1984 Honda Magna 700 hesitation problem

Discussion in 'Motorbike Technical Discussion' started by SG, Apr 28, 2005.

  1. SG

    SG Guest

    I have an '84 Magna 700C that has hesitation to it. Let me give you a
    brief history. Bought the bike in '96. I'm in the Navy thus transfer from
    time to time. In 2000 went to Japan. Bike sat in father's garage until 2004.
    When I got it, it would start up, but not idle properly. I performed all
    repairs that did NOT have to do with the engine directly, e.g. clutch and
    brake seals, front and rear brake cleanings, fluid changes, cable
    replacements. I put the bike in the shop for a full tune-up, fork seal
    replacement, carb cleaning, and gas tank cleaning. My bike ran great after I
    picked it up, just like old times. About two weeks later I hop on it to take
    my daughter for a ride. I notice that it's slow to accelerate (my daughter
    is only 80 lbs). It's been like that all winter. Whether starting out from a
    stop, or accelerating on the highway to pass, it is slow. I cannot even get
    above 9000 RPMs in first gear at full throttle. I've tried different gas and
    carb cleaner. Nothing seems to work. I don't have any fluid leaks. Could an
    exhaust leak cause cause this? Any help would be greatly appreciated.

    SG, Apr 28, 2005
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  2. SG

    SG Guest

    Thanks for the reply Mike.

    All cylinders are firing. I've even removed one plug wire at a time while
    the bike was idling and there was a noticeable drop in RPM. It even stalled
    at one point. I was on the highway today doing 75. I could barely make it to
    80. This is not normal. I remember doing 120 on this bike (8 years ago, when
    I was 23 and indestructible). I noticed that when I had the bike at 75, then
    opened the throttle all the way, I would actually start to slow down. There
    was definitely NO acceleration, in fact, my body would move FORWARD on the
    bike indicating that it was DEcelerating. HELP!

    SG, Apr 28, 2005
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  3. SG

    PQ Guest

    Some thots...
    I would investigate:
    vapor lock-air needs to enter the fuel tank as gas leaves or a vacuum is
    created.Vacuum gets strong enough fuel will not leave the tank or flow is
    reduced. Check any tank venting.
    blocked or partially blocked fuel line or fuel filter - dirt or kinked hose
    maybe the extra weight pinched a hose?
    dirty air filter or blocked air intake (leaves, nesting birds...)
    PQ, Apr 29, 2005
  4. Did you pull out the spark plugs and look inside them at the insulators
    for color? If the insulator is all covered with black fluffy carbon,
    the fuel air mixture is too rich. The engine would run with a heavy,
    thudding exhaust sound, and be unwilling to take full throttle. If the
    spark plug insulators are bone white and rough looking, the center
    electrode and the ground electrode would probably be oxidized blue.
    That means the fuel air mixture is too lean, the engine won't develop
    full power and will overheat...

    There's not much to go wrong with the electronic ignition systems of
    modern motorcycles anymore. Some engines did have mechanical advancers
    in the mid-1980's and the advancer could have a broken spring or one
    spring would fly off, or the advancer weight would rust and stick in
    the fully advanced position and that would affect the starting and mid
    range power and the engine would grind on the starter when the bike was
    hot and the mixture would seem to be too lean.
    Check the parts diagrams at www.partsfish.com to see if you have a
    mechanical advancer. The ignition advance curve is built into most
    modern electronic ignition control units. They just count the number of
    times the pulser coil sends a signal to the spark unit and that signals
    the unit to advance the ignition according to RPM only, no vacuum
    advance like on a car engine...
    Years ago, when motorbikes had ignition points that had to be cleaned
    all the time, you could blame many problems on the points, and the
    carburetors seemed very reliable. As motorcycles all got electronic
    ignition systems, riders didn't have to maintain the ignition points,
    there weren't any. They paid less attention to the engine and the
    carburetors would get dirty inside, especially the idle jets. Riders
    compensate for dirty idle jets by turning up the idle speed knob and
    using more "choke". As I have explained hundreds of times, modern
    constant vacuum carburetors don't have a real choke plate, they have a
    tiny carburetor in the side of each main carb and that tiny carb sucks
    gasoline right out of the float bowl. But the tiny carb, called a
    "starting enrichenment device" needs all the help it can get from the
    fuel passing through the idle bypass ports and the idle mixture screw.
    The idle bypass ports and the idle mixture screw have to get their gas
    through the idle jet and the idle jet has such a tiny orifice (only
    0.012 to 0.016 inches in diameter) it easily gets plugged up and
    sometimes the plugging is so bad it can't be cleaned out using powerful
    carburetor cleaners like the liquid version of Berryman's B-12
    Chemtool(3 ounces per tank of gas, repeated 4 times)...

    You can tell you need your idle jets cleaned when your engine is hard
    to start, needs a lot of choking, is slow to warm up, and tends to
    stall when you pull the clutch in to slow down. When you roll off the
    throttle, the exhaust pipe goes fartBANG!

    If Berryman's B-12 doesn't clean out your idle jets, diassembly is
    required. I've explained the process hundreds of times. Google up
    "[email protected] +idle jet"...

    Other fuel, carburetor and air induction problems might include:

    A clogged air filter that need to be cleaned or replaced;

    A clogged external fuel filter in the fuel hose that needs to be
    cleaned or replaced;

    A clogged fuel filter *inside* the gas tank on the petcock standpipe;

    A clogged fuel filter just above the float valves on the side of each

    Carburetors far out of synchronization;

    A fuel hose that is kinked due to improper routing (I had that happen
    to me once, drove me absolutely nuts, took three weeks to locate the

    Rust and crud in the petcock that has settled down into the fuel line,
    blocking the float valves and the jets;

    Water in the gas that sits in the bottom of the float bowls (you can
    drain the float bowls into a clear glass container and see the water
    settle to the bottom);

    Vacuum piston or slide sticking. Sometimes evaporated gasoline will gum
    up the
    slides so they can't move at all. The carb can't suck much gas through
    the needle jet, it has to run on the idle jets only, and the engine
    starves for fuel;

    Vacuum piston or slide diaphragm torn. It can't lift the jet needle out
    of the needle jet. Same symptom as above;

    Incorrect float level, the floats shut off the gasoline prematurely and
    idle jets and main jets have to work too hard to suck up what little
    fuel is in the float bowls. Floats don't just change their adjustment
    that much, even over a period of years. They get out of adjustment when
    a mechanic bends the adjustment tab trying to set the float levels;

    Float leaking. If a brass float leaks, it fills up with gasoline and
    settle to the bottom of the float bowl. The float doesn't shut off the
    flow of gasoline into the float and that float bowl overflows. Gasoline
    leaks onto the ground under the motorbike.

    Conditions relating to the mechanical condition of the engine:

    Worn out or stuck piston rings reduce the compression of the engine, it
    won't develop full power. Rings stick when hot oil collects in the
    piston ring grooves and hardens. Some chemical nostrum like Rislone,
    Marvel Mystery Oil or Techron added to the gasoline will free up stuck
    rings. This is more of a problem with 2-stroke engines, but an
    oil-burning 4-stroke can have sticky rings;

    Valves out of adjustment or burned. Even the intake valves can burn, if
    the valve guide oil seals leak oil onto the hot valve head. This oil
    turns into hard wet carbon and keeps the intake valves from closing.
    The compression of the engine blows air backwards through the
    carburetors. The engine can't hold in fuel/air mixture that it sucked
    in on the intake stroke because the piston
    blows it back out on the compression stroke. The engine pings and
    detonates like crazy when this happens...

    Blown head gasket between cylinders. If the pistons are on opposite
    strokes, the piston coming up on compression leaks high pressure air to
    the cylinder going down on intake stroke. That cylinder can't fill up
    with fresh fuel air mixture. I saw that happen recently on a pick up
    truck engine. Drove me absolutely nuts for three weeks troubleshooting
    the problem...

    Don't be afraid to do a compression test on your engine. Warm it up,
    pull out the spark plugs, check the compression with the throttles wide
    open so the rings will get enough air behind them to compress the air
    in the cylinder. If the compression is less than 135 to 140 pounds,
    something needs to be done, it's
    worn or stuck rings or burned valves, or out of adjustment or sticky

    Dribble a few drops of motor oil into the spark plug hole of the
    cylinder(s) that is low on compression. Turn the engine over several
    turns to spread the oil around. If the compression increases with the
    oil sealing the rings, that's what it is, a ring problem. If the
    compression doesn't come up, it's a valve problem...

    Engine repair manuals will probably tell you that the specified
    compression for that engine is around 175 to 190 pounds or so. If your
    engine still have 135 to 140 psi, it will go 100 mph. It just won't get
    there as quickly. And I wouldn't diassemble an engine that had 135 psi
    compression, it's a lot of work to pull the engine out of the frame,
    split the cases and re-ring a V-4 that's not worth a whole lot of money
    at this point...
    krusty kritter, Apr 29, 2005
  5. WOW!!! I think you covered every possible contingency. Thanks VERY much for
    taking the time to type all this. Looks like I got some work ahead of me for
    the weekend. I'll start with the easy stuff and work my way up. I'll let you
    know what I find.

    Thanks again
    Shawn Goodwin, Apr 29, 2005
  6. One thing I didn't mention was regapping the spark plugs to about 0.025
    inches. If the engine runs a lot better with a smaller gap, your spark
    plugs might need changing. Something changes the internal resistance of
    spark plugs, even the modern copper core plugs that can withstand a lot
    of heat cycling. When the plugs change inside, for whatever reason, a
    new set works so much better it's amazing. But by gapping the old plugs
    tighter to see if that fixes the problem, you can save yourself about
    $10.00. I have changed plugs twice on my pickup truck troubleshooting
    problems. It takes 8 plugs for a dual plug 4 cylinder, and that turned
    out not to be the problem...
    krusty kritter, Apr 29, 2005
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