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Thread: Winter Layup - 1995 Ducati 900 Supersport

  1. #646
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
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    Lower Hutt
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    1,091

    more fun and games tuning

    Well, I've been annoying the neighbors all weekend. Still trying to get the bike running acceptably, am finally feeling like I'm getting somewhere... I'm writing this up now while I remember what I've been doing.

    I noticed a while ago that the bike has different length headers prior to the cross. The exhaust system is fairly simple: a curved header runs from each cylinder head into a four-way cross, which then feeds twin silencers. The headers enter the cross facing each other, but there are curved guides halving and feeding each header ninety degrees each way into the twin exhaust outlets. The header pulses end up running side by side in the same directions, not into each other. There is no provision for expansion, as more modern systems tend to do where two pipes join. Effectively it looks like the outgoing blast from each exhaust pulse is intended to venturi / siphon the other header pipe whilst keeping header diameter constant all the way to the muffler, also halving the resistance to flow by using both mufflers simultaneously. This may not be what's actually happening though. I've dredged up a few old photos, hopefully these show what the cross looks like.

    It's quite different to modern exhausts. Most current performance V-twins use a two-into-one-into-two configuration, with Y joints and pipe expansion. Headers appear to be equal length - curled if necessary - and the Y joint angle is fairly shallow, not 90 degrees. Ducati themselves have moved away from the cross, finally.

    I've spent a few days reading The Scientific Design of Exhaust and Intake Systems, Philip H. Smith. The book's rather dated but good. There were some key points I took from it concerning this particular exhaust system:

    1) The whole system will pressurise every time there's an exhaust pulse, from header right through to tailpipe. The piping gives a flow resistance, the silencers as well, plus there's pressure wave behaviour to consider.

    2) The waves can be around a third of an atmosphere in pressure, both positive and negative magnitudes. Negative pulses in the exhaust help to start to fill cylinders when there's valve overlap, positive pulses on both induction and exhaust sides help to tamp cylinders extra-full just before inlet valves close. The effect on tuning can be pronounced and timing is critical.

    3) Getting accurate charts of these waves (pressure vs crank angle) could be done in 1967 or earlier, via a very clever arrangement of crank-driven disc valve, single pressure tap on exhaust piping, and around 32 mercury manometers being fed one by one off the disc valve.

    4) The cross may not be functioning as a one-pipe-helps-the-other venturi properly. Smith was very clear that pressure pulses can go around corners, and certainly any pressure pulse coming back from the silencers and tailpipes would go to both cylinders.

    5) The different header lengths may have been an attempt to compensate for the different dwell intervals inherent to V-twins in the exhaust timings. This would work perfectly at one engine speed, although it might be possible to broaden the response over a fairly wide range of RPM via tricks with conical megaphones (visible on the front of the silencers, with parallel perforated pipe inside) and tuned interference or resonance with the non-flowing exhaust header.

    6) If there are differences in exhaust vacuum at the start of the induction cycle, when the valves overlap, this would directly affect fuel/air intake and thus carburettor tuning. The two carburettors won't be tuned the same.

    With this last in mind, I connected the vacuum gauges to the inlet manifolds and tried winding the damping valves all the way out so that the needles tracked variations in vacuum, not the average. This very clearly showed that the vertical cylinder has a wider variation in vacuum than the horizontal, at least at idle. The gauges couldn't respond quickly enough to keep up with variation at higher RPM. I swapped gauges between cylinders to confirm. I tried photographing this: the vertical cylinder's gauge is the lower one in the photo. The needle motions are blurred but there's an impression of the angles they have both swung over.

    I've been wondering why the vertical cylinder keeps on fouling spark plugs, while the horizontal comes through just fine. It's been doing this the whole time I've had the bike. I've checked or changed pretty well everything except the exhaust system. High vacuum spikes coupling through to the induction would tend to pull fuel, maybe quite sharply. No wonder it's had problems with running rich on the vertical for apparently identical settings and timings.

    While running more tuning attempts, I swapped the wide-band O2 sensor over and confirmed the effect. It's a difference of at least 1 full AFR point. This will only show up if the sensor is connected directly into a header, it wouldn't show on tailpipe sensors since the mufflers are shared and thus carburettor tuning becomes averaged out.

    It now looks like I have to road-tune one cylinder, swap the sensor, tune the other, then confirm that tuning on the first is still OK.
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  2. #647
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,091

    Tank breather valve

    Further progress (of sorts)...

    The throttle gauge has turned out to be very worthwhile. The cardboard gauge pictured previously has been replaced with a one-part disc cut from some coreboard (old real estate sale sign), cable tied to the Oxford hot grip and marked out to line up with the split in the throttle twistgrip mount. This is very readable while riding, and is also possible to lock exactly in one place due to friction between the cable ties and the twistgrip clamp.

    This precise readability of the throttle gave an unexpected result: the tuning is changing while at constant throttle and constant engine load.

    Initially I thought it was a headwind affecting pressure of the breather line, since this is now tucked inside the front of the fairing. I'd gone out to Wainui Beach with a tailwind and the bike running on the lean side. Coming back into the wind, the bike riched up by at least a couple of AFR points, then suddenly leaned out and stopped. Engine stoppages happened a couple more times on the way home. I'd pull over, wait about ten seconds, and then it'd restart just fine. This had happened earlier after Shiny Side Up (just once) and I'd been blaming it on the ignition, but this time a look at the transparent Y joint in the fuel lines showed the fuel bubbling merrily backwards out of the carburettors.

    Ah ha. Vapor lock. Maybe the fuel was being overheated due to slipstream off the oil cooler. I moved the fuel line upwards so that it is in ambient slipstream and went out for another evening ride, this time over the 'takas.

    Same thing. Just fine going over, then multiple stoppages on the way home. Whatever was doing it, it was getting worse. At the same time the AFR gauge was clearly showing the fuelling moving all over the place - sometimes rich, sometimes lean, and every time the bike stopped it was showing a super lean condition leading up to the stoppage. I finally twigged that there was a pattern: the bike would run consistently rich at low fuel demand, but start running lean and then have intermittent lean outs at higher fuel demands. It was about fuel consumption.

    I'm not quite sure now how I noticed this, but the roadside test was simple: open the fill cap. If there's a pop on opening and the cap jumps upward, the tank's been at positive pressure. If there's a rush of air inward and the cap doesn't jump, it's been at negative pressure. These were linked with what had been going on. Rich: positive tank pressure. Lean: slight negative tank pressure. Stopped: definite negative tank pressure.

    So, the breather valve clearly wasn't working properly. I took it apart and found a design mistake: it's possible for the thing to latch down and close up completely under sustained high fuel demand. This is where the engine stoppages and apparent vapor lock issues come from. I've tried to show how it is assembled in the attached photos. Something the photos don't show well is the slightly raised thin wall spigot on the tankward side of the assembly, the one with locates and centers the thinner, softer spring. Once the flat disc is sucked into this spigot, and then stuck on there by sticky old fuel, it isn't really coming off again. The spring isn't powerful enough to lift it off. Positive pressure in the tank will open a small gap but not blow it off completely, so it'll sit there until the next time. Problem. I found a fix: take a Dremel to the spigot and castellate the thing, get the finest burr possible and cut in-line slots into it so that even with the disc in contact, it'll still breathe.

    This appears to have sorted the stoppages. It hasn't sorted the variable tuning though.

    It's simple enough: if there's an increase in the tank's ambient pressure, there's an increase in the supply line pressure. That means that the needle valves in the carburettors have to seal harder. They'll only do that if there's a higher fuel level to force them closed. So changes in the tank's above fuel surface pressure will couple through directly to carbie tuning, by altering the float height.

    The breather valve, as designed, basically causes this. It features spring loaded rubber seals in direct contact with metal. It requires a slight vacuum to open, and a positive pressure to open again the other way. The tank vacuum comes from fuel being drawn to feed the engine. The positive pressure comes from fuel evaporating and increasing gas pressure inside the tank. The two effects appear to have a cross over point where evaporation will match fuel draw; I think it's strongly ambient temperature dependent and in real world riding this seems to happen at around 80 km/h.

    I can't be totally sure what these positive and negative pressures are, or what difference they're making in float height. I am sure that the pressure changes are quite significant compared to the very low supply pressure to the carbs. In a sense they don't matter anyway; the important bit is the difference in tuning, which I can see. I think it's around 2.5 to 3 full AFR points, with a time lag, which effectively means that fine tuning the carbs is impossible.

    The tank breather valve isn't something I'm happy running without. Every time fuel sloshes forward, under braking or hill descents, this valve has got to stop overflows. If I remove the thing and reroute the breather line down, I'll end up dumping fuel either over hot stuff under the engine, or losing tank contents all over back country roads and rapidly running out of petrol.

    I've spent a while thinking about this and the only solution really is to move away from contact seals. I had a play with a slider made from Teflon, deliberately not gas tight, but this weeps petrol and really won't work without catch bottles and roadside emptying of same. The slider also plugs gas flow so introduces a pressure gradient in either direction. After some thought, the only breather type that'll keep the tank connected to ambient pressure, with nothing in the way, is a type using floats and sinks, instead of spring loaded one way valves. I've got this on order:

    https://www.rallynuts.com/motorsport...ent-valve.html

    We'll see how this works out.
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  3. #648
    Join Date
    23rd February 2007 - 08:47
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    Suzuki gsxr 600k7. Hayabusa K9 DRZ250 K
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    Hopefully this will be the root cause of your tuning issues.

  4. #649
    Join Date
    24th July 2006 - 11:53
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    Quote Originally Posted by OddDuck View Post
    The tank breather valve isn't something I'm happy running without. Every time fuel sloshes forward, under braking or hill descents, this valve has got to stop overflows. If I remove the thing and reroute the breather line down, I'll end up dumping fuel either over hot stuff under the engine, or losing tank contents all over back country roads and rapidly running out of petrol.
    Is this really more of a problem than was/is the case with most dirt bikes, where the completely unrestricted breather tube runs down the steering head?

    If so then you could look at some of the pneumatic silencers, either sintered bronze or plastic with holes so small they don't actually pass fluids at the sort of pressure differential you're talking about...
    Go soothingly on the grease mud, as there lurks the skid demon

  5. #650
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
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    Lower Hutt
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    Quote Originally Posted by Ocean1 View Post
    Is this really more of a problem than was/is the case with most dirt bikes, where the completely unrestricted breather tube runs down the steering head?

    If so then you could look at some of the pneumatic silencers, either sintered bronze or plastic with holes so small they don't actually pass fluids at the sort of pressure differential you're talking about...
    True. The quick low-tech fix is just run the breather line vertically upwards a distance, then down through the steering head and exit low and to the side. Should work 99% or more of the time. If the bike upends or falls over or similar then as long as the outlet of the breather is higher than the tank, it won't free flow petrol. Of course that depends which flank the bike ends up resting on... so the fix then is to zig zag it from one side to the other inside the fairing while making sure there's a minimum gradient on the line with the bike in normal position. Any petrol sloshing around has to drain back into the tank or out or else there's going to be airlocks.

    The only references I could find to sintered bronze etc looked like they'd pass petrol as well as air. If I was going to go the way described above I think an inline filter would be a good idea - don't want dirt sucked into the tank... haven't met sintered that would pass air / vapour but not fuel yet though. I'm not having much luck googling, can you point me towards anything?

    SVboy - thanks, fingers crossed!!

    Anyway fantastic weather today, screw the imperfect running, got the bike out and went for a damn good fang out to Castlepoint. I'd nearly forgotten how good the 900SS is on that road. Had a great time.

    Incidentally I noticed an unexpected bonus of the auxiliary headlights and running lights combo. It turns out that safety meets hooning on this one - drivers were letting me through much more than previous, particularly on the Hill. Less SMIDSY. More VROOM. Yes. It's been a good day.

  6. #651
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Tank breather valve and results

    The new tank breather valve arrived, I fabricated a bracket up to mount it, and have been trialling it over the last few weeks.

    Short answer: it works but looks awful doing it. Next time I'm going with the flanged option, the one that requires drilling a hole in the top of the fuel tank. The outlet hose from that comes out at 90 degrees and so looks very close to the stock breathing arrangement.

    The breather works by having two ball valves running vertically. Both seal on moving upwards, with the valve in a normal vertical orientation. One ball valve floats on fuel and seals on a rising fuel level - ie against slosh forward under heavy braking. The other ball is solid and sinks through fuel, thus sealing against the valve being upturned (ie the bike upside down in a ditch, at which point presumably I've got bigger problems than carb tuning). The rest of the time this breather valve is open, without any obstruction to gas flow forward or back.

    AFR readings have settled right down. There's no longer any progressive moving to a lean condition under sustained back country running, or a rapid move to a super rich condition once getting to a built up area and 50 k zones.
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  7. #652
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Design issue with vertical cylinder valve guide

    I reckon I might finally have worked out what's been going on with the bike. Persistent oil fouling on the vertical cylinder's spark plug leading to rough running, misfires at low throttle, and general nastiness.

    I took a good long hard look at this problem and came to the conclusion that there's a design flaw in the way that the vertical cylinder's inlet valve guide is installed. It's been pressed down into a pocket, at a shallow angle, and the stock valve guide seal is a simple rubber sock.

    The guide, as made, is permanently sitting in a puddle of oil. This puddle, via surface tension and miniscus effect, is deep enough to reach up to the base of the rubber seal, keeping it permanently wet with oil. The seal isn't clamped on tight by a sprung metal band or similar - it's pure rubber tension, subject to engine operating temperatures and exposure to oil. In short the seal is not very tight on the guide. The rubber will flex under valve motion, effectively pumping oil from the puddle upward and inside the seal. Any wear on the valve guide or valve stem, and oil will be drawn under intake vacuum straight into the cylinder head.

    The engine will work fine while stem and guide are new. Clearances will be tight enough to keep oil flow under control, with the seal sort of an optional bonus. The fun and games will start once there's a few K's on the bikes clock and things loosen up a bit.

    I used a rag to soak up the puddle, then noticed that the puddle came back - several times. Interesting... it turned out that the guide seal was completely filled with oil. Voltaire's earlier comment about older opposed-twin BMW's came to mind: valve guides which are at high angles to the vertical, and which don't use seals at all and yet still work just fine. Looks like 'oil-splashed' is OK, 'oil-soaked' is not. I took a chance and carved drainage channels straight into the cylinder head.

    To do this, I used a special centerstand to support the engine, then removed seat, rear shock, fuel tank, battery box and IDI's, then got in to the head with a Dremel and a ball nose burr. I deliberately did not use anything with particle abrasives. This was carried out with the engine still in the frame and took roughly six hours. Cylinder head drain channels were plugged with disposable earplugs threaded onto a knotted piece of high gauge wire, and the camshafts and rockers were wrapped with rags. I still ended up with fine aluminium shavings everywhere so swabbed the area thoroughly with cotton buds pre-soaked in PB Blaster.

    Obviously this didn't protect perfectly or remove every trace of the carving work, there were still aluminium particles left... Perfect work would have required lifting the frame off the engine, taking the cylinder head off, and removing all internals including the camshaft prior to carving and washing. It's an old bike. Perfect left the building some time ago. I got nearly all of it and that's just going to have to do.

    I've subsequently run the bike for a week, still on the same oil fouled plug. Results: an immediate improvement in running, plus the plug came out dry (for the first time ever) when I pulled it last night. The bike still isn't running perfectly but given the black enamelled coating all over the plug nose that's not really surprising. Looks like oil doesn't burn off over time. I've replaced the plug but haven't run with the new one yet.

    A couple of comments after the week:

    1) AFR gauges can be badly misled by oiled plug misfires. These will show up as an apparently lean or super lean mixture; the mixture might actually be correct but the non-ignition will leave lots of unburned O2 present in the exhaust.

    2) These bikes are very sensitive to spark plug condition and ignition timing. Anything wrong with either of these will lead to rough running. It really doesn't take much of a deterioration in sparking to lead to weak ignition, delayed (and weakened) combustion, and a very rough motor.

    The photos show exhaust and intake valves before and after, with a shot of the Dremel cutting setup for the intake. Care was taken in both cases to avoid having the burr hit the valve guide or its seal.

    I'll run for a bit more with the new plug and see how that goes. In the meantime the plan is to get the bike to the next scheduled interval and change out oil plus filter and both plugs. The timing belts are due as well, it's been two years since the last set.
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  8. #653
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    LED headlights / running lights and colour filters

    Have been having a play with the LED auxiliary lights setup. After a couple of rides I've been forced to rethink this arrangement - it's annoying the hell out of other road users (bikers included) and is producing glare off the road surface immediately in front of the bike. The glare is so bad that I found I was seeing more with the auxiliaries completely off, while riding the Rimutaka Hill on a moonless night. I was getting light flare off the visor, filling in shadows and completely losing detail in the middle / far distance... kind of important if looking more than fifteen meters ahead.

    There's a couple of reasons for this. The first is the beam pattern: dip headlights have a squashed semi-circular or rectangular pattern, with light projected forward but not up, and most of the lumens going into the middle distance. The LED auxiliaries have a very simple parabolic reflector which throws a conical pattern of light forward. The only way to not dazzle oncoming road users is to point these things down. That means that the tarmac immediately in front of the bike gets most of the 900 lumens, at short distance.

    The second reason that they're so obnoxious is the colour. These are very high-energy blue-white LEDs. They're very good at dazzling onlookers and leaving streaks across retinae but to my mind, surprisingly bad at lighting the road up so I can ride it. I'd heard somewhere that the human eye is best tuned to yellow-white (ie. sunlight), and after dark a slightly reddish yellow light is what we see best by in terms of judging distance and avoiding fatigue. Blue-white light is too stark: everything gets washed out and looks flat. The LEDs are bright enough alright. It's just that it's not right, at least for my eyes. After I'd turned the auxiliaries off, I found I was seeing easily with old-fashioned tungsten halogen, even with the 900SS's famously crappy front headlight.

    There's also the legalities. Auxiliaries are OK if the main headlight/s are off... we're supposed to keep our mains on at all times, so legally speaking there isn't really the option there. Positioning lights along with mains are OK but aren't defined clearly. Small and not dazzly should be OK. Whatever passes muster with unsympathetic non-biker-friendly traffic cops, really... the original 4-light setup almost certainly wouldn't.

    I've taken the big flank mounted auxiliaries off and changed the tint on the smaller positioning lights under the main headlamp. The tint (filtering) is worth going through: I was trying to change nasty blue-white into softer, friendlier yellow-gold, like the tungsten halogen main headlamp.

    The photos show the mod. Some very cheap plastic folders from Warehouse Stationary provided the colour filters. The wad punch made cutting them easy. Installation was simple, just unscrew the lens or window on the front of the lamp, then try to put the filters in. After some experimentation I settled on three yellow, one pinky-red (magenta? or whatever the colour is) as a close-enough match to tungsten halogen. Something I found interesting was that if the plastic folders are laid on top of each other, the colour that comes out is an orange tint, very similar to the colour of an indicator lamp lens.

    The reason for the mix of colour filters is maybe best shown by the colour wheel. If light of one colour - say deep blue - goes through a colour filter opposing blue, say an orange filter, most of the light will simply be filtered out. Very few light sources are purely one colour, there'll be a spectrum, so there's going to be some light in the green and the magenta, and that'll get through the orange filter. I found with just the yellow filters, the lamp turned whitish-green. Yuck. The folders aren't exactly pure yellow, they do have some green in their tint. The LED must put out something in the greens. The colour wheel shows magenta opposite the greens, so a weak filter around that colour is the way to get rid of it.

    Comparison with the mains shows that it's pretty close. Another thing (which in this case is a plus) is that the plastic folders are definitely not optical material; they're diffuse instead of glass-transparent. They spread light out. This takes the edge off the lamps output very nicely and gives a nice soft uniform light ahead to complement the mains.

    I've installed with the yellow layers closest to the lens, the single magenta closest to the LED. Once lit up the order won't matter; this is so that with the lamp off, it'll look yellow-ish rather than red.
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  9. #654
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    Further notes on tuning

    Just some observations, jotted down before I forget... I've spent another (!) day attempting to tune the 900ss, with what might be progress, or might just be further tail-chasing. I'll start with some thoughts on Pat Burn's FCR Tuning Guide, as found here:

    http://www.factorypro.com/tech/tech_...Burns,Pat.html

    Bear in mind that this is just me, I've got limited instrumentation (one AFR gauge, SOP dyno, plus plug chops) so this really could be just conjecture. But this is a specific version of the guide, aimed at the 900SS and anyone trying to use FCR41's with the bike.

    Procedure:

    0) Clean everything. There's no point in tuning against jets blocked with dirt.

    1) float valve seats. The bike's 34 - 50 HP per cylinder / carbie, with pump-assisted low pressure gravity feed, so go with the 3.2mm seats. Check that these aren't floating loose in the carb bodies, you may have to use shim washers to set them in place tightly.

    2) Float height. 9mm. Done.

    3) Main fuel jet, main air jet. There are a few FAQ's, settings charts and forum posts out there so there are recipes to follow, alternately fit AFR gauge and go WOT somewhere legal and safe, or dyno. These affect everything so there's no point going further until you're confident with them.

    4) Needle selection. Probably you'll be OK with the needles that came with the carbs but maybe you've modded airbox etc and need non-standard. Again, recipes off the web here are your friend, alternately further 1/4, 1/2, 3/4 throttle tuning with AFR and / or dyno. In order: base diameter, clip position, taper. L1 basically is clip position so when you run out of clips you'll need a different L1.

    5) and here's where it starts getting painful, the below 1/4 throttle settings... start with slow air jets screws at 1 1/2 turns out, set idle speed to 1000 RPM or so, and try to get the idle mixture right.

    This is worth further comment so I'll go into detail. Burn's guide discusses setting idle mixture by ear: you rev the engine and listen for rapid settling to stable RPM. If the idle speed hangs a few hundred RPM high before settling, it's lean, if it slows to below idle and then recovers, it's rich. Burns says to adjust for whatever gives you instant return to stable idle RPM on closing the throttle. AFR is a guide but not really reliable. Fine, except there might be a problem: the 900SS has unusually long and large diameter inlet manifolds.

    There's a big problem with large fuel-air inlet manifolds: wetted surface area. Fuel will condense out of mixture onto the inside surface, or evaporate off again. That condensation / evaporation is dependent on air temperature, manifold temperature, and manifold absolute pressure (hence MAPs sensors in injected engines). Petrol evaporates more readily if it's in a high vacuum situation, ie near closed throttle. Open the throttle, fuel drops out of saturated vapor and condenses on the manifold walls. Close the throttle again and all the liquid on the manifold walls starts evaporating in a hurry. Since the manifolds have a large surface area compared to how much fuel the engine sips at idle, when the throttle is closed there's a temporary rich condition which persists for quite a few seconds. This is like briefly pulling a choke: the engine runs faster.

    This might lead to a major difference in tuning compared to Burn's guide. Maybe you're better off tuning idle steady-state, ie reading against no-load AFR or plug chops. Transient behaviour when the throttle is closed might not be an accurate way to do this, and based on past experience I suspect that tuning for instant return to normal idle will give you an overly rich steady state idle mixture. It looked that way today; closing the throttle gave a brief richening of the mixture.

    Anyway, what you're supposed to do next is to turn the Idle Mixture Screws in or out and then change slow fuel jets to suit. If the I.M.S. is less than one turn out from bottomed, you need leaner, so go a size down in the slow fuel jet. If the I.M.S. is more than two turns out, clearly there isn't enough fuel and so you need to go up a size in the slow fuel jet.

    Yeah. I pulled the IMS screws and had a look at them... then had a play. There's four full turns of adjustment before the needle nose of the screw leaves the needle bore. The experience today was that the screws are still affecting mixture out to 3.5 turns, although the effect becomes more gradual as they go further out. I think the range of useful adjustment is much wider than 1 to 2 turns, I'm inclined to think it's more like 0.5 to around 3.

    The other thing about the idle and slow jet circuit is that it handles emulsion, not liquid fuel. The slow air jet passage is aimed at the perforated base of the slow fuel jet. Air and fuel start mixing at this point, inside the body of the carburettor, not in the carb throat. Evaporation starts as early as possible. This might be the main reason why the carb bodies tend to run colder than ambient while in use.

    The idle / slow circuit has two inlets to the carb throat: one behind the vacuum plate, ie on the high vacuum side - this is fed and controlled through the I.M.S. The other jet is between the front of the slide and the vacuum plate. It's hidden behind the main emulsion tube and needle and is only really visible from above with the slide removed. This is simply a fixed, drilled hole leading straight to the slow fuel jet and slow air jet, which are the only controls over the emulsion flowing through this opening.

    The bike's had a problem with running super rich just off idle. Try as I might, I can't seem to tune it out, particularly on the vertical cylinder. I think this second circuit might be very abruptly providing emulsion once there's a slight opening of the throttle. That's guesswork, it's just not possible to see what's going on due to the jet's position. Anyway, this is why I've been taking the IMS out so far. Getting the mixture away from super rich at 1/16th through 1/8th means running a small slow fuel jet, but then I have to open the closed throttle fuelling right up to compensate, and / or tolerate lean or very lean idle.

    And then...

    6) recheck needle straight diameter and clip position. These play off against the slow air circuit between 1/8th and 1/4. There can be a lot of iteration back and forth.

    7) no-load revving should sort out whether the slow air circuit is too rich or lean, adjust as necessary for high RPM and then re-iterate the slow fuel jet and IMS setting. And needle root diameter. Bah.

    Oh, and 8) the cylinders will be different due to assymetric exhaust pulses. It isn't 360-360 timing, it's 270-450. The vertical gets a higher vacuum pulse than the horizontal. These pulses are quite strong in the 1/8th-ish region. Guess where most street riding happens... Oh, and the anti-rattle shims installed earlier seem to have made the carbs much more sensitive to this stuff too, mostly by ensuring that the vacuum plate actually seals properly instead of allowing air leakage due to bouncing around. THere's also a pretty good chance that there's fuel coming up past the needle as well due to a too-small root diameter, but there's not really any way to see this happening.

    9) iterate. There's no way anyone gets this 100% on the first pass. Or the second, or...

    Basically the below 1/4 stuff is a right PITA. No wonder people just say 'it's meant to be rough' and ride it.

  10. #655
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,091
    A wee note about carburettor slide cutaways.

    One tuning parameter is the cutaway; this affects the transition from the idle to the main circuits, ie the range just off the stops. In most throttles it's changeable by changing slides; the FCRs don't offer that option. There's one issued slide and that's it. At least I think so: it's stamped with 15, which would seem to indicate some kind of series. It's just that I haven't seen anyone offering these for sale.

    The basic story is that more cutaway means a leaner mixture, just off throttle. I decided to give this a go since that off-idle rich transition is the bulk of my tuning problems. Doing this means very carefully, controllably, cutting more cutaway. Cut a little, test, cut a little more, test again. I'm taking 0.10 mm at a time, since it's kind of a one-shot deal. It's laborious.

    It also seems to be working, in that the tuning spread has reduced from over 5 AFR points to around 2.5, with 0.4mm off so far.

    The cutting method has to preserve the cutaway geometry and centering. It also has to be controllable and unlikely to catastrophically damage the slide, ie by biting in or flinging the thing. Not having access to a mill and all the goodies, what I've done has been to lathe up a cylindrical former of the same diameter as the slide cutaway, then use double-sided tape to hold some 400-grit wet'n'dry to the diameter. A bolt through the center and I'm good to fix this into my ultra-cheap drill press.

    The drill press isn't being used in any sort of drilling or milling capacity. The cutter doesn't rotate. I'm using it as a vertical file. The wet'n'dry has been greased, in order to keep chips of silicone carbide under control. I don't want abrasives dropping over the slide and ending up in the vacuum plate / carburettor body interface - if that happens, they'll rapidly embed into aluminium and then rip the hell out of the coating on the vacuum plate.

    A key part of this is measurement, being sure of the length I started at and where I've cut to. The verniers can very easily go off-square, hence the socket being used as a guide.

    I'm unsure if this is helping. One thing I don't have is some sort of measuring device for vibration; I'm trying to sort out unacceptably rough running but it's purely by feel.
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  11. #656
    Join Date
    3rd February 2004 - 08:11
    Bike
    1982 Suzuki GS1100GK, 2008 KLR650
    Location
    Wallaceville, Upper hutt
    Posts
    4,265
    You are a long way down the carburettor rabbit hole and the end may not yet be in sight. Have you considered fuel injection from an other-then-Ducati source? One such system I am on the process of building is Speeduino - an Arduino based system. check out F5 Daves thread on two-stroke EFI to gain an insight into the tuning capabilities of the hardware and software.
    it's not a bad thing till you throw a KLR into the mix.
    those cheap ass bitches can do anything with ductape.
    (PostalDave on ADVrider)

  12. #657
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,091

    Alternator rotor damage

    I'd had a thought: the flywheel attaches to the crankshaft not once but twice. There's a splined hub. The flywheel itself bolts to this via seven M6 cap screws, and is located by a pin. If this assembly had loosened after the main bearing failure, maybe that might explain some of what's been going on in terms of vibration and weird running issues. Only way to be sure was to drain the oil, pull the cover, and get in there to have a play.

    The carrier / flywheel interface turned out to be fine. No play, no off-square, and the cap screws were in so tight that I reckon there's a chance the heads would shear if I tried to undo them.

    What was interesting was noticing the damage on the inboard face of the alternator rotor.

    The left had side of the crankshaft carries, in order from main bearing going outward: the half-speed drive to the timing belts shaft; a free-spinning one way clutch for the starter sprag, and inside that a roller bearing and cylindrical shoulder; the flywheel carrier and flywheel; the alternator rotor; and then the domed conical washer and alternator nut. There's a stack of components, in other words.

    This spins at high speed and there's a lot of mass in the flywheel, which is roughly midway between the main bearing and the outboard bearing in the alternator cover. If it goes off center, there's vibration right there. Just to make life more fun, the crankshaft itself will be constantly flexing and twisting due to pistons being shoved around by combustion forces. It's not only simple whirling, like when the washing machine gets all the towels on one side of the drum. The crankshaft will be bowing one way and the other as pistons get shoved and then release, or go through TDC or BDC.

    The stack, going over the outside of the crankshaft, greatly helps the rigidity of the assembly by being of larger diameter. This depends on everything being flat and square. If something's off, that rigidity can be compromised, or worse, the crankshaft could be bowed one way or another once the alternator nut is done up and the flywheel then set off-center right from the start.

    I pulled most of the stack components and had a close look. Everything made out of steel looked alright but it turned out that the alternator rotor wasn't flat and settled on the flywheel, when I tried this on the bench. It'd rock slightly from side to side.

    This does kind of follow. It's only purpose is to hold the permanent magnets for the alternator, in a non-magnetic material, so it's cast in soft aluminium. The crankshaft must have gone through severe flexing during the main bearing failure. The rotor was by far the softest item in the stack, so it got deformed.

    The solution was to make an inner collet to hold it on a lathe, and then very carefully turn both outboard and inboard faces back to flat and square. I didn't have to take much material off (around 30 - 40 microns each side) to do this. Re-testing on the bench quickly confirmed that the rocking was gone, they sit properly together now.

    As a note - for years I'd assumed that the half-circles of black material were some kind of magnetic material, or bonded rubber, so I'd left them alone. Nope. They're combustion ash, nothing more. They come straight out with a toothbrush.

    As to testing... reassembled today, what seems to be a slight improvement, there's still some vibration there. The outboard bearing is a possibility. I'd got wrapped up in dealing with the rotor and had left it. I'll have to have a look with the engine running and the tiny cover removed and see if the shaft and bearing is steady or not.
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  13. #658
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,091
    Quote Originally Posted by pete376403 View Post
    You are a long way down the carburettor rabbit hole and the end may not yet be in sight. Have you considered fuel injection from an other-then-Ducati source? One such system I am on the process of building is Speeduino - an Arduino based system. check out F5 Daves thread on two-stroke EFI to gain an insight into the tuning capabilities of the hardware and software.
    Yep agree. Am getting pretty tired... however (very slow) progress is happening...

    One thing I was toying with was the idea of buying a donor injected 900SS and doing an engine transplant, looting the new bike for motor, shock, forks, ECU, injectors, throttle bodies etc. Apparently it's almost a straight swap in, some minor fabrication needed but nothing major.

    The Speeduino idea is interesting, I'll have to read up.

  14. #659
    Join Date
    20th January 2008 - 17:29
    Bike
    1972 Norton Commando
    Location
    Auckland NZ's Epicentre
    Posts
    3,468
    I shelved my 39mm flat slides, nice bit of shelf bling.
    DeMyer's Laws - an argument that consists primarily of rambling quotes isn't worth bothering with.

  15. #660
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,091

    OEM clutch pressure plate cracking over time

    Just a quick side note... as part of the current engine work, I ended up removing the pump cover. I have to take the clutch apart to do that, while rebuilding I decided to be fussy and clean everything, and I finally noticed something I should have seen earlier.

    The stock clutch pressure plate, which has over 100,000 km's on it now, is covered in fine cracks. It's practically spiderwebbed. These normally hide from view under a coating of clutch dust.

    I've taken a few closeup pics with a dedicated macro lens. These aren't all of the cracks visible, this is just the selection that seemed to photo best.

    This has got me wondering if flex in the pressure plate is a big part of the reason that I've been having disengagement issues with both bikes. They're both running the stock clutch pressure plate. Both are fine (ish) when cold, then have problems with clunky clutches when warm. The 900SS is particularly bad but then it's got twice the mileage of the ST2.

    Aluminium doesn't have a particularly high Modulus of Elasticity (the stress required to elastically deform the material, i.e. how springy or solid it is). Aluminium also has a rapid fall in stiffness with temperature, get it hot and it rapidly goes soft. I'd be very surprised if diecast Al-Zn alloy was better than normal alloy grades in terms of stiffness and strength, either at normal ambient or once hot. Throw in a lot of cracks in an apparently solid object and suddenly flex in the plate really could be an issue.

    The cracks themselves are probably a mixture of as-diecast cracks and stress cracking from cyclic loading, vibration and heat / cool cycles. This component doesn't have an easy life. I can see why Ducati went with diecast alloy - like it or not everything is made to a price point - but why they had to go full weight saver on this when it's got a heavy steel clutch basket is a mystery. It's obvious that some engineer really did spend time on the design, bracing it, buttressing it, putting neat little stress relieving curves and fillets everywhere... but it's clearly made as thin as possible. Its weight is almost trivial next to the rest of the clutch pack, even with aluminium friction plates. There isn't much to be gained here by shaving a few grams.

    At this point the plan is to replace both pressure plates, on both bikes, with something as stiff as possible and see how the change goes. The aftermarket has a lot of CNC machined plate aluminium options available at reasonable prices. The material's good, but I'm not so sure about the designs. They've generally tried to go as thin and light (or lighter) than Ducati OEM, while leaving sharp internal radii everywhere and machining visually interesting cutouts instead of keeping solid material. I'm tempted to have a go at machining my own, but the internal cogging necessary to mate to the clutch hub might be tricky.
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