Resetting Stock Idle Speed

[motorcycles]

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 

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

[koko64]

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).

[ducpainter]

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.

[Howie]

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.

[motorcycles]

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)?

[greencow]

Lever backward (towards the rider) is choked.

[Howie]

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. 

[koko64]

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

[koko64]

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. 

 

[motorcycles]

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?

[Speeddog]

Just cover the holes with tape, as an experiment.

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

[koko64]

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.

[motorcycles]

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.

[koko64]

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?

[motorcycles]

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.