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Misfiring, No power

Discussion in 'Norton Commando Motorcycles (Classic)' started by Stephen Hill, Jul 12, 2018.

  1. Stephen Hill

    Stephen Hill

    Oct 28, 2009
    I'm trying to sort out my 72 Commando for the ride to the INOA rally in Elma.
    The bike starts fine, idles, and runs well at low to mid-range rpm. Until I get into the 2/3 throttle range and above. At that point the engine stumbles badly, and if I twist the throttle all the way open, the engine loses all power and de-accelerates. If I back off the throttle from full, it stumbles again, and if I back it off more, the engine starts pulling strongly.
    What the hell is going on here?
    The carb is clean. Slides and jets are all correct and in good nick (new needles, needle jets, slides). I tried moving the needle down, then back to the middle position: no difference. It had 200 main jets, which I switched to 220 main jet, no difference. If I turn the choke on, it doesn't change the stumbling or power loss.

    I am starting to think the problem is electrical, not carburation. I am going to try swapping the boyer box, and coils.

    Any thoughts???

    Stephen Hill
  2. nortonspeed


    Jun 29, 2008
    It is a common problem with Boyer ignition; the wires from the stator plate tend to break near where they are glued to the stator plate. You probably won’t see it because the copper wire inside the insulation breaks resulting in an irregular connection. I did repair the wires a couple of times but after I changed to Pazon Sure Fire this problem never occurred again.
  3. Stephen Hill

    Stephen Hill

    Oct 28, 2009
    The broken Boyer wire thing happened to me in the past, and is definitely worth looking into again. Some time ago, after I fixed the broken wire coming from the stator, I packed the space around the stator with poly packing material to remove any vibration, and hoped that would avoid the problem returning. Maybe it didn't work as I planned.

  4. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    Pull the fuel lines off the carbs and check for full fuel flow through the petcocks. Some of them are known to plug up when used with alcohol fuel. Jim
  5. maylar


    May 13, 2007
    I agree with Jim - check fuel flow. Also check that your air cleaner isn't wet with oil (assuming stock paper filter).
  6. o0norton0o

    o0norton0o VIP MEMBER

    Apr 27, 2015
    Pull the plugs and have a look at them. Are they clean or dark colored? Are they different colored from each other?

    So,... have you tried spraying some carb cleaner around the intake manifold while the bike is running to check for manifold air leaks??

    Have you pulled the float bowl plugs to see if the mains jets are still threaded on to the jet bodies??

    From your discription, I would say it's a carb or both carbs. If it's not carb related, I'd check the coils. If one of your coils is bad, then maybe it can't keep up with producing a hot spark at higher rpms.
    Last edited: Jul 12, 2018
  7. Stephen Hill

    Stephen Hill

    Oct 28, 2009
    Today I swapped out the coils, with no improvement.
    Also swapped out the Boyer black box, with no improvement.
    Yes, the main jets are still threaded in the jet bodies.

    Tomorrow I will:
    -check the timing.
    -check for leaks around the intake manifolds.
    -check the fuel flow through the petcocks.
    -remove the air cleaner to ensure it it not clogged with oil.
    -And check the wires where they attach to the stator.

  8. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    Check the fuel flow right through to the float bowl bungs.

    I must say, I’d have started with the advised checks that don’t cost anything and done a full service on the engine.

    And looked into the suggestion in the first reply.
  9. pommie john

    pommie john

    Nov 18, 2005
    Those symptoms sound like fouled spark plugs. Try a fresh set of spark plugs.
  10. Jagbruno


    Jan 6, 2014
    That, or smaller main jets. You said you went from 200 to 220 main jet, but the symptoms you describe are those of over richness at WOT.
    Has the bike worked properly with 200 jet in the past? If not you may want to try a 180...
    (I assume it is a single Mikuni)
    Then again, you could fit a couple of Amal and enjoy the original Norton experience ;-)
  11. DogT


    Jan 20, 2009
    Yes, it does sound like it's too rich at WOT. The symptoms remind me of when I was going down the road with the choke on, ran pretty good at low speed but bogged down opening the throttle and at higher speeds. Problem is finding why.
  12. Stephen Hill

    Stephen Hill

    Oct 28, 2009
    The torture has over! In case anybody is following my efforts to get rid of high throttle/load bogging and misfiring.
    BTW, running dual Amal 30s with stock needles, slides, jets, etc, all in new/clean/tight condition.

    Here is a list of what I tried, with no success:

    Changed needle height.
    Replaced 200 main jet with 220.
    Checked connections and wires to Boyer black box and to magnetic sensor on camshaft.
    Swapped coils.
    Swapped Boyer black box.
    Checked timing: about 8 degrees retarded. Timed engine.
    Checked flow of fuel through petcocks.
    Swapped Boyer stator/pickup.
    Bypassed cutout switch.
    Removed K@N air cleaner, with lots of oil. Problem solved.

    Looks like I over-oiled the K@N. Duh!
    Kudos to Dave from CT, aka maylar, who called it. Dave, if you get to Elma, the beer is on me.
    Note to Triton Thrasher, all the junk I swapped was in my parts bin so cost me nothing. When you have to buy the stuff, it changes the approach.

    Stephen Hill
  13. o0norton0o

    o0norton0o VIP MEMBER

    Apr 27, 2015
    So your timing was retarded 8 degrees??... That sounds like your issue right there. The faster your piston moves, the more you need to advance the spark. Being 8 degrees retarded is the culprit IMO.

    If it was an over oiled air filter, why didn't it effect your bike's lower RPM range too??...
  14. Stephen Hill

    Stephen Hill

    Oct 28, 2009
    Wasn't the timing. After I retimed it, the misfiring problem was still there.
    Only when I removed the oily air cleaner did the misfiring stop.
    Tomorrow, with a lightly oiled air cleaner, hopefully the problem won't return.
    At least I know it is fuel related, not electrical. And the problem is richness, not leanness.
    If the plugs look black, I might go back to the 200 main jet and lower the needle.

  15. maylar


    May 13, 2007
    Awesome, glad you got it sorted. Here's how I came to be aware of that condition:

    Bought my 850 new in 1975. One day I noticed the bike having trouble revving under load. Worse going uphill, worse with a passenger. It was still under warrantee so I took it to my dealer. They putzed with it for a few days... timing, points, plugs, etc and it still wouldn't go. Then I noticed oil dripping from my air filter. I had overfilled the oil tank and the breather pipe spewed oil into the air cleaner. Removed the filter and all was well again.

    That was the last time anyone but me worked on my bike.

  16. worntorn


    Dec 22, 2006
    Couldn't agree more on doing your own service work.
    As Red Green says, when you take it to a shop, you never know if they are doing bad work.
    Whereas when you do it yourself, ya KNOW!!
    I just overfilled my MK3 a little last week and had the DOh! moment all over again.
    It doesn't like even 1/8" over the full mark.

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