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OT motorcycle won't work in rain, any pilots out there?

Discussion in 'Electronic Design' started by default, Jun 3, 2007.

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  1. default

    default Guest

    I posted the problem with my 81 Honda back in April and it has only
    just rained once since then, so I took the bike out to see if the new
    wires and spark plugs and caps helped.

    I did eliminate one possibility before the rain stopped - venting the
    fuel tank didn't correct the problem so it wasn't a vacuum forming in
    the fuel system.

    This time I was starting out with a cold engine, full tank of gas, and
    it was really raining hard. I didn't go more than two blocks before
    the engine started running rough, applying choke caused it to smooth
    out and run normally, turning off the choke caused it to run rough
    again, that worked through several cycles. Eventually I let it get
    too rough and it died. I coasted for about 30 seconds engaged the
    engine and it fired right up. This seems to work about as well as
    stopping to rest - engine will always start after a short rest.

    I should have tried riding at very slow speed to see if it would keep
    going - but didn't think of it when I had the chance.

    No amount of spraying water over the engine, ignition etc.. will cause
    it to fail in the driveway. I can dump buckets of water on the
    ignition and engine without it dying. Even a mist of water from a wet
    road will cause it to die and rain causes it to die faster.

    At this point I'm almost ready to give up on an ignition problem since
    all the ignition except the transistor switches and pick up coils have
    been replaced. And the likely hood of two independent electrical
    systems failing seems too extreme. All the electrical parts have been
    drenched with water and it runs if the bike is stationary.

    My latest and greatest theory is carburetor icing. From what I read,
    the conditions are favorable - humid between 20-70F sometimes up to 80
    F. The problem is more likely to occur when the engine is cooler. It
    is more likely to occur when backing off on the throttle.

    Where it doesn't fit the theory is that using the choke should
    exacerbate the problem - it doesn't seem to, and while the engine is
    relatively cool and the problem is occurring it actually helps. When
    the engine is at full operating temperature the choke is less likely
    to help it.

    I can't get the problem to occur when the bike is stationary, it has
    to be moving through airborne water in the form of rain or mist (we
    don't get enough fog here but I suspect a heavy fog might kill it

    My engine is adjusted too lean according to the color of the spark
    plugs. With "normal" carburetor icing this shouldn't exacerbate the
    problem - according to what I read. I didn't change the jets but did
    put in a less restrictive exhaust when the old one had holes in it -
    the problem did occur with the old exhaust but with holes in it so I
    can't say for certain mixture isn't involved somehow.

    Any one with more ideas on troubleshooting this problem? Can it be
    carb ice with richening the mixture making it better? All I read says
    it should get worse as the mixture is rich, since they assume that the
    airflow is too restricted. But as the mixture is made rich, I could
    see where there may be less evaporative cooling from the gas since the
    gas should evaporate slower with humidity and gas concentration

    Have gas formulations changed in some way that might account for it?

    I used to be able to keep going in torrential 30 foot visibility
    without it missing a beat - now it won't go more than two blocks in

    Once at highway speed, especially when going fast it seems less
    susceptible but backing off the throttle then, in rain, will almost
    always kill the engine.
  2. Does it have an air filter? If so did you ever replace or clean it?
  3. default

    default Guest

    Will check it tomorrow. I have a couple of new ones I'll change it
    and wait for rain. It is a relatively large pleated paper filter -
    but yeah, I haven't checked it in awhile.

  4. Jim Thompson

    Jim Thompson Guest

    Vacuum leak screwing up distributor advance? Change out all your

    ...Jim Thompson
  5. default

    default Guest

    No vacuum to distributor, no distributor. Two independent ignition
    systems firing four cylinders - wasted spark on one cylinder when the
    other is firing.

    Only one external vacuum line and it is used to operate a fuel shut
    off valve so that gas can't flow down (gravity) unless the engine is
    turning over or running (vacuum turns on the valve).
  6. Jeff L

    Jeff L Guest


    That sounds like icing in the carb is a possibility. It is possible that
    frost buildup in the carb can plug off or cover up a jet, causing it to run
    lean (or rich). It depends on the carb design and where the frost is
    forming. Cool damp air is when carb icing normally occurs, especially since
    the engine is also cooled by the cold air, so the carb does not heat up much
    from the engine. Were any air ducts or fairing removed / modified / added?
    Partial constant throttle just after idle is when most icing occurs. Proper
    jetting is one fix, as is getting heat to the carb. Some carbs (mostly a few
    snowmobiles and a few rare outboards) have electric heaters built in.

    To throw another wrench into things, a rich mixture can be easier to ignite,
    so the choke could also be compensating for a weak ignition system. The next
    time it fails, pull a plug wire before it cools down and stick a spare park
    plug on the wire and crank the engine over to check the spark intensity.
  7. mpm

    mpm Guest

    I know you said you drenched the engine while running and it would not
    I'm curious if you ever tried reving the engine and then aspirating it
    with saturated (wet) air?
    Then let off the throttle.

    If that proves useless, then perhaps it is a combination of things.
    Moist air, engine load, and physical weight on the bike.
    Also, what altitude are you at? Could that be a variable here?

    I'm not a bike mechanic, but had my share of Yahamas in college.
    (Not enough parking on campus for cars, etc.. You know the drill.)

    Good luck!
  8. Jeff L

    Jeff L Guest

    You may be able to mimic the icing conditions in your driveway by applying
    a constant light throttle (high idle - say 3000+ rpm) for a few minutes
    while misting the air intake area with a garden hose.

    Also note that when the carb warms up, the ice melts, and things go back to
    normal. You may try some (eg a bottle or two in the tank) gas line
    antifreeze in the tank for an added performance boost, and to help prevent
    ice from forming in the carb. Note that if the bike is running lean, this
    may lean it out more.
  9. GPG

    GPG Guest

    Check/replace the CDI charging coil.
  10. Fuel filter?



  11. The reason that 'carb ice', is rare in cars and bikes, compared to in
    planes, is twofold. The first is that it is normal in these vehicles to
    heat the carburator to prevent it (this has the 'downside' of slightly
    reducing engine efficiency), and the second is that it is rare to run for
    prolonged periods at steady throttle settings (and particularly at low
    throttle settings).
    Fuels have changed, modern stuff, often contains ethanol, which can make
    this much worse. However your problem sounds 'extreme'.
    Has the fuel system been fully drained (if the fuel was left standing for
    a while, it can pick up moisture).
    Does the bike have any system to heat the carburettor, or the incoming
    air?. Is this working?. If the bike is water cooled, then it may be like
    the common system in car engines, with a thermostatically controlled water
    jacket on the inlet manifold. If this ad stopped being heated, it'd
    explain the problem.
    Some aero engines, accept the slight loss of efficiency involved, and pass
    the inlet in through the warm oil in the sump (Lycoming units), making
    them _much_ less likely to have problems. Compared to a Continental engine
    without this, carb ice is perhaps 50* less common...
    The choke, shouldn't help, but if there was physical ice in the venturi,
    movement here, could simply displace the ice. However I'd be much more
    suspicious of the actual fuel jet being where the problem lay. Perhaps
    icing here, which leans the mixture off even further, letting the choke
    Historically, this was more of a problem in some countries, and in the
    past there was a different carb/inlet setup, supplied in some countries to
    help with this. You might want to look and see if there is such an
    alternative installation on your machine.

    Best Wishes
  12. GPG

    GPG Guest

    I have worked in a honda shop as a mechanic, as I said above the most
    likely problem is the charging coil.(Or its connections) Spraying with
    water while sitting in the driveway under no load will not show the
    problem. When on the road much of the water comes from underneath. The
    information you have supplied definitely makes it an ignition problem,
    you do not say what model.
  13. default

    default Guest

    What are you calling a CDI charging coil? The Clymer's manual calls
    it a CDI system, but it isn't. There is no source of HV or separate
    capacitor charging coil. The alternator is an excited field type with
    electromagnet and brushes - three phase output (and I even saturated
    that with water although it will start and run with no alternator at

    There are two pickup coils that turn on the transistor switches. The
    coils are mounted to what appears to be one end of the crank or
    camshaft and are 180 degrees apart. The switches themselves are small
    (less than a cubic inch) aluminum modules with wires. There is a
    battery connection to both modules, the kill switch interrupts the
    power from the battery to shut the ignition off.
  14. default

    default Guest

    Now that I'm thinking carburetor ice, I'll have to try inducing it
    artificially (misting the air intake while revving and closing the
    throttle) I did mist the air intake while idling - but probably just
    got some water on the air filter.

    I tried water where ever I thought it didn't belong like the
    connections between carbs and engine.

    What gets me is that the carbs and ignition are more or less
    independent systems the only things they share are common gas supply
    and the same +12V to the transistor switches. What ever is stopping
    the engine is managing to stop redundant systems (four carbs and two
  15. default

    default Guest

    Well it does have a fuel filter I added. I was having a problem with
    rust from the tank keeping the float valves from sealing properly.

    It is between the tank shutoff valve and engine shutoff valve and
    horizontal with inlet and outlet on the same plane. When the bike
    sits for an extended period I can see the fuel level in the valve and
    when cranking it fills with fuel.

    What hypothesis suggests it could be clogging when it rains outside?

    When I run out of gas (normal - no fuel gauge) the bike will drop
    power as one carb runs out then slow more as a second one drops out.
    Most of the time that takes 1/4 to a 1/2 mile while I fumble with the
    reserve valve and is likely to happen while going uphill.

    This failure is much quicker than normal running out of gas and it
    recovers faster - just switching to reserve will often take another
    1/4 to 1/2 mile before it smoothes out again.

    There may be a coincidence with going uphill and encountering the
    problem - but there are so few hills here I can't really say.
  16. default

    default Guest

    Altitude is between 34 feet above sea level and at most 71 feet when I
    visit the wife. The land is flat for the most part, under water when
    the earth was formed, sand peat and no rocks to speak of.

    Carbs are "constant velocity" types. Twist grip opens a butterfly
    valve but the slide doesn't go up until the vacuum builds enough to
    raise it, opening the air path, and withdrawing a needle from the main
    jet allowing more gas into the airstream.

    It has been my experience that this type of carb can work 0-11,000
    feet with no adjustments and little loss of power. (unlike the old
    '72 Triumph)
  17. default

    default Guest

    That it is
    Well it is normally very humid here. I just filled the tank two days
    earlier from nearly empty before trying it yesterday in the rain. The
    fuel level was only inches from the cap. Normally the water in the
    fuel, if any, will be at the bottom of the float bowls and each bowl
    has a drain screw and I can see any water globs in the transparent
    tubes when I open it. - but if they are adding alcohol to the fuel it
    may mix with the alcohol and not be so easy to spot?

    No that doesn't sound likely. I remember reading that the way to tell
    if a fuel has alcohol is to put it in a graduated cylinder and add a
    known quantity of water to it. When the water mixes with the alcohol
    it drops out of solution with the gas and the oil / water separation
    level is higher than the specific quantity of water would justify -
    but that may just apply to a lot of water . . .

    I don't use a lot of gas, but take a 250 mile trip once a month and
    use the bike for grocery shopping (12+ miles) at least once a week,
    about 15 gallons a month go through it.
    No heat at all for the carburetors. When the engine is hot, cooling
    air from the fins can flow past the carbs or rise from the hot
    crankcase while standing still. Coupling between the carbs and engine
    ports is via a piece of reinforced rubber hose about two inches long
    with hose clamps at the carb and port. (and I tried spraying water
    around the area of the connections while troubleshooting it
    What I need is a good way to rig a water mist for the intake while
    riding the bike. Some kind of nebulizer or atomizer rigged right to
    the air box opening. Try hard enough and I'll induce a problem even
    if it isn't the one I'm troubleshooting.
  18. default

    default Guest

    1981 Honda Custom 750 DOHC - two pick up coils for each ignition
    system - 12V via the ignition switch, and kill switch is applied to a
    pair of transistor modules that feed the coils.

    No separate HV capacitor charging circuit or energy transfer ignition.

    I've sprayed water around the pick up coil housing - I've a frozen
    screw holding the cover on and no way to get it off short of cutting
    the head of the screw off or drilling it out. It has three counter
    sunk oval head Philips screws holding it on. Two of them I could
    loosen with my impact driver - the third still has the tip of the
    impact driver bit imbedded in it - broke off before the screw turned.

    I even moved the little rubber cable gasket away and misted directly
    into it. The outside of the housing mates closely with the engine
    case and to keep water out I added 360 degrees of aluminum tape around
    the seam. (after waiting weeks for any moisture I induced to dry out)
    This tape is real aluminum metal and has a very aggressive adhesive
    and soft enough that it will smooth over contours for a good seal.

    So, while I can't get to the pickup coils, I doubt they are getting
    any water in them.

    The manual's schematic shows a connector somewhere between the pick up
    coils and connector at the modules. It would seem likely it is in the
    starter motor housing since the PU coil wires are routed through there
    and the schematic shows it picking up another single wire for the oil
    warning light.

    Any idea where that connector is on that model? I'd like to inspect
    it. I had the starter motor cover off - the gaskets are in good shape
    and it appear clean and dry in there - but I'd still like to find that
    connector and look at it.
  19. Greg Neill

    Greg Neill Guest

    Have you checked that the kill switch is not intermittent
    or shorting when wet?
  20. Jim Thompson

    Jim Thompson Guest

    Sounds like you found the problem ;-)

    Certainly fits your starved fuel symptoms.

    ...Jim Thompson
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