Ducati Monster Forum

Well, I've done it now.   Bringing back to life my new to me 1999 m900,  Got it running ok, carbs pretty far out of sync from a bench sync so I thought I'd tweak the idle speed to raise it fast enough to ride 20 miles or so to my old man's garage and use his monometer. Got a little ahead of myself and turned the idle speed adjustment screw in all the way, the thing won't start up any more. I didn't count revolutions as I turned it...sooo I have no idea how to find where it started. 

Super frustrating as now I'll turn it a couple revolutions and try it out, and promptly flood the engine. Pull the plugs, wait 20 minutes and try again  [bang]

Anybody know how to find where the idle speed adjustment screw belongs without the machine running?

Idle mixture screws or the idle speed screw?
Set idle mixture screws 3-3.5 turns out from lightly seated.

Hold idle with the throttle till warm up and see if it dies when you release the throttle. Idle speed screw needs to be adjusted until you can let go if the throttle for a decent tick over of 1100-1200 rpm.

You're not playing with the mixture screws per chance?

Look into the carbs and back off the idle speed screw until the butterflies shut as they may be gaping open ( or so closed it can't idle). Shouldn't be flooding or fouling unless the mixture screws are out or the choke is left on too long (or it's just flooding from another cause).

I don't think he's playing with the mixture screws, but it's possible, when standing on your head to see the adjustment screws, that he's turning the synch screw.

AMHIK.

Good possibility.

Thanks all. 

Referring to the idle speed screw (not the sync screw, not the idle mixture screws, but I'm aware that these exist and of where to find them). I was able to get it back to presumably an ok spot by holding the throttle open a bit, half choke, and playing with it with the throttle held in place.  Once I had it running I sycned the two carbs with vacuum gauges.


I'm having trouble getting the bike to idle, however, and I am now turning my attention to the idle mixture screws.  When I began my carb rebuild, I found they were way out-- I don't remember exactly, but on the order of 7 full turns.  I set them back to stock and it seems like the bikes not getting fuel at idle.  Seems like the previous owner had the same problem.

Plugs are brand new so I can't get a read on them yet. But I notice there's been 20 or so additional nickel sized holes drilled into the airbox, snorkels are also MIA (I've been tinkering on the bike more than a month now and I'm just realizing that these holes are DIY...).  So I suspect a lean condition.  My jets are stock and my filter is a new K&N.

Ultra basic question here, but I'm new to ducati's and have primarily been working on 70's and 80's jap bikes in my garage in the past.  I'm used to choke butterflies on my old honda's, but I see these mikuni BDST 38's have a little port which opens or closed with the choke operation.  Which direction of the choke lever has the bike choked and which direction is for general operation?  Lever forward (port closed) or lever backward (port open)?

Lever backward (towards the rider) is choked.

Did you notice the size of the pilot jets?  The drilled holes are essentially an open air box.  You could cover the holes and see what difference it makes. 
That means you want stage 1 jetting.  Worry about that when you get the idle straightened out. 

"1 turn" is 360 degrees just to be clear. I'm sure you know that, but just checking in.

 [thumbsup]

Great tips, thanks

Yep-- 1 turn is 360 degrees.  I've got them 5 turns out now and I still cant get an idle without choke.  There's intermittent backfiring out the airbox, so I think I'm still lean.

I'm not sure the size of the pilots, I'll have a look and see if I can see anything stamped on them. I have a second set from a cheap e-bay rebuild kit I've not installed, but I think they're stock as well. 

Haven't figured a way to cover the air holes, but I looked into stage one jetting a bit.  Name brand Dynokits run around $120 (ouch!).  Seems like they come with lots of bells and whistles for installation that I've already got in my tool kit.  Does anyone have a link where I can get some cheap stage 1 jets alone?

Just cover the holes with tape, as an experiment.

You can get jets from http://www.factorypro.com/

5 turns out is a lot for sure. 3 1/2 should be a base setting.

Throttle butterfies may still be too open so turn down that idle speed screw while you control idle speed with the throttle and continue the "idle test" until it idles. Then you may increase it a smidge when you go too low (if you havent already).

Pilot jets from 37.5 to 42.5 are often used with size 40 common. 37.5 for smooth economy verses the responsive but thirstier 42.5's. Obviously 40's are the compromise.


With a drilled airbox its no wonder its running lean with standard jetting, but the worst effects are usually up the rev range with the spitting and popping, etc. The stock needles and main jets will be too lean. Taping the holes up as suggested is a good test. The drilled/chopped airboxes run crap with stock jetting so its a stock airbox lid or a jet kit. Performance is better with a stage 2 jet kit and opened up lid, but if a stock lid is cheaper than a jet kit then thats an option.

Thanks--  dug into it a bit more tonight and I feel like I got more questions than answers. 

I taped the airbox and ran the bike for awhile; no notable differences by my measure.  Pulled the plugs and they looked normal, rich if anything--wet even; but bearing in mind I'm only able to run it choked, this might make sense.

Tried turning the idle speed up again to see if I could get it idling with the choke off, again to no avail.  I honestly can't hear a differnce in speed when I adjust the idle speed screw, which may be a red flag there.  Drained the carbs and took them off the bike to check jet sizes.  Butterflies seemed pretty wet, and there was a small but noticeable amount of liquid gas on the intake boots.  Holes in the airbox make me still want to say lean, maybe the combination of the richened mix screws, increased idle speed, and choke is causing all this extra fuel.

Main jets are size 70, pilots size 40. Main air is 155....wait what?

I've read conflicting info as to the main jet sizing.  Haynes says main 70, main air 140.  Ducati desmodue says 140 main, 140 main air.  The rebuild kit I bought came with a 70 main and a 140 main air, so I'm inclined to think this is the way to go.   Does anybody have any insights to this?  Should I put in the 140 main air, turn pilots out to 3.5, lean it way out and see what happens?

P.S. snapping the throttle kills the engine, even with the choke on, but if I ease onto it, I can get it revving up.

70 jet is the starter jet for the choke. Main jet protrudes from behind the little clamp holder screwed in place by the 70 starter jet. Pilot jet is deep in the second drillway cavity.

155ish main jets
40 pilot jets
Custom needle from DJ or FP on notch #3 or #4
These would be fine for a drilled airbox lid. Up to 165 main jets for completely cut out airbox lid.

I would run the standard air jets (located in the carb bellmouth) as most tuning data and kits work off those.
Edit: Baseline jetting may vary for a W head motor.

Are the diaphragm slides rising evenly when the bike is revved?

I see.  I'm not sure, how can I tell that the diaphragm slides rise evenly?

The diaphragms new as I found a tear in one of the old ones.  I'll see if I can give it a look tonight.  As for reassembly, I'm thinking about trying two things: 

1. Keeping jets as is, and raising the needle to the 4th position-- richen the mixture within the parameters suitable for my air/exhaust setup

2. Switch from a 155 main to a 140, leave the needle as is -- lean the mixture.

Is it crazy to lean the mix farther? I've been thinking it's been lean all along, but seeing all that fuel yesterday has me second guessing.

If the slides don't lift via vacuum they'll run crazy rich. Verify slide lift before you go further.

Remove the airbox lid & filter and watch the slides as you rev it.

Update: I've taken the carbs all apart and reassembled once more.  I know you're not supposed to change more than one thing at a time for proper diagnostics, but--well I'm tired of taking the carbs on and off at this point hah. 

I changed everything. New jets all around from my jet kit, I had previously left them in a little bag in the hopes that the previous owner had it tuned to run.

When I pulled the pilot jet (a size 40) I noted holes in the sides of the jet which I had thought were stock.  upon closer inspection, it looks like these are DIY... I presume this was the culprit for the improper fuel flow in the pilot circuit. 

Here are the jet sizes now (Previous  / current):

Main Jet:  155 / 140
Pilot Jet: 40 (with holes)  / 40 (no holes)
Starting Jet: 70 / 70
Needle:  3rd position down / 4th position down
Idle mixture screws: 3.5 turns out

Also, the airbox "swiss cheese" holes are masking taped over to better emulate stock

Tweaked the idle speed a bit and Eureaka! The bike now idles!  But it's still not running quite right... I get a bit of backfiring through the airbox, I haven't run it that much yet, but I noticed it at about half throttle.  The 155 main jet might have been better after all.  I'm going to run it for awhile first and put fresh gas in before I make any other changes though, maybe I can get a better read out of the plugs after a few miles.

Well done [thumbsup].
A good baseline now. People get those pilot jets confused as there are two types as you found.

Ancient history at this point, but after a winter of break time, I'm back at it.  Swapped the main jets back from a 140 to the 155 and wow, what a world of difference.  Throttle response is vastly improved and, while it's not running perfect just yet, it's damn close.  It idles fairly well after a quick warmup with the choke, though it definitely sounds slow. 

I think I may need to adjust the needle position.  First I'll probably just increase the idle speed a hair and take it for a cruise and see how it feels--

Thanks everyone who helped out so far!  I wouldn't have had a clue where to pick back up if this thread of our conversations didn't exist for me to reference!

 [thumbsup] Was the jet kit a Dynojet kit?

No, I just used some jets I had lying around the garage; most of the jets I have come from cheap carb rebuild kits ordered online.  Used stock size jets from a rebuild kit and a 155 from another set of carbs I had

I asked due to DJ jets different calibration to oem, a DJ155 is about oem 145. Also try and use genuine Mikuni jets instead of no name jets. So you are using 155 Mikuni jets with a drilled airbox or stock? 140 normally suits stock closed lid, 155 drilled or half open and 160-165 for fully open lid. Are the needles oem or from a DJ or Factory Pro kit? If your fuel has significant ethanol content then that may explain the need to jet up as ethanol leans things out.

People have run into tuning issues with cheap import or pirate jet kits (and carbs for that matter). With those kits (and pirate carbs), established tuning data may not translate across. I don't touch the stuff.

Thanks, I'll keep that in mind.  I'm working on a budget, so I'm gonna make the no name jets work. 

Probably 155 Mikuni's but I suppose they could be from a DJ kit.  They are used and appear to be pretty good quality brass.  Airbox is drilled with a K&N filter, but, as mentioned earlier, I've taped off the holes to better emulate stock settings. 

I was reading up a bit on the particulars of mikuni carbs via this handy webpage: http://www.iwt.com.au/mikunicarb.htm#:~:text=The%20pilot%20jet%20is%20the,to%20around%201%2F4%20throttle. (http://www.iwt.com.au/mikunicarb.htm#:~:text=The%20pilot%20jet%20is%20the,to%20around%201%2F4%20throttle.)

It sounds like at this point a little tweaking of the pilot circuit should get me where I wanna be.  I believe I need a richer mixture there.  Could someone tell me: Would I turn the pilot adjustment screws in or out to achieve a richer mix?

Thanks!

Out [thumbsup]

So you/we are guesssing here, same w pilots. Hard to advice when not knowing the parts....

1. Give us pics, all jets & needles and we`ll sort out what is Mikuni or DJ.
2. Or, buy, despite budget, a DJkit for open airbox.

Alright folks, another update!  So I raised the needle and I'm now running on the 5th position from the top.  These needles are not stock. Cheepos from some kit. Currently running:

Main Jet:  155
Pilot Jet: 40 (no holes on the sides)
Starting Jet: 70
Needle:  5th position down
Idle mixture screws: 4 turns out

Airbox DIY holes are taped off

Bike idles strong now and sounds good.  But, I'm getting too much gas.  When I choke it (which it does not need to start) it smokes to bejesus.  after awhile of idling and a bit of throttle, there is a small small amount of smoke.  Its whitish greyish and I presume it to be excess gas burning-- not oil.  The throttle response is swift and snappy, but it continues to rev high 2 or 3 seconds longer than it should.  Pulling the clutch lever, strangely enough, lowers the high rev.  Any Ideas which circuit is running too rich?  I'm thinking idle screws or needle. 

4th position needle and 3.5 turns out had my engine cutting out when I snapped the throttle and only starting with choke.  I think the carbs are likely a little out of sync, so I'm trying to find a time to run out and borrow a decent monometer setup.  When it smokes, I can also see a bit of smoke wofting from the lower cylinder exhaust where it meets the engine head---I think this will need a new crush gasket; this is the side also smokes much more prominently in the instance of smoke.

Thanks all

Making progress on this, tried several configurations of needle position over the weekend.  Can someone tell me what the stock needle is for these BDST 38 Mikunis for the M900? 


I have a couple different needle types from various sets of these carbs and rebuild kits and I don't know which is which...

I have one that has a long skinny taper and another that is wider with a short stout taper at the end.  (can't quite get my image uploaded...). 

More info for those who want to dig in:
So far, I've had the best performance with:

-the longer skinny needle at 4th position from the top
-155 main
-70 secondary
-3 turns out

with this configuration, the bike runs great in 1st gear, pretty good in 2nd/3rd, but bogs when I snap the throttle twice on the second snap. 

My hypothesis was that this could be a symptom of rich condition (I have a hard time believing the floats could be emptying so fast and starving the engine for fuel).  Maybe the second snap was flooding?  I never did stall though...

Switched needles to see if I could get any better performance, but the bike was real jerky at quarter throttle and worse at half with the fat needle. (much too lean, I think)

Went back to the long skinny taper, and switched the main to a 140... fired up right away, but when I snapped the throttle it died.  Hasn't stared back up yet from there. strange since I don't think the main has much to do with the starter circuit...I'll check my diaphragms and try again with this config.

Still guessing w needles & jets.
And guessing rich/lean.
Try plug reading: When you reach a problem throttle/RPM,
kill engine, check plugs, black=rich, white=lean.

Thanks,

no tach, so RPMs are by ear only.  Plugs say lean sometimes, rich other times.  I'm trying to find the right balance with what I've got. 

I can see it running rich with the smoke out the exhaust when I raise the needle way up.  I can also see excess fuel residue in the airbox in these instances.  I haven't found a middle ground yet.  Slight adjustments from rich condition throw me into territory of white plugs and starving engine... 

I'm thinking I may have a washer on the wrong side of the needle's e-clip.  I've also got a washer below the needle assembly in the slide (or is this a shim?). looking at the stage 2 dynojet kit instructions I'm gonna try to replicate with my 155 main... wish I could figure out which of these needles is stock.

Sharp tapered needles are not stock for a 900 or 750 so are aftermarket and may have markings. The fatter needle with a slight taper is most likely stock and it may have a Mikuni insignia stamped on it.
Does the air box lid have the snorkel? If not, then stock 140 jets may be a little lean.
Worn needle jet tubes will make cruise/midrange too rich.
What do your plugs say?

Thanks koko, you're correct they do have markings.  Y218's or something close to that (I can't recall the last 2 digits). 

No snorkels, and the lid is "swiss cheesed"  I masking taped the DIY holes to better emulate stock.

I have also a set of needles with mikuni markings on them that have only one position on them.  I tried these last night with both the 140s and 150 main jets.  same results each time: starts and idles fine, dies at the first crack of the throttle.  I'm thinking super lean.  I cant for the life of me figure out why the stock needles are giving me such lean results... maybe the pilot air jets with the side holes were appropriate after all?  (I switched these out for jets without side holes when I discovered them, presuming this to be a modification).

I went back to long skinny needles at 4th position last night.  This time I removed the washers (shims?) at the bottom of the slides and moved the washer on the needle to the topside of the e-clip.  This needle is consistently giving me the best throttle response so far.  Looks like I need to sync again though; the the slides are not rising evenly anymore.  Left side lags and backfires into the airbox when I watch it run without the lid (does this indicate rich?). 

At this point I'm considering coughing up the change for a stage 2 dyno kit... unless I can emulate it by just buying 165 main jets?

Update: these washers are indeed Sudco carburetor needle shims.  Pictured here under the mikuni N208.099 press-in main jets:

http://www.pjmotorsports.com/mikuni-jets.html#main%20jets (http://www.pjmotorsports.com/mikuni-jets.html#main%20jets)

The pilot's without the bleed holes are correct. 40 is in the ball park.
The bikes were on the lean side in the midrange (needles) when dead stock for emissions, so no snorkles is having an effect leaning out some more.
The stock needles were often replaced to add some fuel if nothing else was. Lean US needles were substituted for not so lean EU needles or an aftermarket kit was used.

You have a concurrent issue with the slides not rising together and correctly as ducpainter mentioned. It doesn't take much for a diaphragm to not seal correctly. Pinching a lip with cap is common and a ring not seating happens. Test before fitting by blowing in the vacuum hoses and manually lift for even spring return.

The tape would bug me as is not sustainable. I'd commit to an open airbox or a stock one. Drilled de snorkeled lids need  sporty needles and 150-155ish main jets, open lids 162.5- 165's, stock 140''s, but local fuel varies. Ducpainter ran a drilled or partial lid so try his jetting if you want to remove the tape but not chop the lid.

Thanks Koko, this is helpful.  I'll make sure that diaphragm is rising evenly before I move on.  got any idea where I can find Ducpainter's jetting configuration?

PM him or ask the question here.

What markings does the sporty needle have?

When in doubt, start with the middle notch on an adjustable needle.

If you can identify that sporty needle, there will be a guide to its setting for your mods.

Groovy, thanks koko, I'm getting close.  Ordered 165 and 170 mains and sent him a PM.  I'll check the markings when I get a chance to open up the carbs again-- I recall it starts with Y2 and there are 2 more digits I don't recall after the 2. Is there a site you use to find these guides?

Look up Dynojet for their US kits.  I lost my link but the instructions for each kit are on line. The fat needles are stock so go with the sporty needles.

By 165 mains I take it that you're going to open the air box lid. Try a needle position midway or a notch richer, so how many notches on the sporty aftermarket needles?

My drilled air box lid...

(https://live.staticflickr.com/2832/8972570465_7e93a02d3f_z.jpg)[/url]

 [thumbsup]

https://www.dynojet.com/jet-kit-for-1998-1999-ducati-m900-monster-7209/
Click on the download for the installation instructions.

The kits for the earlier bikes were the same except for the nnedles. Probably due to the smaller valves in the 98/99.

Yep 729 Vs 730 needles. You're correct, the 729 needle was also used in the 750 which adds up. [thumbsup]

Shows that valve size and cams have as much to say as displacement.

Awesome, thanks folks--

Got my hands on some 165 and 170 mikuni mains from the aforementioned site, still not sure what the exact specs are on this tapered needle since it has no markings, but I've got it running strong!  Final jetting configuration:

165 main
70 secondary
Pilot air screw 4 turns out
long skinny tapered needle 4th position down + 1 needle shim (so ~4 1/2 down)
40 pilots without bleed holes

My airbox is drilled just about identically to ducpainter's shown earlier, and I'm running stock exhaust.

Thanks to everybody who helped out with this! The plugs look great and bike is extremely fun!

 [beer]