Yamaha XS650 backfire

Discussion in 'Motorbike Technical Discussion' started by Dave, Jun 9, 2009.

  1. Dave

    Dave Guest

    I started up my 1980 XS650G last night after fitting new mufflers (long long
    overdue). By the time I finished it was pretty dark in my garage, so I
    couldn't help but notice that the left cylinder was backfiring... I could
    see the blue flame inside the left muffler. My old rusty pipes were so
    friggin' loud that the backfire may have been present last year, I'm just
    not sure. What might cause this? I swapped the plugs between L & R
    cylinders, but the backfire stayed on the left (damn!). The bike seems to
    rev up okay, could it possibly just be an adjustment of my air mixture
    screw? I'll throw a tachometer on the left plug lead tonight just to make
    sure I'm getting spark.

    Dave, Jun 9, 2009
    1. Advertisements

  2. Dave

    ¿ Guest

    Maybe your carbs are getting a little plugged up inside. You might try
    adding a few ounces of Berryman B12 Choke and Carburetor Cleaner to a
    full tank of gas and going for a slow ride to clean out the idle

    There are two kinds of backfires related to dirty carbs. One is the
    loud *bang!* associated with high fuel levels caused by dirty float

    The other kind of backfire is a wimpy *piffle-piffle-snap!* on
    deceleration, That's caused by dirty idle jets and passages.
    ¿, Jun 9, 2009
    1. Advertisements

  3. Dave

    Dave Guest

    I'll try carb cleaner, I've got a bottle someplace, I recall I ran some
    through last year or the year before. The other cylinder runs fine. I
    think I'm going to end up pulling these carbs apart anyways at some point.
    I would think that a plugged air jet might cause backfiring at idle, but I
    really haven't worked on carbs enough to know...
    Dave, Jun 9, 2009
  4. Dave

    ¿ Guest

    If you're fairly sure that the air jet has gotten plugged up, a shot
    of aerosol carb cleaner would easily clean it out.

    I buy both types, B12 in aerosol form and B12 in the handy liquid form
    for adding to the gas tank.

    Many Japanese ignition systems will fire both spark plugs at the same
    time, using one sensor or one set of points to trigger the ignition.

    When you have a waste spark firing, you'll hear rumbles in the exhaust
    pipe on deceleration and you'll also get backfires when the carbs are
    dirty because unburned mixture will build up in the exhaust pipe over
    two or three exhaust strokes.
    ¿, Jun 9, 2009
  5. But the XS650 *doesn't*.
    The Older Gentleman, Jun 9, 2009
  6. Incorrect mixture. Incorrect timing (I am not sure if the 1980 model XS
    had points or electronic ignition, and I cba to check right now).
    The Older Gentleman, Jun 9, 2009
  7. Dave

    ¿ Guest

    Well, take the time to *look it up*, you silly ****.

    BTW, I'm a cracker. Eat me.
    ¿, Jun 9, 2009
  8. Don't need to. I've rebuilt two.

    But carry on posting erroneous information if you want to.
    The Older Gentleman, Jun 10, 2009
  9. Dave

    Dave Guest

    The XS650G (1980) has electronic ignition, timing cannot be adjusted.

    As for incorrect mixture, I'll try re-setting the air mixture screw on the
    problem side for best idle although last time I used the "dead cylinder"
    method I had quite a time just keeping the bike from stalling. I added some
    carb cleaner and went for a spin last night, the bike runs just fine at
    higher than idle rpm's.


    Dave, Jun 10, 2009
  10. Dave

    ¿ Guest

    Yes, and it only has *one* timing sensor and a 360 degree crankshaft,
    both coils have to fire at the same time. One of the sparks is wasted
    because the exhaust valve on that cylinder is open.

    If there is a buildup of unburned mixture in the exhaust pipe on that
    side, you'll hear a bang! out of that cylinder when the plug fires.

    I advise you not to waste a lot of time debating with "The Turd That
    Won't Flush". He loves to debate and argue instead of helping riders
    find solutions to their problems.

    Before you know it, he'll have you apologizing to him and thanking him
    for giving you a ration of shit.
    OK, just remember that you can adjust the idle mixture screw all day
    and it won't change a thing if the idle jets and air jets and passages
    are plugged up with gum and varnish from evaporated gasoline.
    ¿, Jun 10, 2009
  11. Dave

    Dave Guest

    Yeah, I'm just hoping. I don't really want to pull & clean the carbs if I
    don't have to.
    Dave, Jun 10, 2009
  12. It'll be the carb, then.
    The Older Gentleman, Jun 10, 2009
  13. If they're really gummed, then all the magic snake oil in the world
    won't help. Ultrasound cleaning works wonders. I used it on a carb for
    an old Honda 125 single last year: brilliant.

    Generally, carbs only get bunged up if they've been left for a long,
    long time (like well over a year). In a hot climate, obviously, the fuel
    inside them will evaporate and gum them up in less time than that.

    If the bike hasn't been standing for ages, then it's not going to be
    gummed carbs anyway, but just bad adjustment.
    The Older Gentleman, Jun 10, 2009
  14. Dave

    Dave Guest

    Deep down I already knew this. I'll try to fit it in on the weekend. It's
    just such a pain in the you know where to adjust and balance them. I've got
    to take it out for some highway rides and start the painful guess 'n check
    process of re-jetting for my new exhaust, so the carbs will be coming off
    anyways, one more time won't kill me.
    I've got the manual, it's available online for free. Thanks for your

    Dave, Jun 10, 2009
  15. Listen to this man. He speaks the truth. Lots of people put their faith
    in all these magic potions and cures, but ultimately, fixing something
    properly is the only sure way.
    The Older Gentleman, Jun 10, 2009
  16. Dave

    ¿ Guest

    Do NOT listen to The Turd That Won't Flush. He's more interested in
    proving himself to be "right" (by his own rules) that he cannot accept
    that there are quick fixes to simple problems by using inexpensive
    chemicals designed to cure such problems.

    I go through this business of plugged up carburetors *every* spring
    when I take the motorcycles out of winter storage, and the idle jets
    are seldom as plugged up
    as Mark Olson suggests, but the engine stalls, won't idle, or
    ¿, Jun 10, 2009
  17. Dave

    ¿ Guest

    Steer clear of Dynojet kits. Dynojet is trying to enhance their bank
    account by keeping speed secrets *secret*.

    I recommend going to www.factorypro.com and studying what they have to
    say about jetting CV carburetors. The owner of Factory Pro is *not*
    trying to keep secrets, he's telling how to rejet a carburetor using
    stock Mikuni or Keihin parts.

    Your carbs are probably made by Hitachi, but Mikuni main jets should
    fit just fine.

    The problem with re-jetting is that most home mechanics do not
    understand how the size #'s relate to orifice area.

    Since I've explained this concept dozens of times in this NG, I won't
    go into it again, but you can google up "round jet" and find the
    ¿, Jun 10, 2009
  18. Mikuni, actually.

    You really are preaching from a position of ignorance here.
    The Older Gentleman, Jun 10, 2009
  19. Which raises the question: why don't you just drain the carbs before
    storing the bike?
    The Older Gentleman, Jun 10, 2009
  20. Dave

    ¿ Guest

    On Jun 10, 2:04 pm, (The Older
    Inaccessability of drain screws behind fairings, gas tanks, etc.

    Easier to add B12 to gasoline and ride the carbs clean.
    ¿, Jun 10, 2009
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.