Backfiring

Discussion in 'Motorcycle Technical Discussion' started by Masospaghetti, Sep 11, 2005.

  1. My KZ750 backfires (or that's what i'm guessing...a pop-pop-pop when I
    coast at higher speeds). Every once in a while there'll be a loud one.
    Is this likely to be caused by a carb that needs adjusting? Maybe too
    rich or lean?

    The carbs were just rebuilt and adjusted but I don't completely trust
    the mech who did it.

    The bike also has trouble idling at anything below ~1300 RPM - it'll
    just stall.

    Any ideas? Thanks in advance.
     
    Masospaghetti, Sep 11, 2005
    #1
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  2. You need to clean out your idle jets and idle passages and adjust the
    pilot mixture screws, which are probably concealed under EPA
    anti-tamper
    plugs that need to be drilled out.

    As I have covered this subject a bazillion times in this NG, I won't
    write all that up again, you can google for it.

    Alternatively, you might try running 3 ounces of Berryman's B-12
    Chemtool Choke and Carburetor Cleaner through a full tank of gas.
    repeat until you've used up the whole 15 ounce can. You can get the
    liquid B-12 at Wal*Mart or any good auto parts store for about $3.00.
    You'll know the stuff is working when you have to keep turning the idle
    speed down...
     
    krusty kritter, Sep 12, 2005
    #2
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  3. Maso,

    How long had the bike been sitting?

    BTW krusty is spot on. Although I have not tried Berryman's liquid, I have
    tried the Yamaha liquid carb cleaner. It did a great job and it seems
    fairly gentle on plastic parts. it just took a long time to dissolve that
    ancient varnish.

    Krusty,

    Have you tried the Yamaha liquid carb cleaner? It smells unlike anything I
    have encountered in the automotive field. I think it smells something like
    SpotShot(tm) carpet cleaner which is very mild. BTW SpotShot is a great
    product. I consider it the best carpet cleaner available.

    pierce
     
    R. Pierce Butler, Sep 12, 2005
    #3
  4. Mark Olson recently said that the Yamaha cleaner contains xylene.
    Berryman's
    B-12 also contains xylene and acetone.

    There's no Yamaha $tealer$hip here in Cowpoop...

    If I went to Yamaha to buy their product, I'd have to drive 60 miles
    round trip and probably spend five times as much money to get what I
    can buy locally from Wal*Mart for $3.00...
     
    krusty kritter, Sep 12, 2005
    #4
  5. The bike hasn't sat long since I had the carbs rebuilt.

    Do the idle mixtures usually need to be adjusted after the carbs are
    rebuilt?
     
    Masospaghetti, Sep 12, 2005
    #5
  6. I think i am confusing two products. MarineClean and Yamaha carb cleaner.
    Yamaha may indeed contain Xylene and Acetone.

    The Berrymans B-12 Chemtool I picked up contains Methanol, Tolulene,
    Acetone, mixed Xylenes (I didn't know there was more than one kind), MEK,
    and Butoxyethanol, and Isopropanol.

    If that doesn't clean it, nothing will.
     
    R. Pierce Butler, Sep 12, 2005
    #6
  7. How long did it sit before the carbs were rebuilt?

    It depends on who did the rebuild and if they noted where the mixture
    settings were and if they know where a good starting point is. Generally a
    one turn or a fracton thereof is all it should need to make the idle miture
    optimum.

    Krusty is right. The idle jets are clogged completely. The cheapest way
    to fix this is to follow Krusty's instructions to the letter.

    Once the bike idles good, then generally the bike will run pretty good
    under load and at speed.


    pierce
     
    R. Pierce Butler, Sep 12, 2005
    #7
  8. Masospaghetti

    Battleax Guest

    Be sure the headers and exaust are not leaking
     
    Battleax, Sep 12, 2005
    #8
  9. Masospaghetti

    Matt Guest

    Then it wasn't much of a rebuild.
     
    Matt, Sep 12, 2005
    #9
  10. Thats what I was thinking, shouldn't they have cleaned the carbs when
    they rebuilt them?

    But who knows, I don't really trust them.
     
    Masospaghetti, Sep 12, 2005
    #10
  11. That I have no idea. The guy I bought it from turned out to be a
    complete con-man. He told me (and said in the auction) that the carbs
    had been "gone through and were better than new" although they leaked
    gas like no other and one had a punctured float.

    He said he had it only for a year, only had ridden in a couple of times,
    but who knows.
     
    Masospaghetti, Sep 12, 2005
    #11
  12. Well, a lot of guys work on cars and they don't realize how tiny the
    idle passages and ports are. They've heard about "throwing a kit into
    it" all their shade tree mechanic lives, so they think a kit is some
    kind of magical cure, that a one of bunch of a bunch parts is bound to
    cure the problem.

    I looked at the carburetor drawing for the 1980 750 LTD-2 on
    www.buykawasaki.com

    You can look at the parts diagrams there without registering if you
    click on the link that talks about ordering owners manuals. It leads
    you to a series of screens where you can select whether you want to
    look at motorcycles, watercraft, atv's, whatever. Once you get to the
    motorcycle list, you can select the year 1980, and the engine size,
    750. The list led me to the 1980 750 LTD-2 which has two carburetors,
    so it must gots to be a twin, no?

    Anyway, on the carburetors diagram, you can see #79, called Plug, Pilot
    Screw on the left side of the lefthand carburetor. If you look at your
    motorbike and you see that plug has been removed, you know somebody has
    tampered with the carburetors, that's the anti-tamper plug.

    What you need to do if the plug has not been drilled out is to follow
    the instructions I wrote up about that. Google for "kaybearjr@aol +EPA
    plug". You can follow the instructions for how to remove the pilot
    screws and not lose the original settings.

    The whole object of adjusting those pilot mixture screws is to get the
    engine to idle at the specified RPM without stumbling and idling
    erratically at the lean end, or sooting up the plugs and making the
    exhaust smell like unburned gasoline at the rich end...

    You go for the lean end to save gasoline, make your engine easy to
    start and warm up and have good acceleration just as you open the
    throttle.

    What most shadetree tuners do is open the screws to far, they think the
    engine should speed up when they turn the screws counterclockwise, and
    the engine doesn't do that. They turn the screws so far and the exhaust
    sound starts thudding instead of having a crisp sound to it and maybe
    they even smell raw unburned gasoline coming out the exhaust.

    The idea of adjusting the pilot screws is to turn them *clockwise* a
    little and watch the RPM rise. Then you turn down the RPM using the
    master idle knob and turn the pilot screws clockwise a little more and
    turn the master idle knob down a little more, and so on.

    Eventually, you'll find a point where the pilot mixture screws are
    making the mixture so lean the engine idle starts stumbling. Then you
    can turn the pilot mixture screw counterclockwise 1/4 of a turn and
    that should be good enough.

    Then you test ride the motorbike and see if it has good acceleration as
    you just open the throttle. It shouldn't spit back out the carbs,
    that's too lean. It shouldn't pop out the exhaust, that's too lean
    also, you need to open the pilot mixture screws about another 1/4 of a
    turn...
     
    krusty kritter, Sep 12, 2005
    #12

  13. Sorry to hear you have been taken. I hate liars.

    On the plus side, if you are careful and have some patience and some
    mechanical abilities, you are likely to be able to get the bike running
    well for just a few dollars additional expense.

    You might as well plan on doing all the routine maintenance. Valves,
    brakes, etc.

    pierce
     
    R. Pierce Butler, Sep 12, 2005
    #13
  14. Thanks for the info, krusty. I will try that. The plugs were gone.

    Should the screws necessarily be in the same position between the carbs?
    I noticed one is turned more than the other.
     
    Masospaghetti, Sep 13, 2005
    #14
  15. I hope so. I did some maintenance already, it's shaping up pretty well
    overall. I can tell the bike was neglected though - the chain is crusty,
    the oil was low and god knows how old, the brake fluid was black syrup,
    indicator lights burned out...

    I did notice after I bought it that the head gasket is leaking oil,
    though :( That one won't be fun.
     
    Masospaghetti, Sep 13, 2005
    #15
  16. Masospaghetti

    Matt Guest

    What about acceleration from a stop when cold? If it stalls, I would
    suspect bad plugs.
     
    Matt, Sep 13, 2005
    #16
  17. Masospaghetti

    Matt Guest

    I would do a compression test (buy or borrow a $20 gauge) and replace
    the plugs if you haven't.

    I would pull off one or more of the bowls and inspect the idle jets,
    maybe before and after the carb-cleaner-in-the-fuel routine. Maybe you
    need a stubby phillips screwdriver or maybe one of those S-shaped
    lever-handled screwdrivers ($2 at Sears). That way you don't have to
    remove the carbs. I expect you can get a look at the idle jets in five
    minutes each. You can get low-cost replacement idle jets (aka pilot
    jets) at denniskirk.com or siriusconinc.com.
     
    Matt, Sep 13, 2005
    #17
  18. I would expect the screws to be pretty close to each other in the
    number of turns out from lightly bottomed.

    The carburetors are precision instruments, some of the parts are made
    to fantastically close tolerances. When the carbs were brand new, the
    only reason there was for setting the pilot screws any different from
    each other was they were using an exhaust gas analyzer to adjust the
    screws to get the emissions within spec.

    With separate exhaust pipes, you can run the probe right up each pipe,
    but watch out for systems with equalizer pipes between the two main
    pipes, one cylinder will be feeding excessively rich mixture while
    you're turning the other screw tighter to lean up one carb...

    The average shade tree mechanic doesn't have an exhaust gas analyzer or
    any other instrument except his screwdriver and his ears and nose and
    his brain.

    If you start out with both of the screws about 1.0 turns out and start
    up the engine from cold and the idle RPM runs away and you have to keep
    fiddling with the choke to get the RPM down, you know that the pilot
    mixture screws are too far open and the idle speed is set too high to
    compensate for the rich idle mixture, so you turn the idle RPM down and
    screw the two pilot mixture screws in about half a turn and try
    again...

    If the engine RPM doesn't run away when the engine is cold and you warm
    the engine up and twist the throttle quickly and release it, if the
    idle RPM hangs up and takes too long to come down to the specified
    idle, you turn the pilot mixture screws in another 1/4 of a turn and go
    ride the motorcycle. By this time, if you've been paying attention, the
    pilot mixture screws are only open about 1/4 of a turn...

    If you go for a ride and the exhaust doesn't go fartPOP! when you roll
    off the throttle and the idle RPM doesn't run away when the engine is
    hot, the
    pilot mixture screws are adjusted about right...
     
    krusty kritter, Sep 13, 2005
    #18
  19. Hope the jets are removable. The ones on my 1981 are not. They are
    pressed in.

    pierce
     
    R. Pierce Butler, Sep 13, 2005
    #19
  20. Masospaghetti

    Matt Guest

    wow
     
    Matt, Sep 13, 2005
    #20
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