a couple of odd symptoms, honda cb350 twin

Discussion in 'Motorbike Technical Discussion' started by Sean J Kelly, May 8, 2006.

  1. Sean J Kelly

    Sean J Kelly Guest

    hi guys,

    the bike's running pretty well, but there are a couple of odd things going
    on. i've just recently started riding this bike again after it sat for a
    few months - so the carbs may be a bit dirty, and it's probably due for a
    valve adjustment.

    after an evening ride, i stopped back at my garage to adjust the idle
    speed ( to ~1200rpm).

    first, i'd noticed a bit of smoke coming out of the bottom of the bike
    while sitting at a stoplight. it turns out that this is coming out of the
    vent tube that connects to the valve cover. the mist seems to be oil, not
    gas - but i can't really tell. it just doesn't smell like gas, and is a
    bit slippery. the "smoke" seems to come out in pulses, timed with the
    pistons' rise and fall.

    since there are no seals on the valve guide/stem interface, perhaps this
    is regular exhaust blowing by through that gap? maybe vaporizing a little
    oil on its way through?

    second, while i was idling the bike with its headlight off in the dark, i
    noticed that the spark plug wires (stock, with rubber caps over the
    sparkplug end) are arcing to the cylinder head. there are actually dull
    blue sparks from that rubber insulation to the cylinder head on both
    sides. has anyone noticed this before?

    Sean J Kelly, May 8, 2006
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  2. More or less, yes.
    The Older Gentleman, May 8, 2006
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  3. Sean J Kelly

    John Johnson Guest

    Doesn't sound like a problem.
    Replace the spark plug wires. You shouldn't get arcing anywhere but at
    the plug itself, and that shouldn't be visible with most plugs.


    'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.
    John Johnson, May 8, 2006
  4. Sean J Kelly

    FB Guest

    Your crankcase ventilation system doesn't even rise to the level of a
    road draft tube.

    A road draft tube has a bevel on the backside so a vacuum will be
    generated as the vehicle passes through the air. That sucks oil fumes
    right out of the engine.

    But your crankcase ventilation system could be best described as a road
    drip tube.

    A bit of the hot compressed gasses are leaking past the rings on the
    power stroke, not the exhaust stroke. That pressurizes the crankcase
    and blow some oil out the breather. The oil condenses and drips onto
    the road.

    The earliest smog devices on cars replaced the road draft tube that
    came down from the engine and sucked oil fumes out of the engine with a
    hose that went to a fitting under the carburetor in order to suck the
    blowby through the engine instead. Intake manifolds started looking
    really grody inside with all those fumes and engines with a lot of
    blowby would deposit oil fumes on the heads of the intake valves,
    causing sealing problems.

    This early smog device required a one way check valve called a positive
    crankcase ventilation valve.

    Motorcycle designers started just running the hose from the valve cover
    directly to the
    air box in the 1970's and 1980's.

    They didn't bother with a PCV valve because they were running the oil
    fumes into the airbox upstream of the air filters. The downside of that
    system was that a paper air filter might be ruined by oil droplets if
    you ran the engine really hard.

    The engineers often added an appendix tube to the bottom of the airbox.
    It was supposed to collect excess oil droplets and the rider was
    supposed to know he had to remove the appendix and drain it every year
    or so. But the engineers never bothered to tell the owners manual
    writers what the appendix was for.

    We used to see large diameter clear plastic hoses coming from the
    crankcases of road racing Ducatis back in the 1960's. The hose would
    rise straight up from the rear of the transmission and go past the rear
    fender bracket. That sure looked trick. Oil vapors would rise, and oil
    droplets would fall back down the tube.

    But, if the rider blew his engine from running it too hard, a big glob
    of oil might fly out the breather tube and splatter on the goggles of a
    following rider.

    It's really a rather nice touch to contain oil blow by. Modern racing
    organizations specify that a catch bottle be installed to contain any
    little drops of oil. Beer cans, coke cans and plastic bottles were
    pressed into service. I particularly like a Lawry's plastic spice
    container that has a necked down section so it can be held with a

    But there are still lessons to be learned with catch bottles. I made
    the mistake of running my vent hose all the way to the bottom of the
    catch bottle. As soon as the end of the hose was covered with oil, a
    vapor seal was formed.

    I started noticing that oil was dripping out from under my sprocket
    cover. I cleaned it up and saw more oil dripping out of the electric
    starter cavity behind the cylinder block.

    I removed the starter cover. The cavity was filling up with oil that
    couldn't get past the vapor seal.
    Oh, yes, when we had a lot of time back in the 1950's and 1960's we'd
    wonder why a tuneup didn't ressurect all the original power of our
    car's engine, so we'd go out into the garage and start the engine up
    and raise the hood and turn off the lights in the garage to see if the
    spark plug wires were going bad.

    We would see eerie blue corona effects around the spark plug wires and
    think about buying a new set of wires
    when we got some spare coins.

    Corona is like shit. Corona happens. Did you ever stand under a power
    pole and listen to the corona making static and humming sounds?

    If you see a lot of bright blue sparks making loud snapping noises, you
    have definite arcing to ground. But a little corona is nothing to worry
    about if it isn't so bad it's making a cylinder misfire.

    Look at the center electrode of your spark plug. Is the firing tip of
    the center electrode a uniform gray all the way across? If it is,
    you're getting enough voltage to do the ignition deed.
    FB, May 9, 2006
  5. Had that happen on my old 360, with the oil coming out the tube. Time for
    new rings oversize and piston oversize. That will fix it. Did mine.
    ROBERT MILLER, May 9, 2006
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