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Spark Plug Wire Interference

Discussion in 'Norton Commando Motorcycles (Classic)' started by marshg246, Sep 8, 2019.

  1. marshg246

    marshg246 VIP MEMBER

    Joined:
    Jul 12, 2015
    I'm trying to finish rebuilding a 1974 850 and I've run into a silly problem. The spark plug wires at the coil end are too close to the top exhaust valve cover nut. It's way too much to show here in pictures and comments so I've made a quick web page to show the problems: https://www.gregmarsh.com/MC/Norton/Norton_Spark_Plug_Lead.aspx

    One thing I didn't discuss in the web page is that if the coils are spaced wider by a lot, the problem can go away, but then getting the already difficult front tank mount nuts on and off becomes a real problem.

    I imagine that others have had this problem and that some have solved it in some way that escapes me.
     
  2. lazyeye6

    lazyeye6

    Joined:
    Feb 28, 2014
    I had read years ago that due to the close proximity of the spark plug wire to the top exhaust valve cover
    nut, that the spark could "jump" and ground to the nut. Now, I never experienced that, but it certainly
    could happen. I've never had a problem getting the front tank mounting nuts on and off. It's a Norton!
     
  3. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    I have had the replacement coils you show in the link - with the boots almost touching the nuts - for more than 10 years now with no arcing.

    The "gap" formed by the thickness of the plug wire insulation plus the boot is more than enough to prevent arcing at that point IF the wire/boot/coil tower is in good condition and not carbon-tracked.

    There are many applications on cars/motos where the spark plug wires run directly on/through grounded metal components. If there is arcing, there is a defect in some part of the secondary system - carbon tracking is the first of the 'usual suspects'..
     
  4. marshg246

    marshg246 VIP MEMBER

    Joined:
    Jul 12, 2015
    Yes, but there's a big difference in almost touching and pressing on. With a rubber mounted engine and no gap the engine rubbing/hitting on the wire, something is bound to break over time. Also, I didn't mention arcing although over time, that's a concern in some of the combinations I showed.
     
  5. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    True, if it is actually touching/pressing then, over time, it will wear the boot/wire, possibly enough to cause a problem. Mine do not touch, though they are very close and I have the correct Commando reflectors which, as you noted, force the coil tower to be quite close to the valve cover nut.

    But again, I have had no trouble at all with any rubbing. I suppose there is sufficient variation in frame alignment/dimensions and isolastic rubbers to cause a problem in some cases but not in others...
     
  6. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Looks like it is just the AN coil end cable boot that is different enough in dimension compared to OEM? You can just change to boots if you are concerned.
    I do like the solution you showed with placing the side reflector further forward and downward on a homemade bracket sothe coil can move forward. I need to do similar on my 850. My repo roadster tank interferes with reflector on right side. Had to remove reflector to fit tank. My bike right Frame downtube has been distorted from a DPO running highway bars and there is even less clearance for the right coil.
     
  7. batrider

    batrider

    Joined:
    Aug 9, 2008
    The HV output terminal is closer to the center of the coil on the problem coils... So it can't rotate away from interfering like the stock Lucas coils do. It would also help to just use ring terminals on the coil primary posts to reduce the interference plus it is just good practice anyway.
     
    DevonNorton and Craig like this.
  8. marshg246

    marshg246 VIP MEMBER

    Joined:
    Jul 12, 2015
    Yes, it is the boots. They are shaped like the originals, but much thicker. They appear to be molded on - they certainly don't move with reasonable force. The OEM boot would come off, but the whole point of expensive new wires was not to use 45 year old wires or parts of them but I'm going to because I tested them in the dark and saw no leakage.

    It's a shame because the AN wires are very good quality and exactly the same length as the OEM wires so otherwise, they fit great.
     
  9. batrider

    batrider

    Joined:
    Aug 9, 2008
    Try a mist bottle in the dark for a good test of the boots. I had the spark traveling almost 2 inches in a sudden thunderstorm which made the ignition quit and me have to push. When it dried out it was fine.
     
  10. seattle##gs

    seattle##gs

    Joined:
    Oct 28, 2014
    I've found that the sparkplug wires do not stand up to any abrasion especially in the headsteady region. Even if they are touching rubber. I carved out a coil holder from a block of aluminum and moved the coils inward about 1/4" . It is thick so I could thread every hole...where the coils bolt on and where the bracket bolts to the frame. No more fiddling with nuts. I also cut a trough where the Pazon black box fits secured by a couple of zip ties. It works very well.
     
    marshg246 likes this.

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