1975 Honda CB750K5 uneven idle, stalls below 1000 rpm

Discussion in 'Motorbike Technical Discussion' started by Sergeant Tibbs, May 25, 2007.

  1. Hello again,

    This is the same bike as my previous question; it idles fairly steady at
    about 1500 rpm but won't go lower without chugging and after a few
    seconds it will stall if it gets under 1000 rpm.

    I've removed and cleaned the carburetors. The jets, valves, slides,
    ports, and screws are all as clean as I can get them without invoking
    deities. I've replaced the excessively old spark plugs (the previous
    owner had at least two different types on it) and confirmed solid
    regular spark on each plug, seemingly good contact on the points, and
    good clean new gas flow to the carburetors.

    If I'm really careful I can check the manifold temperature; after a few
    minutes of running, pipes 1 and 3 feel somewhat colder than 2 and 4.

    I don't need the choke to start it, and it starts /almost/ immediately
    when cold. When it's warmer, though still well below operating
    temperature, it starts and runs immediately. After a minute or two the
    idle speed rises by at least 400 rpm though I'm using the aftermarket
    cruise control lever to fix the throttle position. It fluctuates with
    audible difference by about 50-100 rpm in short intermittent bursts when
    it's been running for more than a few minutes.

    The air filter isn't present right now, since mice chewed part of it and
    replacements need time to arrive.

    Revving the engine is smooth, though if the engine speed is jumpy just
    before opening the throttle, it'll backfire in very light pops. While
    idling, even while jumpy, the mufflers have a firm steady popping
    pressure (but this may be masked since cylinders 1-2 and 3-4 share
    aftermarket mufflers).

    What are the likely culprits, and which should I inspect first? It has
    been sitting for over two years and I'm wondering if riding it around
    for a while after I get my motorcycle endorsement would straighten out
    this problem.

    Thanks again for everyone's help!

    Sergeant Tibbs
    Sergeant Tibbs, May 25, 2007
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  2. Sergeant Tibbs

    MadDogR75 Guest

    My first nomination goes to an intake manifold leak somewhere between
    the carb and the engine.
    If the mice chewed the filter, they may have also done a number on any
    plastic manifolding.
    Also Check the lines to those #%$^%@^%^^ vacuum petcocks if
    (A cracked one did something similar to this to a friends yamaha 650
    A small leak will not be sensible at higher rev.s but lean you out at
    low rev.
    It would also be the cause of setting the mix a bit too rich, (that
    'popping` you mention, and
    the fact that you don't need choke to start point this way).
    Failing this it's all carb symptoms so thats where to look.

    Best of luck,

    P.S.It definately takes second place, but just for jolleys, check the
    ignition for intermittant
    operation of one coil.

    " They tell me that if a worm is cut in half,
    both halves continue to live.
    The half of the worm in the apple seems to live.
    but I have never checked on the other half."
    MadDogR75, May 27, 2007
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  3. I've taken the air screws all the way in and set them each in the middle
    of the 3/4 - 1 1/2 turn range as specified in the 750K manual. This
    seems to have smoothed out idling a little and it isn't nearly so smoky.
    It still lopes at slow speeds.
    It doesn't really idle at all with the choke on, as it'll stall out quickly.

    I noticed when I pulled the carburetors the throttle slides do not fully
    close; there's about a 3mm gap at the bottom, as set by the previous
    owner. Should the gap be there?
    The RPM is far steadier now. However, I noticed plugs 1 and 4 have
    turned black, and 2 and 3 only mildly tan. Do cylinders run rich at
    idle (thus blackening the plugs) or should they stay clean? I get spark
    on every wire.
    I don't have tools for these tests. My guess is, somebody with enough
    knowledge to be dangerous fiddled with the bike and I'm going back
    through (with the help of r.m.t) setting it all back to rights.

    That reminds me though; sometimes when starting the bike I can hear
    hissing somewhere inside the left part of the engine, below cylinder 1.
    It doesn't sound like it's coming from the head, and it stops when the
    engine starts.
    Very likely any gasket material is going to be as old as the engine. If
    I do use carb cleaner or gas additives, how likely am I to damage
    gaskets, rubber seals, or the non-metal floats?

    Thanks, guys; this is helping very much.

    Sergeant Tibbs
    Sergeant Tibbs, May 29, 2007
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