The trouble(s) with this '81 R100 Airhead

Discussion in 'Airheads' started by Rapid Dog, Feb 6, 2011.

  1. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    1981 R100RS recently gone thru, fresh heads, coils, wires, plugs, dual plugged.

    Weird buzz in '81 R100 on start.
    I've been fiddling with the carbs, starting the motor alot.
    I've had the starter rebuilt as a note. Also a new ICU under the tank.

    I noticed a few times (and this is a new one) that when I push the start button I hear a buzzing sound under the tank, like a relay or something, just before it wants to actual turn the starter motor. This is intermittent.

    What's that? Is it a starter relay going south?

    Rebuilt carbs won't adjust.
    Soaked the carbs in Pinesol for 48 hours (disassembled).
    Blew out all the passages with compressed air and reassembled with al new o-rings and float needles (shot).
    The right side is still sputtering and backfiring at idle.
    Anything below 2500 RPM and it surges and sputters.
    WFO it seems to run good but of course, backfires when you shut the throttle.
    I even ran it around the local loop with the synch port rubbers of by mistake, didn't seem to make much diff in the way it ran. Weird.

    While synching I see that the right side idle screw doesn't seem to have any effect on the Twinmax readout when adjusting it. Checked cables for free play etc, cables are new. Diaphrams are good.

    Worn out idle screw? I'm going to order up a new one for both sides Monday.

    Generator stays on light dimly.

    This is minor but the gen light never seems to go completely off even at highway speeds. I've cleaned all the contacts, battery is charging.

    Haven't figured this one out at all.
    As a note, the voltmeter goes whack and pins of and on on the + side, I have a fresh one coming this week, we'll see how that pans out...


    Neutral light inconsistent.

    Half the time I have to pull in the clutch to start as the neutral light does come on., sometimes it does. Totally intermittent.
    I cleaned the clutch switch at the lever, it works fine. Cleaned contacts at both ends of the neutral switch wire.

    Do these switches act up or just die?

    Any insight from those in the know is appreciado..
    .:ear
    #1
  2. fishkens

    fishkens Long timer

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    Weird buzz in '81 R100 on start.
    Starter relay? Sure sounds like it. Remove the tank and put your finger on the relay when trying to start. If the buzzing is coming from the relay it's probably on its last leg. You can take it apart and clean it but it's best to replace it.

    Rebuilt carbs won't adjust.
    Idle screw? If turning it doesn't affect the idle speed then the circuit seems to be clogged. Try blowing it out.

    Generator light on dimly.
    Dim alt lamp? I had that condition for years. Tried cleaning contacts, etc. It finally went away when I replaced the diode board with Rick's. Not certain if it was the board or contacts but that fixed it.

    Neutral light inconsistent. Do these switches act up or just die?
    Yep. My guess is that you'll be pulling in the clutch lever more and more until it's 100% of the time and the neutral light never lights. Bad neutral switch at the tranny. They go bad regularly.
    #2
  3. Country Doc

    Country Doc Wanderer

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    I had almost that exact same carb issue on my slash 5. Turned out that there was a tiny, tiny, tiny little passageway JUST behind the butterfly that sprays fuel for idle and just off idle, up to about 2k rpm where the main part of the needle takes over most of the fuel delivery.

    I forget the name of this passage. I could not even see it on my carb, as it was just gummed up flush to the surface of the carb throat, and was invisible. I needed a magnifying glass, tiny sharp needle, and I had to compare side by side with the other carb to figure out where the hole was supposed to be.

    Ok, correction , all this troubleshooting to find the passage was actually done by bmwrench, after I tried to sync my carbs and adjust them a thousand times and couldn't get the thing running right.

    maybe worth a look.

    dc
    #3
  4. supershaft

    supershaft because I can

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    Take the tank of and see which one it is. It can't get much simpler than that.

    Did you make sure the two small holes in the carb for the idle circuit are positively clear? It takes compressed air IMO. IF your carbs are good , you have got leaking valves. Listen with a leak down tester. They will not idle right with leaking valves.

    Test the diodes. Make sure nothing is grounding on the hot side of the diode board. Make sure none of the other wires are shorting out somewhere they are rubbing on something. Just LOOKING is a good test for this. Make sure your brushes are making good contact. Find a good rotor to try. In this situation, testing it might not show the problem.

    When the light is acting up, test the switch at the leads. If it tests good you have got to trace the wires. It's probably the switch.

    Good luck!
    #4
  5. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    Take the tank of and see which one it is.
    Trouble is, the buzzing is new, definitely under the tank, but doesn't happen all the time.
    Charged the battery overnight.
    Just went out there and removed the tank. Hit the starter several times, no buzz...weird. I have a feeling it's got to do with lack of juice from the battery.

    It takes compressed air IMO.
    Yes, shot a good blast straight up he idle screw hole, gas sprayed everywhere..:D

    test the switch at the leads

    I'll give that a try for sure.
    #5
  6. supershaft

    supershaft because I can

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    That in no way guarantees that those wholes are open. You have to block the mixture screw and the idle intake at the back of the carb at the same time and then squirt air up the pilot jet boss look and see it BOTH holes are working.

    Good luck!
    #6
  7. Solo Lobo

    Solo Lobo airhead or nothing

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    Another way to tackle this is to use carb (or brake cleaner), insert the plastic extension tube into the various carb body passages and spray.. then block the hole the spray exits with your finger and spray again, and then reverse direction.


    IIRC, the idle passage exits in two places, in the throat of the carb as well into the body.. you need to pull our the needle jet and mains for that one.

    But, as said above, if the idle screw doesn't do anything, you do have a blocked passage. No if, ands or buts about that.
    #7
  8. supershaft

    supershaft because I can

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    Actually the idle circuit is completely separate from the passage way that goes to the atomizer on top of the main jet stack. The other whole in the back of the carb is for the idle circuit.
    #8
  9. boxerboy81

    boxerboy81 Stay Horizontal

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    [​IMG]I would use something soft that won't damage the jets and circuit, like fishing line, and feed it in and out of the two holes that effect the idle circuit, #1 & #2 in the picture. Then I'd spray it thru with carb cleaner and compressed air.

    [​IMG]
    #9
  10. supershaft

    supershaft because I can

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    That diagram doesn't show the air jet at the back of the carb and its path to the idle jet. It's very important!
    #10
  11. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    ...cconcerning the 'slow jet' or idle jet (internal), it has a spring like the idle mix screw (external). When I installed this I screwed it all the way in.
    Is that correct?
    #11
  12. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    Generator light stays on dimly.

    I've removed the alternator and rotor, checking all connections to be clean in the interim.

    According to Antoine's website and others, resistance should be between 3.4 and 3.74 if the rotor is 'good'. Rotor measures resistance at 3.9 ohms.

    Quote
    "Lower rotor resistance means that charging can begin at lower RPM. There's no reason no reason to use earlier, higher resistance rotors except that the larger diameter of the /5 rotors (smaller air gap between rotor and stator) gives more power.

    Interestingly enough, the bulb for the charge light (which is in series with the rotor) changed in wattage over the years in keeping with the varying rotor resistance."


    I was told by the PO that the bulbs had been replaced in the gauge cluster with new bulbs.
    Could it possibly be that the bulb is the wrong type so it reads wrong due to the 3.9 ohm the rotor is showing?
    I am suspect of the replacement bulbs because I can see that the high beam blue lens on the tachometer looks melted...
    #12
  13. supershaft

    supershaft because I can

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    The isle jets don't have springs.

    It sounds like you need some manuals RD?

    Your rotor is testing good but they very often test good when they are not. That is why I recommended that you test it last because you often need a known good rotor to try to see if that is it. Rotors can work when they are static. The problem is that they need to rotate to work.

    I don't know about the instrument lights. I do know that too hot is too hot and probably too bright? I have never experimented with different watt charge bulbs.
    #13
  14. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    ...looking at the Clymer manual it shows a spring on the idle jet (internal), which indeed mine has...check your manual Super.

    I understand that they rotate and can become non working dyamically. I don't understand what you mean by 'test it last'.

    I have the parts out of the bike now...so, rotor measures as good per the Clymer manual.
    #14
  15. supershaft

    supershaft because I can

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    That is the mixture screw, not the idle jet.

    Test it last because you don't need a known good part to test the rest.
    #15
  16. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Rotors and diode boards are kind of hard to test statically since under load they may not stand up. They may test good statically, but act oddly in use - seem to be working, but not quite right, or not work at all. The rotor windings may fling outward and short, the diode may function with the meter current flowing through it but not with actual power from the alternator at operating temps.

    One of the places to look for bad connections (and quite often overlooked) is the light board in the gauge cluster. Two out of three I see have high resistance and my handling is to solder the foil to the input pins.

    I doubt the high beam indicator blue lens melted due to too high wattage bulb. There's a long tunnel from the bulb that channels the light to it, and besides, those sockets will take only one size bulb. It's more likely the lens deformed over time from the sun and getting pretty hot in there.
    #16
  17. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    Thanks Wirespokes. I have the gauge cluster apart right now and I can see that the lightbulb socket contacts on the strip looks corroded.
    Possibly a source of a blinking gen light?
    One bulb (oil) is burned out as well.
    #17
  18. lkchris

    lkchris Albuquerque

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    When "carbs won't adjust," check compression.
    #18
  19. Rapid Dog

    Rapid Dog bikes, booze, broads...

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    Ordered a new starter relay just to eliminate that question.
    Had my NASA tech buddy run a static test on the diode board, rotor, and aternator. They all check out good.

    Checked all the grounds and connections for good contact.
    I'm thinking of doing an extra earth cable as described here for good measure: http://robfrankham.co.uk/bike/earthing_article.htm

    In reference to the carbs, I checked the two tiny passages at the inlet , one where the idle screw comes in. I ran fishing line up thru the passages, no blockage there.
    As a note, compression is good, newly rebuilt heads.
    I have new idle screws and o-rings ordered.

    If all comes together this weekend I might have some results...

    I do have one question. The 'safety pins' for the float needles., do those just lay there or are they suppose to be hooked up to the floats somehow?
    I seem to remember pulling my floats on the old /7 and the valves hanging from them...
    Can't remember what went on when I pulled these apart.
    #19
  20. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Those safety clips go through the hole in the float valve (of course) and around the float tang the valve presses against. It helps hold the two together, and the purpose, I presume, is to ensure the valve opens the instant fuel level drops.

    The bulb contacts are important, but another place to check is where the large wiring harness plug attaches to the light board. Those pins are just a press-fit connection to the light board traces and corrosion builds up at that point. Checking with an ohm meter can show thousands of ohms resistance when it should be close to zero.
    #20