Possibly knackered CDI unit? Circuit diag anywhere?

Discussion in 'Motorbike Technical Discussion' started by Chris N Deuchar, May 23, 2006.

  1. Being an electronicky person I have no qualms whatsoever about delving
    into the tinny depths of the Unit of my 1981 Yamaha XV750SE, however,
    to save me working out the circuit diag, has anybody done this already
    - or know where I can find a copy?

    The problem is that, once running, the beast goes fine, but on
    starting (and overrun), it appears "over advanced". Elsewhere I have
    heard that something in the CDI retards things under these conditions
    so I would like to find out which 'bit' this is and sort it. Its
    really struggling to turn over against the first firing - despite new
    Yuasa batt.

    A thorough electrical (and valve clearance) check has revealed no
    other issues. I never had this problem when I first got the bike - any
    help gratefully received.


    Chris D
    Chris N Deuchar, May 23, 2006
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  2. Chris N Deuchar

    TOG Guest

    In my experience, CDI units either work or they don't - there's no
    halfway house. And as the XV750 is notorious for its self-destructing
    starter motor clutch, I'd suggest the problem lies here. I'd also check
    things like air intakes, carb diaphragms etc to sort out the over-run
    thing. Either way, I bet it's not the CDI.
    TOG, May 23, 2006
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  3. Chris N Deuchar

    FB Guest

    CDI has become a generic term, like "aspirin" and "coke". People
    diagnose problems
    as being a "bad CDI unit" even when the motorbike in question does not
    have a *capacitor discharge ignition syetem*.

    I remember the first real CDI units. They would charge up a great big
    capacitor with a
    DC to DC high voltage converter. I would hear the oscillator inside the
    converter whining at high frequency when I turned the ignition switch

    I don't think your Yamaha even has a CDI, but Hondas have CDI's that
    get their needed high voltage from an extra exciter coil in the stator.

    If there are only three wires coming out of your alternator stator, you
    don't have a CDI.

    Yamahas and Suzukis use a simple transistorized ingition module with a
    built in "advance curve" which is seems actually to be a built in
    *retarding circuit*.

    The retarding circuit would count the number of pulses from the trigger
    coils and retard the ignition accordingly.

    Two transistors take the place of ignition points. Power goes from the
    ignition switch, through the coils, through the transistors, to ground,
    and the signal from each pulser coil has to be delayed by the retarding
    Well, if your starter is struggling to start the engine when it's warm,
    I would suspect a lean idle mixture, low fuel level in the float bowl,
    or a malfunction in the starting enrichener system. As I have discussed
    the cleaning and adjustment of constant vacuum carburetors endlessly in
    this forum, you can google for such things as "transition ports" and
    "EPA anti-tamper plugs".

    As I said to Checkers in a previous post just a few minutes ago,
    constant vacuum carbs are adjusted marginally lean at the factory and
    the screws are concealed by an anti-tamper plug.

    Constant vacuum carburetors are very prone to gumming up and this
    causes all sorts of starting problems that can be avoided by running
    carburetor cleaner through the fuel a few times a year.

    It's not impossible that a previous owner has tinkered with the
    ignition timing though.

    Inline fours have a nasty flat spot between 5000 and 7000 RPM and
    owners can somewhat overcome this by slotting the screws that secure
    the pulser coil plate and advancing the timing by about 5 degrees.

    I wouldn't think a Vee-Twin owner would feel he needed to do this
    FB, May 23, 2006
  4. Chris N Deuchar

    Frinton Boy Guest

    IRTA electropanicky

    Frinton Boy, May 23, 2006
  5. Chris N Deuchar

    e Guest

    i have a cdi from one of the 78-80 triples in my pile
    e, May 23, 2006
  6. Chris N Deuchar

    e Guest

    there are fixes for the starter. check the yahoo groups.
    e, May 23, 2006
  7. sort out the over-run

    There are indeed.
    The Older Gentleman, May 23, 2006

  8. Read the OP. It's not struggling to *start* it - it's struggling to turn
    the engine over. Slightly different.

    This is a common fault on these engines and, as I've said, is usually
    down to the crap starter motor design on the early vee-twins. Some
    dreadful Bendix thing, IIRC. And as another poster has said, there are
    fixes posted on the interwebthingy.

    I'd investigate the starter motor before pulling the carbs apart.
    The Older Gentleman, May 23, 2006
  9. Chris N Deuchar

    Moon Badger Guest

    Yahoo groups is quite good for finding owners groups who may be able to

    It is unlikely to be the ignitor BTW.

    If you thinks its struggling to start, take the plug caps off, insert spare
    spark plugs in to the caps and ground them. If the motor still struggles to
    turn, it has to be the starter as the ignitor is just firing the plugs to
    the open air.

    The ignitor in bikes of this vintage is quite simple ( usually ). No
    throttle position sensor to worry about for one thing. It careth not a
    flying ferk about throttle position and knows nowt about overrun. All it
    'knows' is RPM and a preset amount of advance. Most don't have a curve,
    just a series of steps.

    Find the base advance from a manual and start the engine. Use a strobe lamp
    to see the timing marks. Some dismantlement may be needed to find them.

    Next, get someone to hold the throttle at 500 RPM above the step for the
    next amount of advance, ie, manual says 6 deg [email protected], 12 deg
    [email protected] and gives no other values, you may only have two advance
    settings. If these values match what is seen, the ignitor is OK.

    My money is on an air leak twixt carb(s) and head(s).

    Moon Badger, May 23, 2006
  10. Trouble with posting to as many groups as you have, you get as many
    opinions, some of them utter bollocks.

    These bikes are known for weak starters, as has been said, so it's best
    to check the obvious first. If the starter's weak, there's a mod which
    involves throwing the original away and fitting an XS750 one, though
    these are getting rarer nowadays.
    Grimly Curmudgeon, May 24, 2006
  11. Chris N Deuchar

    e Guest

    barstow is not the edge of the desert, it's 25 miles up the
    desert's ass.
    e, May 24, 2006
  12. Chris N Deuchar

    platypus Guest

    Your revelation has turned up a little too late. HST is dead.
    platypus, May 24, 2006
  13. :) I have reached the 'sorting the wheat from the chaff' stage of
    Interesting ta

    Chris D
    Chris N Deuchar, May 24, 2006
  14. Ta, will do

    Chris D
    Chris N Deuchar, May 24, 2006
  15. I remember building one :)
    Acknowledged. In some places in the manual it is called a TCI
    (=Transistor Controlled Ignition?)
    By 'number' did you mean 'frequency'? If so this is getting
    Even more interesting. So what (I suspect) we have here is a circuit
    where the transistor is basically triggered - probably via a
    capacitor. A simple RC combination could then be added as an
    integrator to modify things at low revs (ie 0 > starting)...

    If the integrator components were 'ineffective' then that would mean
    that firing under normal running conditions would still be ok but not
    starting. That could be it - if only for a circuit diagram...
    Might be worth doing anyway as the beast stood a while before I got it
    - and after I first used it.
    No, this is fine. When I first looked at the spinning disc in the
    timing light, I thought it was way too advanced, then I realised that
    what I thought was a displaced 'F' (=Firing) mark was actually the 'T'
    (=TDC) mark and that the 'F' was invisible when lit 'head on'. When
    not running, putting the T where I had seen it by the timing light
    puts the F exactly right.

    As an additional check I put the timing light on the other (front)
    cylinder and saw the TDC line in exactly same place as I had
    previously seen T mark. Therefore both cylinders are apparently firing

    Thanks for all the ideas.

    Chris D
    Chris N Deuchar, May 24, 2006
  16. Chris N Deuchar

    A.Clews Guest

    Grimly Curmudgeon's weak starter advice aside, what about checking the
    battery connections to the starter relay for corrosion? My Trident 900
    suffered from a sluggish (and then dead) starter last year, and the good
    folk of this NG suggested I check the relay connections. Once they were
    cleaned, the starter had a whole new lease of life.
    A.Clews, May 24, 2006
  17. Chris N Deuchar

    e Guest

    e, May 24, 2006
  18. Chris N Deuchar

    FB Guest

    HST is not Harry S. Truman, it's Hunter S. Thompson, the "gonzo
    journalist" whose style introduced himself into his stories. You can
    google for "The Sausage Creature", which is about his ride on a Ducati.
    It was orinially published in Cycle or Cycle World magazine. Half the
    writers of letters to the editor threatened to cancel their
    subscriptions, the other half wanted more HST stories.

    HST's other major involvement with motorcycling was "Hell's Angels",
    written around 1967 after the HA's were involved in a gang rape on a
    beach in Monterey or Santa Cruz. HST also wrote a newspaper story in
    defense of the HA's and how the gang rape was really just a bunch of
    people partying hearty and that the victims were willing, up to a
    point. That's also available, in the rec.motorcycles NG.

    HST was personal friends with controversial San Francisco intellectuals
    such as William S. Burroughs, who served a prison sentence for
    murdering a man who made sexual advances to him, and Alan Ginsburg who
    wrote the banned poem "Howl!".

    The line that got "Howl!" banned involved how "the finest minds of a
    generation" squealed with delight when being sodomized by the "saintly
    motorcylists" who were obviously the Oakland Hell's Angels.

    HST committed suicide about two years ago, in his Colorado home. His
    friends promised to fulfill his wish to be shot out of a cannon by
    shooting his ashes out of a cannon, but I haven't heard anything since.
    FB, May 24, 2006
  19. Chris N Deuchar

    TOG Guest

    I'm amazed that anyone can do a biog, however brief, of HST and not
    mention 'Fear & Loathing'.
    TOG, May 24, 2006
  20. Chris N Deuchar

    TOG Guest

    Worth doing simply as a matter of course, but i'm still sure the
    reluctance to turn on the starter means the motor assembly itself is

    They *all* do that, sir.
    TOG, May 24, 2006
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