? I think that's part of my prob too. Surging at highway speed/throttle openings, which is just off idle.

As always, THANKS!
 

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European 900s use 42.5 idle jets. How is your carb sysnchronisation? If it is out that will make setting the idle mixture difficult.
 

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Raising the needles could be worth a try if the surging happens in a high gear.
I would agree and suggest you change out the needle to one from a dynojet kit.

bob
 

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Discussion Starter #5
It's got a dynojet needle kit in it already but I'll go up a clip or 2 on the needle and see, there are 2 clip positions left so that indicates that the needle jet that's in there is a tad small, I think. It's idling fine but just falls on its face and bucks right off idle, I have to throttle past that "dead" spot to take off from lights, etc. Also very surgey when at 60-65mph in 4th or 5th, which is within the first 1/4 throttle opening range. If raising the needle doesn't remedy it I'll try a 45 or 47.5 pilot, they're cheap enough on ebay.
 

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Discussion Starter #6
Needles raised (two clips - as far rich as they'll get) and they look to be in good condition so we'll give that a try. Also, I ordered a set of 45 and 47.5 pilots just in case. This is a US spec bike so it's got a 37.5 or 40 pilot, which is probably not enough for this motor configuration. Thanks for the tips, guys!
 

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There is a lot of overlap between the pilot jet and the needle position. If it starts running rough at the bottom, you may have gone too big. Don't be afraid to go farther out with your low speed screws, as they affect mixture out to about 6 turns out.
 

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Discussion Starter #8
Okay thanks Duc96cr, that's good info.

I moved the needle the last two remaining clips (raised needle) and it improved 1/4-1/2 throttle surging dramatically. I'm out of clip positions so my choices to get the last little bit of surge tuned out is to go with a needle with more taper or wait 'til the mufflers are re-installed, which I have a feeling will richen me up enough to do the trick.

I'm at 5 turns out on the low speed/pilot screw, which I believe is the stock US 37.5 It's idling fine but falls on its face right off idle, about 1/8th throttle. Bucks and sputters then clears when I twist the grip to 1/4 of the full travel or further and then it starts pulling fine. Still waiting on the bigger pilots. I'll start with the 45 pilot, turn the screw back in to 2.5 turns out or thereabouts and tune from there. Wish me luck!
 

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I went from 40 to 45 under similar circumstances. Way overkill, went back to 40. Dealer didn't have 42.5. Added another washer under my needle. Pulling that air cleaner gets to be a pain, doesn't it ? I run a little lean cold but ok warmed up. I was fouling plugs with 45 s. screws 5 1/2 out now. Runs good down to 2500. Stick with it, good luck.
 

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Discussion Starter #10
I went from 40 to 45 under similar circumstances. Way overkill, went back to 40. Dealer didn't have 42.5. Added another washer under my needle. Pulling that air cleaner gets to be a pain, doesn't it ? I run a little lean cold but ok warmed up. I was fouling plugs with 45 s. screws 5 1/2 out now. Runs good down to 2500. Stick with it, good luck.
Oh that's a good idea re: the washer under the needle. I'm going to install the new exhaust before I tinker with it too much more or I'll just end up having to do it twice (or 5 times, realistically). I have another turn out on the 37.5's if the 45's prove to be too rich. I'll probably order a set of 42.5's too just to complete my collection. I'm deadset on getting this thing to run right on stock carbs, I've already turned my attention to 2 new projects (fuel injected CB550, hot-rod GL1000) so the likelihood of me getting around to FCR's or injection on this beast anytime soon is significantly reduced!

Thanks again for the tips!
 

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If you can't get the needles rich enough, another trick to try is go even richer on the mains - that'll richen things up backwards through the range to the needles. It's cheap insurance against leaning out too much and melting holes in things like pistons. I'm assuming you don't go everywhere at full throttle? ;)

BUY some main jet's though, rather than drilling - they're not that pricey, and spares often come in handy...

When I put the FCR's on mine, they came with 155 mains, and it ran like a pig through the upper end. I ended up with 182 mains - worked out by changing one size (OK, I jumped a few initially) at a time and going for a run (I live rural), and seeing if it felt 'better' than the previous run, then checking plugs and how hot things were. Just kept richening it up until I couldn't notice the difference. Oddly enough, I get better economy than from the standard (CV) setup. About 15%!

I also (IIRC) went one clip raised on the needles.

One day, I'll take it to a dyno, and get it checked out with a gas analyser - but for now, it runs like a charm, so I'm in no rush. Crisp, responsive, and no coughs, hiccups, surges or burbles.
 

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Discussion Starter #12
If you can't get the needles rich enough, another trick to try is go even richer on the mains - that'll richen things up backwards through the range to the needles. It's cheap insurance against leaning out too much and melting holes in things like pistons. I'm assuming you don't go everywhere at full throttle? ;)

BUY some main jet's though, rather than drilling - they're not that pricey, and spares often come in handy...

When I put the FCR's on mine, they came with 155 mains, and it ran like a pig through the upper end. I ended up with 182 mains - worked out by changing one size (OK, I jumped a few initially) at a time and going for a run (I live rural), and seeing if it felt 'better' than the previous run, then checking plugs and how hot things were. Just kept richening it up until I couldn't notice the difference. Oddly enough, I get better economy than from the standard (CV) setup. About 15%!

I also (IIRC) went one clip raised on the needles.

One day, I'll take it to a dyno, and get it checked out with a gas analyser - but for now, it runs like a charm, so I'm in no rush. Crisp, responsive, and no coughs, hiccups, surges or burbles.
Whoa, 182's! I've got mine drilled to 162's and I thought that was ridiculous. I'll order some bigger ones and give 'em a try just for fun. :)
 

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Whoa, 182's! I've got mine drilled to 162's and I thought that was ridiculous. I'll order some bigger ones and give 'em a try just for fun. :)
Yeah - I was kinda surprised too! But I kept going bigger, and it kept getting better. Till it was 'good enough' - and I was thinking "This is crazy..." But it works good!

The only thing that I can think of, is that I've heard (some article -somewhere- years back) that leaving the airbox lid on, with the 'horns' removed, allows a 'ram' effect from the air flow from the front of the bike, at speed. Apparently, it doesn't happen with the lid completely removed, but with it on, it causes a pressure build up.

I guess the proof will be in testing on a dyno - unless they have some BIG fans blowing into the front, it'll show up really rich at full throttle...

Mind you - I think the 155's were for standard headers, with slip-on's, and my (other...) guess is that the SMI headers flow quite a bit better...

Besides, I found riding it significantly quieter (less induction noise) on-board with the lid on, as opposed to it completely off. Don't think it makes much difference to the noise that bystanders hear... ;)

Found this: BikeBoy.org - Ducati 900SS Carb model with Keihin FCR 39 and 41mm carbs

The first 900 (“Michael”) came in with Staintune slip on mufflers, a K&N air filter and std air box lid with the snorkels removed. It was the first bike I’d run on the Dynobike’s new Dynojet 250 and the power readings on this dyno are a bit lower than the previous one. So we can’t really compare these runs to previous 900SS I’ve run. Also it was a bit lean, as I expected with the snorkels out. Since running this bike I fitted a lid without snorkels to my 750M to replace the full open lid and was surprised by how little difference it made, both to noise and performance. So with the snorkels removed the larger holes must give enough extra airflow to have almost the same performance as an open lid, but without being that lean (or more to the point, without upsetting the carbs that much) that it all falls over when you open that throttle fully. Although it’s still too lean for max power.

The second 900 (“Ashley”) came in with old Gia.Co.Moto slip on mufflers – the ones where the mufflers slip on to separate pipes held on by springs, std air filter and an open air box lid. For the baseline dyno run I fitted the K&N filter from the FCR kit and the std lid without snorkels from my 750M, meaning that apart from muffler brands it was pretty much identical to the previous 900. The only other difference was that this second 900SS was in fact a 900SS SP, one of about 20 (from memory) bought into Australia in late 1995 (to fill a local shortage of 900SS?). Spec wise for us it was the same as the 900SL, but with a dual seat. So it has USA spec carbs – different needles, same main jets, and some different internal drillings.
Errr - one other thing - mine are Keihin jets! The sizing is different to Mikuni!
 

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Discussion Starter #14
Aaaaarrgh. Ready to throw these mikunis in the trash.
 

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I ended up with 2 rebuild kits for a Yamaha Super Tenere which had all the jets & O-rings plus gaskets & I used every jet & gasket that was included...took a little fiddling around..mainly because it's such a PITA with the bloody airbox/battery lump ...but now my Mikunis are sweet :D ..It's really smooth throughout the whole range & gotten rid of that dreaded low rev stumble..Lotsa lowend grunt & cruises pretty smoothly too..it probably runs a little rich but isn't fouling plugs..I reckon the Mikunis aren't a bad bit of kit if you have the patience to fiddle around a bit with them. The most time's spent taking on & off that airbox:mad:

The jetting i used is as follows:

Pilot Jet #42.5
Main Jet #140
Main starter Jet #70
Needle jet - Y4
Jet needle 2nd down from the top
Open airbox with Ramair Foam panel
 

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which dynojet needles do you have? the stage 2 needles are similar to the stage 1 needles, but the root diameter is smaller by 0.05mm from memory, and the same as the factory pro needles. which will impact low speed mixture.

also the american carbs i believe are leaner in some circuit drillings than the euro ones. not exactly sure how, but that could also be impacting it.

i found on my 750 that i got stuck in a situation at very low throttle where it was just nice hot, but bad when cold, or going up 1 pilot or 1 needle notch made me too rich when hot and made a big difference to fuel economy. that was with 600 carbs too. with the 900 usa carbs it was definitely richer at low throttle with the same jets, but i never bothered playing with it. with the fcr39 i ended up with 45 pilot jets i think to remove low throttle richness.
 

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Discussion Starter #17
I'm getting close.... Went back to stock slide springs (probably irrelevant) and goofed with the clip settings. I'm amazed how fat the whole midrange got from going up 2.5 on the pilot. Once I leaned it out in the mid it's back to its old wheelie happy self. And the hiccup off idle is completely gone. I'll turn some more screws tomorrow and try to dial in that last bit of hp. Loving the bike again.. I was ready to put it up for sale 3 days ago. Ha!
 

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the stock slide springs are much higher rate, so you'll have less slide lift for a given downstream vacuum and a higher dynamic vacuum over the needle jet so it should be richer just due to them. i expect that will have made the midrange difference.
 

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Drilled main jets?

Sorry but that might be good enough accuracy for a Hardley Ableson but it could easily lead to a blown piston on something as highly strung as a 12.5:1 944. I suggest you either find a selection of jets above and bellow the '160's you have and do the job properly or take the bike to a dealer who can. Likely to save you money in the long run.
 

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Discussion Starter #20
Drilled main jets?

Sorry but that might be good enough accuracy for a Hardley Ableson but it could easily lead to a blown piston on something as highly strung as a 12.5:1 944. I suggest you either find a selection of jets above and bellow the '160's you have and do the job properly or take the bike to a dealer who can. Likely to save you money in the long run.
Probably not a bad idea. Dude who did some of the motor work and about half of the tuning before i got impatient and picked it up to finish myself is a racecar fabricator kind of guy who's not afraid of anything. Drilling mains seemed nuts to me but what the hell do i know? I feel like they're a little large/fat now with the affect of the spring and pilot changes, it's not hitting as hard "on the pipe" as it was once, so it's a good opportunity to swap them out for a slightly smaller (and factory manufacturered) size. The correct size mains that I order will represent the LAST $10 I spend on these stock Mikuni's. :) I really need to keep some focus on this bike to finish it up, I'm much more interested in another project at the moment, this poor bike spends far too much time over in the dark corner of my garage venting race gas and collecting dust.
 
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