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850 Bad running, engine noise --gas cap?

Discussion in 'Norton Commando Motorcycles (Classic)' started by 998cc, Oct 14, 2018.

  1. 998cc

    998cc

    Joined:
    Oct 1, 2018
    After a couple of weeks de-bugging the 850, I finally took it onto the freeway Saturday. Earlier in the day, the new Ceandess flip gas cap arrived in the mail, so I installed it just before leaving.

    The bike ran well at about 72 MPH heading uphill into the mountains with both fuel taps on. About 12 miles from home, traffic was coming up behind and passing, so I took it right up to about 77 MPH--no worries for about a half mile until the bike started bogging down with the engine emitting what sounded like a rod knocking. Dropped it to 50 MPH, took the first exit; the noise went away and smooth running returned. Back home at low speed. Oil and fuel levels are good, no excess slack in the primary chain. I pulled the external oil filter checking for metal fragments or particles and found nothing. The assumption is the noise was pre-ignition or perhaps detonation due to a lean mixture.

    The sequence of events suggests lack of fuel cap venting or other fuel restriction but I want to get input here. Has anyone used these new Ceandess caps with ill effects or no effects?

    Also, it is still running points ignition which will convert to electronic shortly.

    Thanks.
    ~998cc
     
  2. triumph2

    triumph2

    Joined:
    Aug 13, 2008
    What carb set up do you have?
     
  3. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    pull the plugs out and inspect them. If they are dark, maybe your main jet size is too large and your mixture was getting richer the more you opened the throttle until it was so rich you were getting a misfire at WOT. Maybe one of your enrichment lever cables popped and one carb's choke is "on" hanging down into the carb opening.

    Either way, the plug color might indicate what is happening. Needless to say white or super clean plugs could indicate the opposite situation (lean) that could be caused by any number of things. Maybe a needle came unclipped and isn't moving up and down with the slide body.... Ya just gotta slow down and slowly go through stuff. Things like changing the spark plugs because they are cheap is a worthwhile expense to eliminate an intermittently bad plug... yes, I was tortured by that...

    The good thing is that a running bike can usually be diagnosed and sorted out because it gives you symptoms to steer you in the direction you need to go...
     
    998cc likes this.
  4. dynodave

    dynodave

    Joined:
    May 28, 2003
    Which Candess cap?
    Some older but emission era lotus' had charcoal canisters which meant sealed gas tanks. ie sealed/nonventing candess caps.
    Lean out the carbs and seize the pistons, especially when "at speed"
     
  5. batrider

    batrider

    Joined:
    Aug 9, 2008
    There should be a small vent hole in the metal disk with the rubber seal on the underside of the cap. You can check and see if it passes air.
     
    dynodave likes this.
  6. cash

    cash

    Joined:
    Nov 10, 2006
    Remove the fuel pipes from the carbs and run the fuel into a container you'll soon see if there is a flow problem.
    Dave
     
    998cc likes this.
  7. dynodave

    dynodave

    Joined:
    May 28, 2003
    Yes the tanks side disc has a hole in it. Then there is the upper metal disc with a hole 180 degrees from the tank side disc. The rubber is sandwiched between them. This is the vent baffle area and air runs within ID of the rubber.
    The rubber tube/seal prevents gas sloshing out from under the cap
     
  8. 998cc

    998cc

    Joined:
    Oct 1, 2018
    To answer a couple of questions, yes, there is a vent hole in the disc; the carbs are Amal 932's. I'll look at the plugs and chokes asap.

    I like the idea of testing the fuel flow by opening the taps into a container. That should easily expose a venting problem! :)

    Thanks.
    ~998cc
     
  9. jsnorton

    jsnorton

    Joined:
    Oct 10, 2018
    If you suspect a venting problem, just pop the cap open when it starts to act up, that should tell you.
     
  10. 998cc

    998cc

    Joined:
    Oct 1, 2018
    Thanks Jsnorton.
    I did think about that very thing and would consider it, but the knocking sound was severe, and I wish to avoid any engine damage while in the process of working out the problem. It may come to that at some point, but I'll start with open taps.
    ~998cc
     
  11. cash

    cash

    Joined:
    Nov 10, 2006
    A long shot, do you have a anti-drain valve in the oil line?
     
    snowdr likes this.
  12. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    What RPM range were you at when the knocking occurred? Could it be the ignition timing/auto advance sticking, as this seems to be a common issue on the points setup.
    BTW, when setting timing, be aware that timing scale under the primary inspection cap can off for accurate position by quite a few degrees...need to locate TDC and then position crank to 28 BTDC via a timing wheel, then check rotator mark against timing scale to determine how far off yours is.
     
  13. 998cc

    998cc

    Joined:
    Oct 1, 2018
    Cash, Tornado. To answer your questions,
    1. There is no anti-drain valve in the oil line, but hey, long shots occasionally hit the mark. ;)
    2. I wasn't watching the tach, but was at 75 MPH or so in 4th gear on an uphill grade. I did replace the points with the Pazon system last Saturday which was a breeze to install. Put about 110 miles on the bike Sunday riding backroads and secondary highways with no issues riding in various terrain topping out at 70+MPH for short durations. Before the ride, I pulled the carb bowl drains and found a small amount of unidentifiable matter that was large enough to plug a main jet.
    Following up on the flow test, the gas cap vent seems to function properly; the flow from the left tap does seem a little low; the right tap is flowing very well. Both taps were open during the incident. I have another carb question I cannot find an answer to. Will post a new thread.

    Thanks.
    998cc
     
  14. batrider

    batrider

    Joined:
    Aug 9, 2008
    Be sure there is gas at the bottom of the float bowls not just at the petcocks. I have found gray sticky goop plugging my banjo fittings on the carb bowls twice this past season. (This could be coming from my Emgo tank but need to dig into it a bit more.)
     
    snowdr likes this.