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Stump the Chumps-intermittent misfiring (2015)

Discussion in 'Norton Commando Motorcycles (Classic)' started by Milo of Vt., Aug 10, 2015.

  1. Milo of Vt.

    Milo of Vt.

    Dec 11, 2012
    Okay, I need help...and not the kind my wife says I need.

    I completely rebuilt my '69 Commando a couple of years ago. Engine down to last nut and bolt. After a couple minor break-in issues, it has been running strong. Taken several hundred + mile trips without incident. About 2,500 miles total since rebuild. When rebuilt went with electronic ignition (Pazon), Mecuni carb, new German canister coils, new wiring to coils, new spark plug wires.

    I seem to have developed an intermittent firing issue. Happens in a funny way. First time it happened I took it to work some 14 + miles. Fired up first kick in morning and went like a bear to the office. End of day fired up great (with choke, as always) but then when warmed up and choke off, ran very poorly, backfiring, barely could keep it running for about two miles. Then all of a sudden, it started firing well and ran great all way home. I attributed to bit of bad gas (hopeful and not very logical I know)

    Next time out, same thing...ran great to work, but on way out at end of day, terrible missing etc. for about 2-3 miles and then bang...starts running fine again. That evening stopped at friend's shop. When went to start it, backfiring so bad flames shooting out of back...had to get a ride home.

    Went back to shop next day, took tank off and checked all wires to coils, plugs. Found a couple loose connections and tightened them but no smoking gun. Check timing. Dead on. Plugs sooty (as might expect) so blasted them. Bike started like a charm and took out for a good run. No problem.

    Next day checked plugs. Still a little sooty so leaned out mixture. Started great and ran great on another test. Plugs looked better after ... so all fixed! (Right)

    Few days later ran 20 miles and was great. Later in day, started back up and again backfiring, shooting flames (think mostly on right side).

    A friend says that he thinks I am fouling plugs by running rich...he suggests dropping the needle in the carb.

    I say how could it run fine for 2,500 miles and then run great or not, intermittently, if that was the issue?

    When I first rebuilt the bike, I had intermittent running, esp. when bike was hot. Turned out the coils were bad so I bought the new ones
    that are on the bike and that ended that problem. This intermittent misfiring acts like the same issue but the periods of running good and bad are longer and often when cold, not when hot. I could just swap out the coils for new ones, but I hate throwing new parts at a problem. Can a coil be intermittent in both hot and cold state?

    Any thoughts... :?
  2. concours

    concours VIP MEMBER

    Dec 29, 2011
    Check (by testing) for broken wire at the flexure of the ignition wiring where it exits the timing chest, and also flaky kill switch contacts/wiring.
    If your MIKuni starts with no choke, you've got a too rich setup, or failed enrichener plunger seal/cable misadjustment(if it has one) or incorrect float height,etc.
  3. MikeG


    May 31, 2012
    I had a similar problem with mine a while back. It would run fine one minute and pop, fart and bang the next. Turned out to be a loose ground wire in ignition circuit due to broken female gang connector. They can be hard to detect if you don't know the failure mode.
  4. Anonymous

    Anonymous Guest

    Hi Milo.
    Check the coil terminals if still using spade type. Crimp tighter or replace with eyelet type directly screwed to the post.
    I found that modern coils come with thinner spade gangs and the original wiring terminals do not grip them as tightly.
  5. bill


    Jun 1, 2003
    +1 even if this is not your trouble I do not trust spade terminals when ever there is another option.

  6. Milo of Vt.

    Milo of Vt.

    Dec 11, 2012
    All good ideas. Thanks to all.

    One of the "loose connections" I found when I thought I had licked the problem was one of the two-pronged spade connectors had become loose where it is attached to the coil by a nut on threaded stud. I tightened that, although I doubted it was the problem as the nut was only loose. The other problem I found was that one of the female to spade connectors was not actually on the spade but held up against it by the plastic cover due to the opening being crushed. I replaced that. So, I looked that those connections pretty well and thought I'd found two good possible causes. Alas, not the case. My colleague did recommend replacing the spade connectors with round eyelets which I think I'll do now. I am intrigued by the possibility that it could be the wire coming from the Pazon through the case. Still what bothers me about a faulty wire being the problem is that it will go great for a long ride, and only be bad when the bike cools off and is restarted. I'd think a faulty wire would tend to be intermittent during a ride, due to vibration effects.

    No, the bike won't start without the choke when cold, and actually is a little cold-blooded. Takes 3-4 minutes at least to get it to run without choke at idle. The carb was brand new 2,500 miles ago, so I doubt it is broken, but if I don't find a short, I will need to look.

    No one likes my coil idea. :(
  7. worntorn


    Dec 22, 2006
    Internally corroded ignition switch? Check by bypassing with a fused jumper supply wire directly from battery to Pazon.

  8. concours

    concours VIP MEMBER

    Dec 29, 2011
    Swap it out, it's only a few bucks, relatively speaking. :mrgreen:
  9. concours

    concours VIP MEMBER

    Dec 29, 2011
    Spade terminals, like all connectors, must be inspected as they are assembled/connected.

    Have you replaced ALL THE FEMALE BULLET connecters on the bike? They have a pattern failure where they crack and do not make good contact, but inside the rubber boot are invisible. Some images here: twin-output-coil-t19939-30.html
  10. Milo of Vt.

    Milo of Vt.

    Dec 11, 2012
    Thanks guys...I'll try all reasonable solution and report back.
  11. concours

    concours VIP MEMBER

    Dec 29, 2011
    Jumper leads are a good way to test for flaky wiring/connections... bypass 'em
  12. Brly


    Dec 3, 2009
    I have a single Mikuni on my 850. If I leave the choke on that long, it will foul the plugs every time. In fact, I don't let it run with the choke on at all. I turn the choke on, kick it through a few times, take the choke off and then turn the ignition on and start it. Just have to stay on the throttle for a bit when it fires. Mine can take several minutes to warm up also and with the choke on, the plugs are guaranteed to foul.
    Doesn't explain why it only runs rough for a while and then runs fine, but fouled plugs will not help your trouble shooting any.
    Get some new plugs so you have a fresh start while you work through potential wiring issues.

  13. pete.v


    Oct 31, 2009
    Yes, I have a big pumper and just a couple twists of the throttle is enough, sort of like a tickle I guess. Choke for extreme cold only.

    A quick turn on the big fat throttle stop and a quick flip back when warm if needed.

    The point is, use the choke only if needed or at least get off it as soon as you're started.

  14. concours

    concours VIP MEMBER

    Dec 29, 2011
    Any updates?
  15. desmo


    Aug 16, 2019

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