1976 BMW R90/6 left cylinder not working

Discussion in 'Motorbike Technical Discussion' started by gabe2004, Aug 3, 2005.

  1. It is a big problem. This popping can turn into stalling in dangerous
    situations, like when you roll off the gas to approach a stop sign, and
    the idiot behind you in a car expects you to immediately move away from
    the stop and he hits you from behind. Or, you could be in a real slow
    tight turn and be depending on the engine to keep running to keep the
    bike upright, and it will stall and you'll tip over.

    Your idle jets and idle mixture passages have dirt in them. That's
    because you did the carb cleaning half-assed with WD40.

    Find the idle mixture screw underneath the carb. Turn it all the way IN
    until it's lightly seated. Count the number of turns and parts of a
    turn. Write it down. Unscrew the idle mixture screw. Pay attention to
    the assembly of springs, washers, and little rubber o-rings if there
    are any. Don't lose the o-rings up in the hole.

    Once the screws are out, you can squirt real aerosol carb cleaner like
    Berryman's B-12, STP, or Gumout up through the idle mixture hole.
    Forget about WD40, it won't do the job.

    Also squirt it through the pilot air jet hole which may be in the
    intake bell or under the diaphragm. There are online drawings for Bing
    carburetors if you need to look at diagrams to find the parts I'm
    talking about.

    Squirting carb cleaner through the pilot air jet hole should cause carb
    cleaner to squirt freely out through the idle jet in the float bowl, as
    well as some transition holes downstream of the throttle butterflies.
    If you take the carb off for a proper cleaning with aerosol carb
    cleaner you can remove the float bowls and squirt carb cleaner directly
    through the idle jet.

    When you go to reinstall the idle mixture screws, springs, washers,
    and o-rings, make sure it's in the exact order them came out. And screw
    the *same* idle mixture screw back into the same hole it came out of
    until it's lightly seated. Then screw it back out the exact same number
    of turns you counted when you removed it.

    When you ride the bike the exhaust popping will be gone...
    krusty kritter, Aug 20, 2005
    1. Advertisements

  2. Might be running slightly lean. What color are the plugs ?
    Both the same ? Might also want to take a good look for
    leaks in the intake and exhaust. Does it still have all
    the emissions control crud on it ?
    Rob Kleinschmidt, Aug 22, 2005
    1. Advertisements

  3. gabe2004

    gabe2004 Guest

    Hey guys. I'm back with another question. First of all, did what you
    said and did a THOURGH carb cleaning, unscrewing mixture screws, etc.
    and it now runs perfectly, no backfireing. But, I think i now have
    some sort of electrical problem. A while ago, i was riding and the
    engine suddenly died. Tried to start, wouldnt crank. So i left it and
    came back later and was able to kick start it. When i as just about
    home, i turned on the headlight and the engine suddleny diead. Left it
    for night and put the charger on the battery. Now, it sometimes starts
    fine, and other times the solenoid will click and then nothing happens
    and the indicator lights all day. Seems like it's shorting out
    somewhere, Other times, it will crank and start up fine. Any ideas?
    gabe2004, Sep 9, 2005
  4. Dead battery?

    Does it start and run Ok with jump leads from another decent battery? If
    so, that's your problem.
    The Older Gentleman, Sep 9, 2005
  5. wrote in @z14g2000cwz.googlegroups.com:
    Change the battery.

    Did you balance the carbs too? If not, do it. You will be amazed how much
    bettter it will run.

    R. Pierce Butler, Sep 9, 2005
  6. Batteries do not last forever. I got two years MAXIMUM out of original
    equipment Yuasa batteries before the invention of sulf-stop, which is
    their anti-sulfation technology. Yuasa seems to have bought up the US
    battery manufacturers, at least those who make motorcycle batteries...

    If you discharge a battery (maybe because your charging system isn't
    working right, or maybe you rode the motorcycle in hot weather and a
    lot of water evaporated out of the electrolyte) and you don't
    immediately recharge it, lead sulfate forms on the plates, killing the

    You might as well figure on getting a new battery if you inspect all
    the wiring and don't find any loose connections.

    The best service life I got out of a Yuasa battery with sulf-stop
    technology was five years. Then they came along with maintenance-free
    batteries that never need water. The first one of those lasted eight

    Even a maintenance-free battery will eventually evaporate all of its
    water. Then it will sulfate the plates.

    Battery plates used in small starting batteries are made of spongy lead
    so the electrolyte can penetrate deep into the plate for more cranking
    amp capacity out of a small battery. That's how car and motorcycle
    batteries package a lot of power in a small space. If they didn't use
    spongy lead, your car battery would be as big as your engine.

    But, if the spongy lead plates are allowed to sulfate, the electrolyte
    cannot get deep into the plates and, when you do charge the battery,
    you only get a charge on the surface of the plates.

    When your battery has a surface charge, it will have the correct
    voltage and the specific gravity of the electrolyte will be correct.
    But, when
    you turn on the ignition switch, maybe the lights will come on for a
    second and then the whole ignition system goes dead, leaving you
    scratching your head wondering what happened.

    A surface-charged battery is just like a big capacitor. It immediately
    totally discharges and is done, it needs to be immediately recharged.
    As soon as you turn the ignition switch on, it discharges again. It's
    done for.

    Another problem with spongy lead plates is that particles of the lead
    will fall out of the plates and sit on the bottom of the battery case.
    If enough of this lead builds up in the bottom, it can short out
    adjacent plates.

    If your motorcycle battery has a clear plastic case, you can look up at
    it from underneath and see the silvery mass of lead particles in the
    bottom of the case...
    krusty kritter, Sep 9, 2005
  7. gabe2004

    gabe2004 Guest

    No, im postive its not the battery, its fully charged and brand new.
    See, sometimes it wont crank but the soloneid will click. I try mabey
    4, 5 times in a row and it wont crank. Then sixth, boom, cranks
    prefectly and starts up fine. What i think is happending is that
    somehow when its just about to crank, something shorts the whole system
    out, cause all the lights go off to. I then disconect battery and try
    again, sometimes it works right away, sometimes it will short again
    for a few times, then crank.
    gabe2004, Sep 9, 2005
  8. I doubt that "something shorts the whole system out". Most
    non-technical types don't know what a "short" is. They say "short" when
    actually what they have is an OPEN circuit.

    I think that what you have is an *intermittent open circuit" due to bad
    electrical connections. If you had a real "short circuit" in any
    circuit except the starter circuit, you'd be blowing fuses.

    A short circuit occurs when electricity finds a shortcut around the
    load that it is supposed to operate. Suppose you have a simple circuit
    containing a battery, a switch, a light bulb, and wires connecting the
    battery to the switch, the light bulb, and back to the battery.

    Now, suppose the electricity finds a way to go directly through the
    wires and back to the battery. *That's* a short. The light bulb was
    supposed to use the electricity from the battery, turn that electricity
    into heat, the heat was supposed to make light, and almost all the
    electricity except just a tiny few volts were suposed to be used up by
    the light bulb.

    But, suppose the lightbulb socket had a direct connection so all the
    power didn't have to go through the light bulb. There is enough power
    to make the bulb filament hot, but that power doesn't all have to go
    through the bulb filament so IT HEATS UP THE WIRE INSTEAD!

    That's why there are fuses in all the circuits except for the starter
    power circuit...

    When you try to start your BMW, you might have a short inside the
    starter armature. That's a very low resistance piece of electrical
    equipment. it normally gets hotter than hell PDQ. Or, one of the
    starter brushes might
    be grounded out, short circuiting the starter, and using up all the

    That's another thing that confuses electrical newbies. People talk
    electrical "ground" and the newbies figure that the frame is "ground"
    and that all the elctricity just somehow *disappears* into ground when
    the wire goes into the frame. No, no, no, no, no! The motorcycle's
    engine and frame may be "ground", but all the engineers are doing is
    using it for a return wire to the battery. The battery ground cable
    usually comes off the
    battery's negative terminal and it hooks up to the frame. There may
    also be a second ground cable that goes from the frame to the engine,
    making sure that the engine is "grounded", that it can make a complete
    return path from the spark plugs and the starter and anything else that
    is "grounded" to the engine.

    But, even that extra "ground" cable is NO GUARANTEE that you'll get a
    complete circuit from your battery, out the battery cable, through the
    solenoid, through the armature in the starter, through the engine
    block, back out that "ground cable", through the frame, and finally
    travel through the battery negative cable to the battery. A bad
    connection anywhere will stop the starter from working.

    We even had a guy in here complaining his starter wasn't working and it
    turned out that(after he went through all the wiring and cleaned all
    the connections, including the "ground" connections on the frame), his
    engine block was so corroded right where the starter bolted to the
    engine, he wasn't getting a good "ground"...

    So, you need to go through all the electrical connections around the
    battery and especially follow out the main power bus circuit to the
    fuse box. A "bus" is a main power feed that everything else is hooked
    up to. Whatever power that comes out of your generator or alternator
    and goes to the battery will also usually go to the main power bus.
    That's where you get the main "juice" to power your ignition, lights,
    turn signals, etc...

    Look for dirty or loose connections everywhere on the main power bus...

    Loose connections get hot, they melt plastic and discolor the
    insulation on bullet type connectors.
    krusty kritter, Sep 9, 2005
  9. <snip>

    kk, with the best will in the world....

    I bow in respect at your technical knowledge, but - a plea from the
    heart - your posts are in danger of tripping my binary filter ;-))

    Find an editor!
    The Older Gentleman, Sep 9, 2005
  10. My guesses would be either

    your battery is low
    you've got a loose wire or other bad connection
    your starter is in need of some rework
    some combination of the above.

    Incidentally, my guess is that the click you're hearing is
    the starter relay rather than the solenoid. The starter
    relay is what energizes the solenoid.

    I agree with Krusty that it's not a short.

    Get a voltmeter, check the battery voltage, then take a careful
    look at all the battery cables, especially anything you may
    have disconnected recently.
    Rob Kleinschmidt, Sep 9, 2005
  11. gabe2004

    gabe2004 Guest

    Hmm. The reason i was suspecting a short was because everything went
    off, idicator lights, headlight, voltmeter went down, clock stopped. I
    dont't think the starter is normally in series with the lights, clock,
    etc. I was figuring that some how maybe the started relay or solenoid
    was shorting out the system when it was mechanivly moved. The OTHER
    reason i suspected a short is be cause the battery was dead. I've been
    riding it moderatly for a while and boom! One day the battery is
    totally dead. That smells like a short to me. I think we can defintly
    rule out the battery as a CAUSE of the problem. After all, how would a
    battery cause the bike to stop ONCE its running. Btw, I confirmed that
    the alternator works fine by starting it up and then disconnecting the
    battery - it kept running fine. I was thinking of loose wires, but i
    couldnt find anything loose near the coils and voltage regulator under
    the tank. I suspect that if there was some sort of bad connecting, it
    would be near the starter.

    Another idea I had (just a thought) was that something was wrong with
    the points. If the points were bad, the coil could be drawing current
    longer than its supposed to, draining the battery. But then again, the
    alternator works fine so that shouldnt make a difference. Also, if the
    points were bad, I suppose the cylinder firing would sound wierd.
    These are just vauge thoughts. Any other suggestions would be great
    before I start working on it. Thanks.
    gabe2004, Sep 10, 2005
  12. wrote in @g14g2000cwa.googlegroups.com:

    Your chronology of events is not 100% clear to me but be that as it may, I
    will try to help you with your problem.

    These issues lead me to think you have more than one problem.

    Your assesment about a short may be wrong. It could be an open. Check
    your fuses. I am not familiar with BMW, but on many bikes there is a
    rather large "main" fuse. These can develop physical cracks that will
    intermittently cause a total loss of power to the bike (no lights, bike
    stops, etc). You have to check this fuse by removing the fuse as cracks are
    almost always invisible. This failure does not explain the battery being
    dead however.

    Clean your battery connections, particularly the positive terminal. Using
    a suitable tool, scrape the terminals until you see shiny metal. Lead can
    and will corrode and this corrosion transforms lead from a conductor to an
    insulator. Thankfully this corrosion occurs at the surface and it can be
    removed by scraping it off. Lead oxide can be pretty tough so you might
    have to use a small file. This could account for the battery being dead.

    You may be right about the short, but it may be where you least expect
    it... Internal to the battery itself. Batteries can be intermittently bad.
    What happens is that as a battery ages, material comes off the plates and
    collects in the bottom of the battery. When the level of this conductive
    junk gets high enough, it will short out the plates and the battery will
    discharge quite rapidly even though it is disconnected.

    Try this test. If the battery is charged and can start the bike,
    disconnect it from the bike and let it sit for 8 hours or more. The longer
    the better. Connect it and try and start the bike. If all it does is
    click or crank the engine slowly, then your battery is bad.

    Also try the following test. WIth the battery fully charged and with the
    bike not running, turn the headlight on and leave it on for 3 minutes.
    Then try to start the bike. If it doesn't crank or cranks the engine
    slowly, the battery is weak/bad.

    Do you have a voltmeter? That could prove very useful right about now.

    R. Pierce Butler, Sep 10, 2005
  13. gabe2004

    gabe2004 Guest

    Thanks for the advice but let me try to explain the problem better.
    First off, the battery is brand new, about a month old. The terminals
    are in great shape and it is fully charged (is able to crank the engine
    fine and checked voltage with voltmeter). There is absolutly no

    The wierd problems started about a month ago. I was riding and the
    engine suddenly stopped. It wouldn't crank up so i left it. Came back
    a bit later and was able to kick start it. I figured the battery was
    dead. I almost got home when i switched on the headlight and imiedtly
    the engine dies. I disconnect the battery and put it on the charger.

    Few days later:

    Battery is hooked back up to bike and i decied to see if it starts up.
    Press the button and it starts up fine, no problem. Idles perfectly
    too. I come back a few hours later to try it out. I press the button.
    I hear a click and imiedtly all the lights, voltmeter, clock, goes
    off. I imiedtly disconnect the battery and try again: same thing
    happens about 4 times in a row. Finnaly, on the fith time, it cranks
    and starts up perfectly. I stop the engine and try it again: no start,
    click and lights go off. I then try again and it starts up perfectly.

    Now, the first question is WHY did the engine STOP in the first place
    while i was riding. There's no way that a bad or dead battery would
    cause the engine to STOP running once its started. As i said before,
    the alternator works fine because i was able to disconect the battery
    and the engine kept running. Once the engine is started, the alternator
    provides all the current neaded. The other question is why did the
    battery suddenly go dead. One day it's fine, and the next it's dead.
    That's why i suspected a short, not open circuit. I think that
    whatever caused the battery to go dead while the bike was parked
    overnight also caused the engine to stop while running. I mean,
    imagine if the bike's 12 volt system shorted out, lets say close to the
    battery, not blowing a fuse. The battery would quckly drain. A short
    would also stop current from flowing through the coils and no spark,
    stopping engine.
    gabe2004, Sep 11, 2005
  14. wrote in @g14g2000cwa.googlegroups.com:

    I think you should concentrate on one issue at a time. I have seen cases
    where salt and corrosion built up and caused a low resistance short to
    ground. It was odd as the fixture would get warm without the lightbulb
    being in it. This sort of situation could be your short.

    As for your excessive drain on the battery, that is a tough one. It has
    always been tough to trace out even on cars. I have fixed car problems
    very similar to your situation. The short version of the most memorable
    one is that the trunk lights were staying on, draining the battery
    overnight. How did I find it? First I measured the current that was
    being drawn with nothing turned on. Then I pulled fuses one by one until I
    found the circuit affected. I actually had to get in the trunk and have
    someone open and close the lid a few times. Once in a while the lights
    would go off. I adjusted the switch and all was fine. One can use a
    voltmeter to determine if something is drawing current. Disconnect one
    battery lead and connect the voltmeter to the disconnected lead and to the
    battery. If there is nothing drawing current, then the voltage will be
    zero. Conversly if there is a load the voltage will rise to 12 volts.
    This sort of testing will allow you to "divide and conquer."

    R. Pierce Butler, Sep 11, 2005
  15. I was thinking some more about your issue. It could be the starter
    solenoid that is your problem. There could be some contaminant inside
    causing a low resistance path to ground. One cannot discount anything
    especially on something that is as old as your bike.

    R. Pierce Butler, Sep 11, 2005
  16. If you had a bad ground this would cause most of the symptoms you're
    seeing. I don't think disconnecting the battery was such a hot idea.
    A better technique would be to measure the battery voltage.

    In spite of some of what you've seen, I'd still suspect a charging
    system problem where you slowly discharged the battery as you
    were riding. This could be intermittant I suppose so that it
    sometimes worked and sometimes failed. A flakey voltage regulator
    could do this.

    When you first turn on the key, do you see a red "GEN" light, and
    after you've started the engine and revved it to about 1500 RPM,
    does the light then go out ?

    You should have one fat lead running from the battery to the starter
    solenoid, with another lead from the solenoid to the diode board
    which rectifies the current from your alternator.

    You should also have a second lead from the battery to a fusebox
    with a couple of circuits. These would handle everything except
    battery charging and the starter motor.

    On the negative side, you should have one fat lead between the
    battery and the gearbox by the speedo cable.

    I recently installed a $6 Radio Shack LED charging meter, which
    tells me quite a bit about my battery and charging system. You
    might want to buy one of these and rig a connection so you could
    watch the battery and charging system as you were riding. You'd
    also want to be able to disconnect it or shut it off when you
    stopped, so you'd want either a switched connection or an on/off
    switch on the meter itself. For $6, it's a great onboard instrument.
    Rob Kleinschmidt, Sep 13, 2005
  17. gabe2004

    gabe2004 Guest

    Yes, the Gen light does exactly as you say.

    I don't think it's the alternator. I've been riding it for about a
    month, using the battery to start it everytime and never had a problem.
    ALthough it's possible for the alternator connection or voltage
    regulator to be flaky, i still doubt that the battery would discharge
    so quickly, just overnight. I'll try to get my multimeter out tomorow
    and check all those connections. Thanks.
    gabe2004, Sep 13, 2005
  18. When you're running with the headlight on, the battery can
    discharge pretty quickly. If it discharged while sitting idle,
    it's possible you've got a bad cell in the battery even if
    it's almost new. What voltage had it discharged to ?

    Other suspects might include the voltage regulator and
    diode board.

    Good article:

    Rob Kleinschmidt, Sep 13, 2005
    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.