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OT ~ perhaps, advice on ignition system

Discussion in 'Electronic Design' started by default, Apr 27, 2007.

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  1. default

    default Guest

    My '81 Honda motorcycle dies in the rain.

    Obvious choices are the HV ignition system.

    Motorcycle will do fine with just a replacement gas tank - 32 ounce
    soldered tin can. But I haven't tried that in the rain - I have to
    wait for rain . . .

    Motorcycle will work fine as long as the weather is dry. Duh?

    Motorcycle will work for "any" amount of time with just the trial gas
    tank - and water being sprayed over the ignition system - from fine
    spray to jet spray

    MC is a Honda '81 with a transistor switch ignition. The pickup coil
    drives a TX switch. Coils are tied to B+ on one side.

    I tried dumping a bucket (5 quarts) of water on the ignition
    coils/wires/plugs while idling - no change in idle speed.

    MC dies when in rain only, after about 5-20 minutes. MC will work as
    long as there is no rain (hundreds of miles). MC will start after a
    pause - and run for a time, after wet, dependent on rest period and
    rain level. Will always start when wet - but won't go.

    Two completely independent ignition systems - firing two coils.
    Pickup coils detect the fire timing and transfer that to two coils and
    ground them for the dwell time. Coils are three ohms. The reverse EMF
    on the coils is ~200 volts.

    Obviously I am over looking something

    HELP!
     
  2. 1) You are lucky that you don't have to deal with '70s Ducati
    electrics.....

    2)Since you appear to have 2 independent systems, maybe there is a
    problem on the DC source side, is it electronically switched?

    3) Run the MC in the dark, and lookout for corona, but that's a long
    shot.


    martin
     
  3. default

    default Guest

    I haven't ruled out the LV side of things. The two systems don't fail
    exactly coincident with each other - one will go and the bike will
    stagger on for a few seconds then the other goes out.

    The Clymer manual calls it a CDI ignition system, but it is only
    supplied with 12 volts and the switching modules are only ~1 cubic
    inch - I doubt they had any real CDI system back then (and that
    small?) Modules are large enough for a power transistor or two and a
    cap.
    I'll give that a try - I'm grasping at straws now.

    I did break down and replaced the plug connectors and wires - that has
    really made a difference in acceleration and idling so maybe that is
    it . . . The old plug caps measured between 6-9 K ohm resistance and
    all the new ones are exactly 5 K (5K is what it should be).

    I've had a few bikes with some pretty horrible electrics - a Gilera in
    ~'66 and Triumph in '70. Gilera - the engine would die if the stop
    light was blown out and you braked, side draft carb float would cause
    it to accelerate in left hand turns and slow in right turns - Trumpet
    caps would die and the engine would make a burbling sound when
    accelerating. Neither had headlights worth a damn. Neither bike was
    much good above 8,000 feet in altitude.
     
  4. Argh, I rememer dirtbiking just after new year, in the snow in
    Derbyshire on a Montesa, and the magneto packed up. It wasn't even
    Lucas, I was towed 8 Km across the rough, including a river bed, in
    the dark. Not fun


    martin
     
  5. ehsjr

    ehsjr Guest

    Maybe not an electronic problem?
    Some kind of vaporlock in the fuel system
    piping comes to mind as a possibility.

    Ed
     
  6. default

    default Guest

    The temperature may drop a little in rain if anything.

    I did consider a fuel problem - sprayed water on the seals between the
    carbs and cylinders so it isn't sucking some water in there. Runs a
    little lean, but I haven't re jetted since replacing the exhaust.

    Possible that the water on the fuel cap is making the tank vapor
    tight. There's a large rubber washer that is spring loaded to press
    against the tank opening. It must be a perfect seal it always looks
    like polished steel - no rust. Possible that a tiny amount of water
    is just viscous enough completely seal the tank and it tries to pull a
    vacuum - that would account for all the symptoms - and something I
    should be able to check with a water bottle and some driving.

    There are four carbs one for each cylinder - I wouldn't expect them
    all to drop out together - but this isn't like running out of gas,
    there's always a little in the tank that sloshes into the fuel tube -
    a vacuum would mean no carb gets a taste of gas once the vacuum forms.
    Rain cooling the tank would also lower the vapor pressure. (and it
    fits all the other symptoms very well)

    one more thing to try . . .
     
  7. Rich Grise

    Rich Grise Guest

    Is the air filter getting wet and plugging up the intake?

    Good Luck!
    Rich
     
  8. default

    default Guest

    Unlikely. Air box opening faces aft and has no leaks. It will
    tolerate a lot of sprayed water before it starts running rough. I
    sprayed water everywhere even the places that shouldn't cause problems
    - fuse box, headlight nacelle, anywhere wires run or air moves.

    The symptoms when it dies are rather sudden - just like switching off
    the ignition. I sometimes lose one pair of cylinders first then the
    other. Each (brand new) ignition coil feeds two cylinders - one spark
    is wasted firing a cylinder that is not on the compression stroke.

    The coil side has compression ferrules,of rubber, that grip and
    waterproof the HV wire. The plug caps only have rubber boots - but
    have worked well enough in the past.

    When I replaced the caps and wire I used some thick silicone grease
    that is supposed to be for "spark plug boot release."

    Maybe there's more than one cause for the problem. Maybe I should be
    dealing with the lean running . . . that and a weak spark, high
    moisture in the air . . .

    Dies just like switching off the ignition so that's where I'm
    concentrating.
     
  9. Jim Thompson

    Jim Thompson Guest

    I'm late to the discussion, so this may be a repeated question...

    Electronic ignition? I've had those modules in the distributor fail
    with the symptom of like you just momentarily just turned it off.

    Particularly Fords... which I placed on my "Do Not Purchase" list
    almost exactly 30 years ago... scary as hell, tooting down the
    freeway, suddenly no power, slows then resumes ignition :-(

    ...Jim Thompson
     
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