CB125RS sorted

Discussion in 'Classic Motorbikes' started by The Older Gentleman, Nov 23, 2008.

  1. Checked the timing (as best I could - those 125s are a bugger for trying
    to get a static light on[1], and I couldn' shift the screw-in plate on
    the generator cover[2], to access the crankshaft nut for turning the
    engine. So had to stick it in gear and turn it by hand.

    New points installed, gap set, but... still misfiring. The midrange was
    cleaner with the choke on, so OK, it's almost certainly the carb.

    Hm. OK, dig out the £30 used carb I bought, shove that on, and...


    Perfect. The cam that operates the accelerator pump is a bit clogged
    with dirt, and doesn't return freely, but it runs perfectly now.

    I might just clean up this carb and leave it as is, or get the original
    one (which is way cleaner externally) ultrasonically cleaned. There's a
    place down the road that does it for nottalottamoney.

    A couple of hours in the garage well spent.

    [1] The light just *will not* light no matter what you do to the points
    gap. Using a multimeter with an audible buzzer that should stop when the
    points open doesn't work either - it just buzzes all the time.
    Infuriatingly, it works if you test the points off the bike. In theory,
    the points are shorting out, but they ain't as it runs. I'll
    double-check with a fag paper, methinks.
    [2] A round screw-in cover about two inches in diameter with a shallow
    slot in it. Bastard thing's stuck and I don't have an impact screwdriver
    with one-inch blade, which seems to be the tool to shift it.
    The Older Gentleman, Nov 23, 2008
    1. Advertisements

  2. The Older Gentleman

    Timo Geusch Guest

    I'd get it cleaned. You never know how long the other one will play.
    Beats farting about with an ovloV in the pissing rain.
    Timo Geusch, Nov 23, 2008
    1. Advertisements

  3. The Older Gentleman

    Guest Guest

    Remember that sort of thing from years back.

    You might want to try an old 2 pence piece cut in half and held in a
    mole wrench. Being copper it'll do less damage if it slips. Current 2p
    pieces are copper in name only though, to the extent they're magnetic.
    Folded-over water pipe might do it too, but you really need a good
    square edge for it to sit well in the groove.

    T'other thing is warm the engine then use freezer spray on the plate,
    but that's obvious really.

    Got a feeling I'm telling granny...
    Guest, Nov 23, 2008
  4. The Older Gentleman

    SteveH Guest

    Could be worse - you could have been pissing about with an Alfa in the

    Just about managed to get the nose far enough under the garage up and
    over door to stay relatively dry, though.
    SteveH, Nov 23, 2008
  5. The Older Gentleman

    Mark Guest

    Did you disconnect the low-tension wire from points to the coil.

    Low-tension side of coil is earthed one side and feed to points the other.

    Coil resistance is under 2ohm.

    Assuming that the cbr is the same as cb/cg. :(

    Mark, Nov 23, 2008
  6. No. But then you wouldn't get current across the points, would you, so a
    timing light wouldn't work?

    I attach one crocodile clip to the points, where the wire attaches, and
    the other to earth, like a cylinder fin. It works on just about
    everything, except Honda 125s, for some reason.
    The Older Gentleman, Nov 23, 2008
  7. I tried that, with an old half-crown, believe it or not. Well, a whole
    one, not halved. And it bent.
    This is the problem
    I'd thought of warming the engine, but using freezer spray on the plate
    I hadn't thought of. I might try that. I'll see if my local dealer has a
    tool for the job first.
    The Older Gentleman, Nov 23, 2008
  8. Yeah, good popint. It looks so clean it could be nearly new (same with
    the whole bike) but it's obviously got something nasty inside. My guess
    is the needle jet. It starts fine, ticks over like a watch, runs well
    flat out, but the midrange is one big misfire.
    The Older Gentleman, Nov 23, 2008
  9. Using the patented Mavis Beacon "Hunt&Peck" Technique, The Older
    Actually, they are.

    It's a Honda 125. The points are only there to give you something to
    play with. The ignition is actually done by folk memory.
    Wicked Uncle Nigel, Nov 23, 2008
  10. Idiot....
    The Older Gentleman, Nov 23, 2008
  11. The Older Gentleman

    platypus Guest

    Radio 4 method?
    platypus, Nov 23, 2008
  12. Thought about that. Can't find a battery-operated transistor radio in
    the house.
    The Older Gentleman, Nov 23, 2008
  13. Using the patented Mavis Beacon "Hunt&Peck" Technique, The Older
    I've been called worse.
    Wicked Uncle Nigel, Nov 23, 2008
  14. The Older Gentleman

    Rusty_Hinge Guest

    The message <1iqvery.1r0vcaxgx615mN%>
    from (The Older Gentleman) contains these
    I remember someone saying that to my great, great, great, great grandfather.
    Rusty_Hinge, Nov 23, 2008
  15. The Older Gentleman

    Mark Guest

    Well try it again, disconnecting the points lead at a snap connector
    Otherwise you are measuring the resistance of both the flywheel coil and the
    HT primary in parallel when the points are open which antalot. Even then it
    will measure something like 4ohm with the points open so you need a DVM set
    to a low Ohm range not a buzzer
    see "for some reason"
    Its how I do my SL125 without any problems.

    Mark, Nov 23, 2008
  16. Brillitant. I didn't know that, so thanks for the heads-up. Every day's
    a schoolday.
    The Older Gentleman, Nov 24, 2008
  17. Sounds familiar, I think my A100-II was like that.

    Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration,
    Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
    GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
    WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
    KotPT -- "for stupidity above and beyond the call of duty".
    Dr Ivan D. Reid, Nov 26, 2008
  18. The Older Gentleman

    sweller Guest

    I have a special light for this purpose. Special as in an MZ gave up an
    indicator for it.
    sweller, Nov 26, 2008
  19. Thought about that. Can't find a battery-operated transistor radio in
    the house.[/QUOTE]

    Quick and dirty method is the LT spark.

    Set the gap and line up the timing marks, turn the ignition on and
    gently press the points cam sideways. As you do so, you'll see a small
    spark leap across the point faces. Adjust points timing to suit. With
    practice you can get this to be dead on.
    In the early days of using this method, I double checked it against more
    tech ways and it was just as good, given the accuracy limitations of
    points ignition.
    This can be used on any bike or car with no need for any special tools
    but what you've got in your head.
    Grimly Curmudgeon, Dec 26, 2008
  20. I did what someone else suggested, in the end. Used a multimeter, but
    disconnected the wire to the coil. Worked a treat.
    The Older Gentleman, Dec 26, 2008
    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.