1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

1975 Mk III cutting out

Discussion in 'Norton Commando Motorcycles (Classic)' started by Sitman, Sep 12, 2018.

  1. Sitman


    Jun 13, 2018
    Hi all

    may I seek your advice and experience please.

    A Norton 850 Commando Mk III. Picked it up rode it 60 miles and no issues. It's got the period Mikuni carb conversion and behaves fine most times. It idles well but some times riding it along at commuter pace it just cuts out. Usually by the time it coasts to the road side, a quick press of the electric start and she fires up again.

    I try avoiding premium or higher octane fuel and whilst the fuel is 6 months old, to me it doesn't seem to be suffering what I'd expect stale fuel to behave.

    Does it sound like something is overheating in the electrics? I've checked and not found any loose connections. Timing is right. In fact everything I've checked is in my eyes correct.

    What do you guys think? Where to look next or have you experienced this before?

    Any feedback appreciated.

  2. kommando


    May 7, 2005
    Try running with the fuel cap loose, if it runs properly then clean out the fuel cap breather.
  3. MexicoMike


    Jan 31, 2010
    So it was OK running down the road and only cuts out at low speed?

    Could just be the idle speed/mixture setting ? Will it idle in the driveway with no issues after being warmed up? What is your idle RPM setting? If it won't idle well/operate at low speeds, could be an Intake air leak or a gunked-up idle/low speed circuit in the carb.

    I'd like to say, "It's that dang Mik carburetor; put a pair of Amals back on it and that will fix it!" But, though I think you should do that anyway, it's not what's causing the problem - at least not directly. If it's dirty/clogged, that can happen to any carb.

    Of course, the easiest first attempt at a "fix" would be a fresh tank of fuel. Longevity-wise, modern gasoline is pretty grim stuff compared to what that bike saw when new...

    If the carb is internally gummed up, it will probably take disassembly/cleaning to fix it. Fresh fuel won't dissolve the deposits.
  4. jaydee75


    Jul 2, 2012
    The ignition runs thru the Kill Switch, and they corrode very badly. Sometimes it makes a hit and miss connection. I recommend you take it apart and inspect and clean it in any case.
  5. brokeneagle

    brokeneagle VIP MEMBER

    Oct 30, 2014
    Check for any loose electrical fittings, especially the blade type at coils or ignition switch. The old wires become stiff with age and "learn" where they belong. As the vibration or wind increases they fall away enough to kill the motor and when you stop they spring back enough to make a connection.
  6. seattle##gs


    Oct 28, 2014
    I will guess a bad electrical connection. If the bike is new to you it's time to remove the tank and go through all connections and especially cleaning up all grounds. You will probably find the problem somewhere in the rats nest of wire. And if someone has twisted wires together..for instance while installing a Boyer...you need to put on proper connections.
  7. 3 of them

    3 of them

    Jan 22, 2018
    try the rubber manifold for leaks .
  8. Torontonian


    Dec 28, 2009
    Electrics. Kill switch elimination. Did I miss something but what kind of ign. ?
  9. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008

    If the kill switch is presenting a bad connection try aggressively pushing it many times when the problem re-occurs after testing for a plugged tank breather.

    Simple problem usually means simply fix; if you go looking for trouble on a machine that old you'll, probably, end up creating it.
  10. Torontonian


    Dec 28, 2009
    Just leave the cap slightly loose with half a tank of gas or less. If it craps out again then that is eliminated.
    Another option is to go driving with both taps fully on/open.
  11. MexicoMike


    Jan 31, 2010
    It doesn't seem likely to me to be a fuel cap venting issue if the bike runs OK down the highway but not at low speed. Poor/no venting would show up quicker at higher loads. If I understand the symptoms correctly - poor running at low speed, OK at high speed, then I'd say the same re ignition; ignition issues show up as load increases, not the other way around.

    HOWEVER, it IS quite possible, IMO, that the additional vibration at low/idle speed could cause some electrical issues... ;)
    Last edited: Sep 14, 2018
  12. Madnorton

    Madnorton VIP MEMBER

    Jun 9, 2008
    Wiring loom under the tank, good place to look wire worn through.
  13. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    What sort of ignition system does it have?
  14. Sitman


    Jun 13, 2018
    Boyer Mk3.

    Thanks everyone really interesting to look into your suggestions.

Share This Page