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Thread: Winter Layup - 1995 Ducati 900 Supersport

  1. #676
    Join Date
    5th January 2007 - 14:58
    Bike
    motocompo
    Location
    Buttfuck nowhere
    Posts
    5,156
    Quote Originally Posted by OddDuck View Post
    The second issue with the design is the nose cavity and its liability to trap air, just behind the banjo bolt. Once there's an air bubble in there, flow won't knock it out in either direction. It'll stay trapped until the master is taken off the bars, tipped to banjo-down vertical, and the lever flicked a few times.
    Correct. Bolting the master cylinder firmly to the handlebar should be the very last step in bleeding brakes, in fact 90% of the air will naturally come to the top given a little patience & holding the master cylinder on an angle is the best way to achieve that.
    I have a short section of handlebar that I bolt master cylinders to, so I can achieve the required angle.
    It usually results in less than a tablespoon of wasted fluid.

  2. #677
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,274
    While carrying on with the rebuild, I noticed that one of the old plugs has a faint brown line on the insulator, ending at where the spark plug boot starts.

    It looked a wee bit like an arc path, like high voltage has been tracking across the insulator and leaving a trail. There will be some combustion gas blow-by due to differential expansions in the plug body, if the plug has enough k's on it, but there was also something silvery-grey over the insulator. All plug nose insulators were fouled and they haven't been run all that long. I had a look inside the plug boots and found a mess.

    These are the older style that have a spring loaded brass socket which pushes on to the threaded end of the spark plug - the bead at the end has be unscrewed and discarded for these boots. They've had a lot of plugs through them over the years and it looks like particles of brass have been coming off and getting stuck onto the inside walls.

    There's a very clear carbon arc path shown in the photos, plus a few fainter lines visible. They look like someone's been doodling with a pencil. I've been losing significant amounts of spark to this, possibly for some time, and it was like this on both boots. A quick turn with some bits of 360-grit paper, held on a rubber tube serving as a mandrel, cleaned up both boots without too much fuss. The plastic looks alright, if no longer glossy. I haven't attempted an engine start yet though.
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  3. #678
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
    Location
    Lower Hutt
    Posts
    1,274

    Running again

    Finally got there. The bike's going again, vibration sorted.

    Multiple causes...

    Ancient hose clamps between carburettors and inlet manifolds which had worn smooth on screw and band, these just didn't stay tight. At first I tried improvising wire clips to lock the slotted heads on the drives but finally gave up and bought the proper (improved) replacements.

    Rotten wiring and tarnished connectors on the low tension side of the ignition. The grounds for the ignition modules really weren't the best, and the brass crimp connectors were shot too. I found that the same part was used on a Monster so ordered these and replaced the wiring completely with something thicker.

    Misaligned inlet manifolds. They can be moved around a bit on their cylinder heads; this can mean they're too wide or too narrow on the parallel carburettors and the rubber heat mounts can be made to flex a bit far as a result. Simple enough to sort out.

    Gunking in the carburettors themselves. Well, this is embarrassing... of course this is going to happen if the bike's laid up for weeks at a time and only run briefly. Gotta put a couple of tanks of petrol through it before making any calls about fuelling.

    Loose engine mounting bolt. There are only two of these, both very long M10's, and a tight couple to the mass of the frame helps to smooth out any engine vibration. I found the rear bolt had lost most of its torque - reassembly this time was with Loctite 222.

    The biggie was an imbalance in the stock flywheel. I finally put a dial gauge onto the flywheel to check runout and presto, yes this'd explain the shuddering and why it's particularly nasty on the left footpeg. It was showing a runout of 120 microns, aligned with the induction trigger. This increased to 170 microns on a re-fit, i.e. 85 microns off-center. There was some free play in the flywheel mounting spline. Rotating the flywheel mounting position by approx 180 degrees almost cancelled the runout; this would seem to indicate that the off center is present on both the crankshaft and the flywheel. High mileage bike, earlier mains bearing failure, mid-90's Ducati in serious financial trouble, etc etc... it's plausible.

    A quick look at the Wikipedia page about balancing gave me a couple of simple calculations:

    U (unbalance) = mass x radius
    Force (unbalance) = U x w.w (where w is rotational speed in radians)

    Lots of measurements of the crankshaft's component stack with dial gauge, vernier calipers, digital kitchen scales, and then lots of careful adding up of U terms, and I had a possible solution: slightly enlarge the hole drilled through the flywheel already. It looks like Ducati drilled this hole at the factory to compensate for the induction trigger on the flywheel rim. If the entire flywheel moves ever so slightly in the direction of the trigger, leading to out of balance, then enlarging that hole shifts the center of mass back to the crankshaft axis. It's possible to work out how much mass is removed via density of steel and calculated volume prior to machining.

    So I took the flywheel off, put a drill of the calculated size through the hole using a drill press, and remounted it on the carrier. It appears to have worked. The bike (after a couple more fuelling tweaks) is running better than it has at any time since the mains bearing failure.

    It's a dicey procedure but a) it was just the flywheel and b) it's an old, high mileage, fun but not particularly special bike which is not and never will be a 916 or similar. I was OK with taking the risk. Something newer and nicer... for once I'd pay a pro, mostly for the peace of mind.

    The bike's done several all-day rides since reassembly and appears to be going just fine.
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  4. #679
    Join Date
    24th September 2004 - 06:46
    Bike
    '76 CB550 Super Sport
    Location
    On the road to nowhere...
    Posts
    7,414
    Good to see you got things sorted out with the old gal oddduck.

    I found carb/injecter cleaner mixed with fresh tank of feul, Supercheap had the same brand on special recently for around $7 instead of the usual $17. Did a return trip tfrom Palmy-Havelock North and my Suzuki XN650 was running as if it was a totally different motorcycle. Much smoother and no random cutting out,

  5. #680
    Join Date
    7th March 2006 - 21:17
    Bike
    Kawasaki Vulcan
    Location
    New plymouth
    Posts
    288
    Bloody outstanding work, have enjoyed every single word and photo of this marathon. Nice to see ya finally get a win a d fingers crossed some long term reliability too .


    Sent from my iPhone using Tapatalk

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